Feeding the Corn Belt: Intensification of Corn Cultivation in the U.S. Corn Belt, Resource Inputs, Impacts, and Implications
Senior Thesis
Presented to The Faculty of the School of Arts and Sciences Brandeis University
Undergraduate Program in Environmental Studies, Advisor Dr. Dwight Peavey
In partial fulfillment of the requirements for the Degree of Bachelor of Arts
By: Hannah Moshay
May 1st, 2018
1
Abstract
The United States is currently the world’s largest producer, consumer, and exporter of corn. The concentrated cultivation of corn within the U.S. Corn Belt produces a third of the world’s corn. This intensive cultivation, has resulted from a number of resource inputs, namely land conversion, irrigation, and agrochemicals. The current corn management practices have been detrimental to the air, land, and water, and in turn resulted in increased nitrous oxide emissions, soil acidification, loss of carbon sequestration, and eutrophication. This thesis has two principle aims. Firstly, to compile and asses the historic and current practices of land use, water use, fertilizer use, and pesticide use within the U.S. Corn Belt. Secondly, to project global corn production to the year 2050 based on growing demand for livestock and ethanol, as well as the land, water, fertilizer, and pesticide input this will require. The following two facets of this thesis will be used to frame the argument that our current corn-dependent food systems and energy systems are fundamentally unsustainable, and have resulted in a “hungry-production system”.
2 Table of Contents
Cover Page……………………………………………………………………………………pg. 1
Abstract………………………………………………………………………………………..pg. 2
Table of Contents………………………………………………………………………….....pg. 3
Tables and Figures…………………………………………………………………...………pg. 4-6
Introduction……………………………………………………………………………..……..pg. 7-10
Corn in U.S. Agriculture……………………………………………………………....……...pg. 10-23
The Corn Belt…………………………………………………………………………….…....pg. 23-26
Ecoregions and Hydrology of the Corn Belt……………………………………………….pg. 26-37
Land Use………………………………………………………………………………………pg. 37-53
Water Use……………………………………………………………………………………..pg. 53-64
Fertilizer Use………………………………………………………………………………….pg. 64-87
Pesticide Use……………………………………………………………………....…………pg. 87-122
Environmental Impact Summary……………………………………………………………pg. 123
Climate Change………………………………………………………………………………pg. 124-132
Demand……………………………………………………………………………………….pg. 133-143
Calculation……………………………………………………………………………………pg. 143-149
Conclusion……………………………………………………………………………………pg. 149-152
Acknowledgements…………………………………………………………………………pg. 152-153
Sources………………………………………………………………………………………pg. 154-163
3
Tables
Table 1. U.S. Farm Sales Class Distribution………………………………………………..pg. 11 Table 2. U.S. Farm Sales Class and Farmland Operated………………………………...pg. 12 Table 3. Grain and Oilseed Operation Acreage…………...……………………………….pg. 13 Table 4. U.S. Planted Acreage of Field Crops…………………………….……………….pg. 13-14 Table 5. U.S. Production Total of Field Crops……………………………………………...pg. 14 Table 6. U.S. National Yield Average of Field Crops…………………………...………....pg. 15 Table 7. U.S. Commodity Prices of Field Crops…………………………...…………...….pg. 16 Table 8. Laingen’s Parameters………………………………………...…………………….pg. 24 Table 9. Laingen’s County Classification…………………………….…………………..…pg. 24-25 Table 10. State Acreage Granted Under the Swamp Lands Act...…………………...….pg. 38 Table 11. Total Subsurface Drainage Acreage and Percent Cropland with Subsurface Drainage by State…………………………………………………………………………...…………....pg. 42 Table 12. Historic Tallgrass, Mixed-grass, and Shortgrass Prairie Acreage……...…….pg. 45 Table 13. Cadmium Content of Phosphate Rocks…………………………...…………....pg. 83 Table 14. Claimed and Found Contents of NK Fertilizer Mix……………………………..pg. 84 Table 15. Claimed and Found Contents of Iron Amendment……………………………..pg. 84 Table 16. Claimed and Found Contents of Zinc and Zinc Sulfate………………………..pg. 85 Table 17. Glyphosate Resistant Weeds in the Corn Belt………………….…..………….pg. 101 Table 18. Atrazine Ecological Exposure Monitoring Program……………….…………...pg. 109 Table 19. Water Systems with Annual Averages above 3 ppb Atrazine………….……..pg. 117 Table 20. Water Systems with Highest Peak Atrazine Concentrations in Raw Water....pg. 118 Table 21. Water Systems with Highest Peak Atrazine Concentrations in Finished Water..pg. 119 Table 22. Bt-Crops, Cry Proteins and Target Pests………………………………………..pg. 122 Table 23. Summary of Intensive Agriculture Activities………………………………….....pg. 123 Table 24. Summary of Environmental Impacts of Intensive Agriculture Activities……...pg. 123 Table 25. Feed Conversion Ratio…………………………………………………………....pg. 135 Table 26. Projected Corn Production (bushels)...... pg. 144 Table 27. Projected Corn Harvested Acreage (acres)...... pg. 144-145 Table 28. Corn Yield Projections (bu/ac)...... pg. 145 Table 29. Corn Production, Acreage, and Yield Production for 2050…………….…..….pg. 145 Table 30. 2050 Water Requirements………………………………………………..……....pg. 146 Table 31. 2050 Fertilizer Use……………………………………………………..………….pg. 147 Table 32. 2050 Pesticides Use Prediction…………………………………..………...... pg. 148
Figures
Figure 1. U.S. Corn Acreage 1866-2015……………………………………...……………pg. 21 4 Figure 2. U.S. Total Corn Production 1866-2016……………………..……………….....pg. 22 Figure 3. U.S. Average Corn Yield 1866-2016………………………………………...... pg. 23 Figure 4. Map of the Corn Belt…………………………………………………………..….pg. 25 Figure 5. Corn Belt Ecoregions………………………………………………………….….pg. 27 Figure 6. United States River Basins……………………………………………..………..pg. 30 Figure 7. Upper Mississippi River Basin and Ohio Tennessee River Basin..………….pg. 31 Figure 8. Missouri River Basin……………………………………………..……………….pg. 32 Figure 9. United States Principal Aquifers…………………………..…………………….pg. 33 Figure 10. High Plains Aquifer…………………………………..………………………….pg. 36 Figure 11. Subsurface Drainage System……………………………………………..…...pg. 41 Figure 12. Subsurface Tile Drainage System Cover by County……………..………….pg. 42 Figure 13. Historic Boundaries of Tallgrass, Mixed-Grass, and Shortgrass Prairie…...pg. 44 Figure 14. U.S. Wind Speed Map…………………………………………………...……...pg. 47 Figure 15. Prairie dogs………………………………………………………...……………..pg. 48 Figure 16. Changes in CRP Enrollment by County…………………...…………………..pg. 51 Figure 17. Changes in Total CRP Enrollment Area……………...………………………..pg. 52 Figure 18. U.S. Water Withdrawals by Sector……………...……………………………...pg. 54 Figure 19. Irrigated Acreage Share by State………………………………………...…….pg. 56 Figure 20. Irrigated Acreage by Crop, Western and Eastern States…………………….pg. 57 Figure 21. Corn Yield Response to Irrigation……………………………………………....pg. 58 Figure 22. Nebraska and U.S. Irrigated versus Non-Irrigated Corn Yield……………….pg. 59 Figure 23. U.S. Drought 2012………………………………………………………………...pg. 60 Figure 24. Corn Water Use of Growing Season…………………………………………...pg. 61 Figure 25. U.S. Freshwater Withdrawals 2010…………………………………………….pg. 64 Figure 26. Synthetic Fertilizer Use by Four Major Crops 1964-2010…………………....pg. 66 Figure 27. U.S. Soil Nitrogen………………………………………………………………...pg. 67 Figure 28. Corn Nitrogen Use and Uptake…………………………………………...…….pg. 68 Figure 29. Seasonal Nitrogen Use…………………………………………………………..pg. 69 Figure 30. Fish Kill………………………………………………………………………...…..pg. 71 Figure 31. Nitrogen Loading from Corn Belt………………………………………………..pg. 72 Figure 32. Hypoxia in the Gulf of Mexico Dead Zone……………………………………..pg. 73 Figure 33. Nitrogen Inputs by County within Mississippi River Basin………..………....pg. 74 Figure 34. Tile Drainage Concentration by County in Mississippi River Basin…….…...pg. 75 Figure 35. Predicted Nitrogen Loading of Mississippi River Basin by County………….pg. 76 Figure 36. Groundwater Nitrate Contamination Risk……………………………………...pg. 77 Figure 37. Nebraska Groundwater Nitrate Contamination……………………………….pg. 78-79 Figure 38. Cadmium Concentrations in Grains with Manipulations of Soil pH and Soil Cadmium……………………………………………………………………………………….pg. 86-87 Figure 39. U.S. Annual Pesticide Use……………………………………………….……..pg. 88 Figure 40. Pesticide Use By Crop……………………………………………………...…...pg. 89 Figure 41. Corn Acreage Treated with Pesticides……………………………………...... pg. 90 Figure 42. Glyphosate Structure…………………………………………………....……….pg. 91 Figure 43. Glyphosate Use by Crop……………………………………………….………..pg. 92 5 Figure 44. Estimated Agriculture Use of Glyphosate 1992, Low Use Estimate……....pg. 93 Figure 45. Estimated Agriculture Use of Glyphosate 1992, High Use Estimate……...pg. 94 Figure 46. Herbicide Acreage in Relation to HT Corn Acreage………………………...pg. 95 Figure 47. Estimated Agricultural Use for Glyphosate 2015, Low Use Estimate……..pg. 96 Figure 48. Estimated Agricultural Use for Glyphosate 2015, High Use Estimate….....pg. 97 Figure 49. Collembola………………………………………………………………..……...pg. 100 Figure 50. Oniscidea………………………………………………………………………...pg. 100 Figure 51. Glyphosate Resistant Species by State……………………………………....pg. 101 Figure 52. Glyphosate Resistance in Populations………………………………...……..pg. 103 Figure 53. Atrazine Structure……………………………………………………………….pg. 104 Figure 54. Atrazine Use by Crop…………………………………………………………...pg. 105 Figure 55. Estimated Agricultural Use for Atrazine 2015, Low Use Estimate…………pg. 106 Figure 56. Estimated Agricultural Use for Atrazine 2015, High Use Estimate………...pg. 107 Figure 57. Atrazine Levels in West Branch Sugar Creek………………………………..pg. 110 Figure 58. Atrazine Levels in Little Sugar Creek………………………………………….pg. 110 Figure 59. Atrazine Levels in Branch of Boeuf River……………………………….…….pg. 111 Figure 60. Atrazine Levels in South Fabius River…………………………………...…...pg. 111 Figure 61. Atrazine Levels in Young Creek………………………………………………..pg. 112 Figure 62. Atrazine Levels in Honey Creek……………………………………………….pg. 112 Figure 63. Atrazine Levels in West Fork Cuivre River…………………………………...pg. 113 Figure 64. Atrazine Levels in Big Blue River, Upper Gage……………………………...pg. 113 Figure 65. Atrazine Levels in Chocolate Bayou…………………………………………..pg. 114 Figure 66. Atrazine Ecological Risk Map………………………………………………….pg. 115 Figure 67. Atrazine and Groundwater Contamination Risk……………………………..pg. 120 Figure 68. Cry Proteins……………………………………………………………………..pg. 121 Figure 69. Carbon Dioxide Atmospheric Concentrations……………………………….pg. 125 Figure 70. Methane Atmospheric Concentrations……………………………………….pg. 126 Figure 71. Nitrous Oxide Atmospheric Concentrations………………………………….pg. 127 Figure 72. High Plains Aquifer Groundwater Withdrawals for Irrigation……………….pg. 131 Figure 73. U.S. Corn Domestic Use……………………………………………………….pg. 133 Figure 74. U.S. Feed Grain Production…………………………………………………...pg. 134 Figure 75. Corn Use by Livestock Species……………………………………………….pg. 136 Figure 76. Livestock Feed Efficiency……………………………………………………...pg. 137 Figure 77. Corn Feed Grain and DDG/CGF……………………………………………...pg. 138 Figure 78. Corn Prices and Ethanol Use………………………………………………….pg. 139 Figure 79. World Corn Production…………………………………………………….…...pg. 141 Figure 80. World Corn Exports and Imports……………………………………………...pg. 142 Figure 81. Global Pork and Poultry Production Projections…………………………….pg. 143
6
Introduction
Objectives
This thesis aims to describe the resource use and environmental impact of industrial corn production within the U.S. It also aims to use this description to inform a forecast of possible resource use and consequences of industrial corn production globally as population growth and economic development occur. This will be executed by describing the resource input of industrial corn production on two scales: within the U.S. Corn Belt currently, and globally by the year 2050. Resource use will be defined using four key dimensions: land, water, fertilizer, and pesticide. Describing the resource use of current domestic corn production will reference historical progressions of technology, policy and key economic changes and will draw on various studies in multiple regions of the Corn Belt. This will build context to describe current cultivation practices and their impacts. The majority of the thesis will be spent on describing this, in an attempt to frame the mechanisms of industrial corn production using four key resources (land, water, fertilizer, and pesticide), as well as their impact on abiotic resources, biotic communities, and human health. Describing the ways in which corn has altered the ecosystem within a region such as the Corn Belt, which spans from West Ohio to East Nebraska, and from Southern Wisconsin to Missouri requires a limited scope and therefore the examples used aim to strategically build upon one another to describe overall how corn production has fundamentally disrupted natural cycles and ecology both directly in the regions with concentrated corn cultivation as well as proximate regions. Though the majority of this thesis will be spent describing the current state of U.S. corn production and its environmental impacts, these descriptions will serve as valuable examples of potential consequences if other developed and developing nations follow in the footsteps of U.S. corn production. This thesis will not attempt to create a perfect prediction of the environmental impacts of within nations which develop industrialized corn cultivation practices resembling those of the United States Corn Belt. Rather this thesis aims to draw from predictions of the FAO and OECD which estimate the scale at which corn production will need to increase and propose a scenario of resource use in which production would be achieved by the world adopting an industrial agricultural production model similar to that of the U.S. This thesis will extrapolate by drawing from predictions of the FAO and OECD of the world’s future corn production projections to the year 2050. From this year, working from yield projections for developed and developing nations also calculated by the FAO and OECD, this thesis will work back to estimate, based on the inputs of U.S. industrialized systems, how much land, water, fertilizer, and pesticide this will require. This is purely hypothetical, as even within the U.S. resource requirements for corn are highly variable, and it is without doubt new technology will influence resource use. However,
7 these predictions of resource use will be valuable in understanding the potentially devastating impacts of corn production on the environment, and substantiate growing concerns over satisfying the world’s growing demands for food and energy.
Outline
The structure of the thesis will be described in greater detail in the following section as to give some overview and insight into the structure and topics which are covered. Before describing in detail the environmental impacts of the U.S. Corn Belt, it is important to first provide the current structure of U.S. farms and production more broadly. This will be achieved in the section “Corn in U.S. Agriculture”. This section will define agricultural intensification, as well as briefly provide demographic information on individual American farm structure (farm size and sales class), the declining American agricultural workforce, as well as trends in the consolidation of cropland. Following which current American crop acreage dedication, crop commodity price, average yield, and production will be described for field crops which dominate the American agricultural landscape and market. Having described trends in individual farm characteristics, more attention will be given to historical events which greatly influenced corn acreage, production and yield as a means of giving historical context to the current industrial corn cultivation system of today. In the next section, “The Corn Belt”, focus will be placed on examining where the cultivation of corn is concentrated in the U.S., and geographic parameters will be established based on economic output, yield, and production of corn, which will guide the geographical scope of this thesis in focusing on corn production in the U.S. Having given background on historical and current U.S. corn production, as well as having defined the geographic area by economic output and intensive corn based agriculture, it is important to give background to the natural abiotic characteristics of the area in which corn cultivation is concentrated in the U.S. These abiotic characteristics are critical in influencing agricultural practices as well as the environmental impacts of agriculture. Describing these characteristics, and comparing the similarities and differences of abiotic factors throughout the Corn Belt, provides a foundation for the future sections describing resource use and environmental impacts. The next section “Ecoregions and Hydrology of the Corn Belt” distinguishes three regions of the Corn Belt and compares and contrasts each region’s climate, hydrology, geology and soil, as well as historic vegetation. This section also describes the corn belt’s hydrology including its connectivity to various watersheds as well as its aquifers. The following of which will be important in the following sections concerning direct resource use, such as groundwater withdrawal in the water use section, as well as in terms of agrochemical mobility and contamination, described in the following sections of resource use and environmental impacts. The next four sections, “Land Use”, “Water Use”, “Fertilizer Use”, and “Pesticide Use” will look at each of these as resources within U.S. agriculture and investigate how they have been applied to corn production, how they’ve altered natural systems, and how they have in turn impacted the environment. The first section of these four will focus on agricultural practices which altered existing ecosystems and continue to disrupt natural cycles. This section titled
8 “Land Use”, gives both a historical account of the destruction of wetland and prairie ecosystems within the United States, as well as describes the ways in which these ecosystems are still impacted by agricultural practices. Practices described in this section will repeatedly re-appear in other sections, as they have a substantial influence on the movement of materials such as water and agrochemicals, and therefore have far reaching environmental impacts. Following the land use section will be the section “Water Use”. This section will introduce irrigation in U.S. agriculture, including differences in irrigation in Eastern States and Western States, as well as introduce irrigated acreage by crop. Then will introduce the water needs of corn, variables influencing the water needs of corn such as climate, soil, and lifecycle stage and variety, as well as yield response of corn. Describing the water natural requirements and yield response will provide important information on variable water requirements of corn in different regions and conditions as well as incentives to irrigate corn. Later in the section, the environmental impacts of irrigation will be described, namely runoff and unsustainable groundwater water use. Water use plays an important role in agriculture, particularly in cultivation of corn in the Western Corn Belt. Water acts as a resource used directly by crops, a media which transports agrochemical pollutants, as well as an abiotic condition which influences a number of other environmental factors such as nutrient availability. The next section will discuss fertilizer use in U.S. Corn Production, focusing mainly on nitrogen and phosphorus fertilizer use. The section will begin with nitrogen fertilizer, since it is applied in the greatest proportional quantity to corn. Background information on the amount of nitrogen fertilizer applied to corn, as well as the nitrogen requirements and use efficiency of corn will be discussed, as well as how nitrogen fertilizers move through natural systems. Following which, the impact of nitrogen on aquatic systems, soil properties and biotic communities, and air will be discussed. In addition to introducing corn’s use of nitrogen and the impact of nitrogen on the environment, the environmental impact of phosphorus fertilizers will be discussed as well. While nitrogen and phosphorus play different critical roles in natural systems, both are limiting factors in the natural environment. When they cease to become limiting factors, either intentionally through application of fertilizers to cropland or unintentionally through runoff, their introduction is often disruptive, changing community composition in ways which can have important implications for an ecosystems abiotic conditions. While the use of phosphorus fertilizers can have substantial impacts through runoff on aquatic ecosystems, like nitrogen, focus will be primarily placed on phosphorus production and how this influences cadmium content of phosphorus fertilizers and how cadmium accumulates in corn and its impact on soil pH. This will open further discussion to other heavy metal impurities present in fertilizers and how this impacts soil health. The following three sections on land use, water use, and fertilizer have all included adding resources or applying practice which promote crop growth and yield as well as resources and practices which compensate for suboptimal abiotic conditions such as drought or poor drainage, however the next section, pesticide use, will cover chemical intervention which mitigates crop yield loss to biotic actors in ecosystems. While titled “Pesticide Use”, this section will focus on two major herbicides, glyphosate and atrazine. Background will be provided on their chemical properties and the scale at which they are applied to corn, as well as factors
9 influencing their rate of use such as genetic engineering. Understanding their mobility, persistence, and degradation will lead to a discussion of their environmental impacts. Having described the many ways land use, water use, fertilizer use and pesticide use impact the environment. A brief section will be dedicated to summarizing these impacts in terms of first summarizing intensive activities and their desired outcomes as well as grouping the environmental impacts of these activities in four categories: biotic impacts, soil/land impacts, aquatic impacts and atmospheric impacts. The following sections have described current resource use within the U.S. for intensive corn production as well as the environmental impact of these practices. Another dimension will be added to this description within the section “Climate Change”.This section will evaluate and pose how a changing climate might impact resource use and agricultural practices in the U.S. First by giving background information on the climate change and key compounds in climate change known commonly as greenhouse gases, then by describing changes in temperature and precipitation driven by climate change in the Midwest and Great Plains may impact corn production practices and resource use. These sections have carefully studied the resource use of intensive U.S. corn production and its environmental impacts. Analyzing this system requires consideration for agricultural practices, resource use and regional biotic and abiotic conditions of where this corn is grown, however the piece which has not been addressed is where this production goes and what it becomes. In the section “Demand”, production volume and trade will be discussed as well as domestic use of corn, domestic policy influencing use, and global corn demand and production. All of the following are key in motivating mass production of corn, with serious environmental consequences. This discussion of demand will lead into the final section, the calculation, which will come up with a prediction for the world’s corn production, acreage and yield for the year 2050; as well as world corn production use of water, fertilizer, and pesticides. This will be followed by a conclusion section which aims to summarize the findings of this thesis and provide insight for potential opportunities moving forward.
Corn in U.S. Agriculture
Intensification
Intensification of corn production for ethanol and livestock feed within the United States, has led the U.S. to become the world's largest producer, consumer, and exporter of corn.
Intensified agriculture is based on the principle of increasing yield by increasing resource inputs, to maximize profit. Maximum profit is contingent on maximizing gross value of production and minimizing the cost of resource inputs. Therefore, practices of intensified agriculture are highly dependent on prices received per unit of crop relative to the cost of resource inputs per unit of crop.
10 The intensification of US agriculture is dependent on technologies such as mechanized harvest, irrigation, agrochemical (fertilizers and pesticides), and genetically engineered crops. These technologies are economically favorable to large consolidated farming operations.These advancements allow for less labor and land to be allocated for greater yield and in turn, greater profits.
With these technical advances such as mechanized harvest, irrigation, and GE crops, often large capital investment is required and often the application of these technologies is most profitable on large scale farming operations. As a result agriculture within the United States has become increasingly consolidated, with high income farms controlling a disproportionate amount of acreage.
Farm* Size and Sales Class
*farms include operations producing crops and livestock
In 2016, an estimated 2.06 million farms* (crop and livestock operations) were active, operating 911 million acres, and managing an average of 442 acres (NASS USDA, 2017). About half of U.S. farms (50.1%) operated between an annual income of $1,000 to $9,999, and about a third (30.1%) operated between an annual income of $10,000-$99,999. Only 4% of U.S. farms had an annual income between $500,000 to $999,999 or over $1,000,000.
Of the 2.06 million farms*, surveyed by the USDA 50% fell within the sales class range of $1,000 to $9,999, 30% fell into the sales class range of $10,000 to $99,999, 7% fell within the range of $100,000 to $249,000, 4% fell within the range of $250,000 to 499,999, 4% fell within the range of $500,000 to $999,999, and 4% fell within the range of over $1,000,000.
Table 1. U.S. Farm Sales Class Distribution Sales Class Range (annual income) Percent of U.S. Farms*
$1,000-$9,999 50.1%
$10,000-$99,999 30.1%
$100,000-$249,000 7.0%
$250,000-$499,999 4.8%
$500,000-$999,999 4.0%
$1,000,000 or more 4.0%
11 Source: NASS USDA, 2017
It is important to note that these “incomes” do not necessarily indicate the revenue received by farms. Farms receiving millions in income may struggle to break even as they have millions in operating cost. This is further complicated as there are distinct differences in high-income farms with high-value products versus high-acreage. Therefore it is key not to over simplify these figures, or assume that high-income operations are necessarily high revenue operations. However it is important to note that, high-income operations do control a disproportionate amount of American farmland. Therefore their management strategies have important implications.
Although the majority of individual farms*, had a sales class under $99,999 (80.2%), these farms only operated 30.6% of U.S. farmland (NASS USDA, 2017). A quarter of U.S. farmland was managed by only 4% of farms (NASS USDA, 2017).
Table 2. U.S. Farm Sales Class and Farmland Operated Sales Class Range (annual income) Percent of U.S. Farmland* Operated
$1,000-$9,999 9.5%
$10,000-$99,999 21.1%
$100,000-$249,000 14.3%
$250,000-$499,999 14.0%
$500,000-$999,999 17.2%
$1,000,000 or more 24% Source: NASS USDA, 2017
Land area represents 80% of U.S. farm sector assets (McDonald et al., 2018). In 2016, 80.2% of farms managed less than a third (30.6%) of U.S. farmland, while 8% of farms managed 41.2%. These 8% fell in the top sales classes of $500,000 to $1,000,000 or more (NASS USDA, 2017). From 2009 to 2016, the average farm size has continued to increase. (NASS USDA, 2017).
Despite high grossing farms operating large amounts of land, and farms growing in size, the agricultural workforce is decreasing.
Agricultural Workforce
Fewer farms are cultivating more land. This has primarily been attributed to developments in technology and economic conditions which have encouraged mechanization and less labor on
12 the farm. In 1900 41% of the United States workforce was employed by agriculture, however by 2000 1.9% of the workforce was employed by agriculture (MacDonald et al., 2018).
While in general farms are employing fewer people with high income farms growing in size and becoming increasingly consolidated, this is particularly prevalent for grain crops, oilseeds, and livestock.
Consolidation of Corn Cropland
The number of large corn farms has increased over time, while the number of small corn farms has decreased (ERS USDA, 2018). High acreage, high income farms tend to specialize in corn and soybean production (MacDonald et al., 2018).Four-fifths of farms with harvested cropland over 2,000 acres and under 25,000 acres specialize in grain and oil seeds, primarily corn and soybean (MacDonald et al., 2018).
An important note to qualify this data further, is that “farms” include livestock production, and so U.S. farms with over 25,000 acres tend to specialize in beef (MacDonald et al., 2018). However grain and oilseed, corn and soybean namely, account for the majority of large acreage farming operations.
Table 3. Grain and Oilseed Operation Acreage Harvested Cropland Percent of Farms Specializing in Grain and Oilseed Production
2,000-4,9999 acres 80.6%
5,000-9,999 acres 78%
10,0000-24,999 acres 74%
25,000 acres of more 50% Source: MacDonald et al., 2018
Crop Acreage Dedication
In 2014 16.9% of U.S. land was dedicated to cropland (Roser et al., 2018). The crop with the highest acreage dedication in the year 2017-2018 was soybean followed by corn.
The Farm Service Agency found in their 2018 report on 2017 acreage dedication:
Table 4. U.S. Planted Acreage of Field Crops Field Crop Acreage Planted in U.S. 2017 (millions of acres)
13 Soybean 88.7
Corn (grain) 87.3
Wheat 43.2
Sorghum 5
Barley 2.4
Rice 2.4
Oats 1.9
Sugar Beet 1
Sugar Cane 0.889 Source: NASS USDA, 2017
Crop Production
Corn production in the United States is the highest of any other field crop with over 14 billion bushels produced in 2017.
In 2017, the USDA reported 2017 U.S. total crop production as follows for the following field crops:
Table 5. U.S. Production Total of Field Crops Field Crop 2017 U.S. Production Total (1,000 Bushels)
Corn (grain) 14,577,502
Soybean 4,425,279
Wheat 1,740,582
Sorghum (grain) 355,633
Barley 141,923
Oats 49.391
Rice* 178,382 cwt*
Sugar Beet* 36,037 tons*
14 Sugar Cane* 32,124 tons* Source: NASS USDA, 2018
(unit is 1,000 of bushels except for rice* 1,000 cwt*, sugar beets* 1,000 tons*, and sugar cane* 1,000 tons*)
Corn is the U.S. crop with the highest production volume. Despite having more acreage, soybean production fell short of corn by 10 billion bushels, resulting in production which was less than a third of corn. This discrepancy between acreage and production, is due to the extraordinarily high yield per acre of corn. High yield of corn has resulted from intensification of corn production, the specific practices of which will be discussed in further detail later on.
Yield per Acre
Yield per acre of crops is a critical cofactor which relates acreage to production. Different crops have stark differences in their yields based on both natural properties of crops as well as agricultural practices. Furthermore yield varies amongst crops based on practices adopted by farmers as well as the regional differences in the natural conditions. Average national yield for crops serves to approximate the average yield of U.S. cultivation of a of a crop taking in consideration regional differences.
The national yield per acre of the following field crops in 2017 were found to be as follows:
Table 6. U.S. National Yield Average of Field Crops Field Crop National Yield Average (bushels/acre)
Corn (grain) 175.4
Barley 72.6
Sorghum (grain) 70.4
Oats 61.4
Soybean 49.5
Wheat 46.3
Rice* 7,461 cwt/acre*
Sugar Cane* 36.0 tons/acre*
Sugar Beet* 32.4 tons/acre* Source: NASS USDA, 2018 15
Corn has the highest yield per acre by far of these field crops, followed by barley and sorghum. However there is a substantial gap between corn and these other field crops. Take corn and barley: with a difference of 100 bushels/acre between the yields of corn and barley the national yield average of corn is 2.5 times that of barley. However the difference between the yields of barley and sorghum is 2.2 bushels, making the national yield average of barley just 1.03 times that of sorghum. Perhaps the most interesting comparison however is between the yields of corn and soybean. The national average yield of corn is over 3.5 times that of soybean, a disparity which explains why despite comparable acreage, soybean production in the U.S. in terms of volume is overshadowed so greatly by corn.
However while bushels and acres give tangible information about how each crop occupies physical space on cropland, as well as in market, not all bushels are equal. A bushel of one crop can be worth several times more than another. Thus production volume does not translate directly to gross income of producers. Commodity prices are important because they directly influence gross income of producers in relation to production volume.
Crop Commodity Prices
In January 2018 the commodity prices per bushel of the following field crops were as follows:
Table 7. U.S. Commodity Prices of Field Crops Field Crop Commodity Price- January 2018 (per bushel)
Soybean $9.30
Wheat (all) $4.69
Barley (all) $4.45
Corn $3.29
Rice* $12.80/cwt*
Sorghum (grain) $5.77/cwt* Source: NASS USDA, 2018
Soybeans are the field crop with the highest commodity price, at $9.30, followed by wheat and barley. However there is a substantial disparity between soybean commodity price and the commodity prices received for other field crops. The price received for a bushel of soybean is nearly twice that received for a bushel of wheat, and nearly three times that received for a bushel of corn. Corn receives a substantially lower commodity price than soybean and a lower commodity price than other field crops.
16
Corn’s low commodity price seems to contradict it’s high acreage and high production. If low prices are received per bushel, what incentive is there to grow corn when another crop could be cultivated and purchased for more. While corn has a lower commodity price than other field crops, it also experiences significantly higher yield.
Therefore under conditions in which a farmer is posed with growing corn or another crop on a set amount of acreage, theoretically even if corn receives a marginally lower commodity price, if the yield of corn is substantially higher than the alternative crop, it would seem the best crop to grow would be corn. This simple hypothetical shows one relationship between yield and crop commodity price. However in this hypothetical, yield, commodity price, acreage and even choice of crop are described as static. This is not the case.
There are complicated relationships between yield, commodity price, and acreage, all which influence production. The yield of field crops is in some ways naturally constrained however is often highly dependent on farming practices. Resource input and the scale farming practices are largely economically driven, with commodity price being a significant influence in how farmers choose to invest capital and structure their operations. In turn, directly influencing yield and acreage. Commodity price, resource price, and other input costs are all subject to a number of variables. The price of energy, labor, and chemicals all can influence whether certain practices are profitable. Trade relationships between nations can close or open markets, subjecting crops with significant export markets to vulnerability in certain political climates. Economic, political, technological, and natural disruptive events which shift production, yield, and commodity price are virtually constant, with reactions to the following being imperfect. Production and market response does not occur instantaneously and does not occur with exact precision.
These complex and imperfect relationships work to shape the current practices and characteristics of U.S. agriculture. As said prior these relationships are imperfect and therefore current crop acreage, production, and yield cannot be understood as they are currently without acknowledgement and investigation of their history. Therefore to understand the prevalence of corn in U.S. agriculture, it’s environmental impact, it is important to understand its history in U.S. agriculture.
Increased Corn Acreage
Early Origins
Since the beginning of agriculture America, corn has been an important crop. Corn served as an important crop in many different Native American group’s food systems, and served as an important crop in colonial agriculture (Conkin, 2008). English colonist took advantage of open land, and increased acreage in cropland by clearing forests, with corn grown as the staple cereal (Conkin, 2008). Plowing was conventional practices for preparing land for cultivation, however most plows before 1800 were comprised of wooden beams and moldboards, and cast
17 iron shares (Conkin, 2008). These plows dulled with use rapidly, and could only be used effectively on some land (Conkin, 2008). These plows evolved to be cast-iron beam and moldboard, allowing replacement of broken or worn shares, however they were still imperfect (Conkin, 2008). In 1837, John Deere, replaced cast-iron shares with steel, allowed for a plow which was effective in shedding dirt and cutting through thick sod of the midwest opening new land for cultivation (Conkin, 2008).
1860-1917
Increased corn acreage from 1860 to 1917 and was driven by a variety of factors: increased food demand, westward expansion, and an increase in farmers and farming operations all increased corn acreage cultivated (Wallace, 1956). This increase was further supported by westward infrastructure development which allowed access to East Seaboard markets which were growing in population and density (Hillard, 1972).
The Homestead Act of 1862 encouraged agrarian expansion, resulting in 270 million acres of public land turned to private landowners under this act (National Parks Service, 2015). According to the Homestead Act, a homesteader had to be the “head of a household” or 21 years of age (National Parks Service, 2015). The homesteader would be granted 160 acres of property for $18, on the condition that the homesteader would live on the land, build a house, “make improvements” and farm the land for 5 years. Once the individual had undergone this process they would be granted ownership of the land (National Parks Service, 2015).This land-grant program was established by the government to increase land development and promoted westward agrarian expansion (Hilliard, 1971). Corn was an important staple in westward expansion and was grown on nearly every farm, and served as a cereal for human consumption and for livestock feed (Wallace, 1956).
Henry A. Wallace, the former secretary of Agriculture, who served from 1933-1940 described the early development of the Midwest from 1840 to 1900 as an “extraordinary period”...”the population of the thirteen Midwest states increased by five times, corn production by ten times” (Wallace, 1956)
The Sodbusting plow was one of the key technological advancement which allowed for westward agricultural expansion. Prairie sod which had previously been a barrier for cropland development was now able to be broken up which allowed for cultivation of previously unavailable land, increasing corn acreage.
Sod stood as a barrier to cultivating certain land, as well as poorly drained soils. During this period there was a number of federal initiatives which encouraged the draining of wetlands in order to farmland. Both sod busting and wetland drainage served to increase American farmland, specifically corn acreage.
1917-1919
18
Corn acreage in the United States peaked in 1917 due to the increased food demand brought on by World War I (Nielsen, 2017). This caused corn acreage to increase up until the U.S. entered into World War I in April of 1917 (Nielsen, 2017). Farmers rapidly bought up land with optimism for high and rising commodity prices. Corn reached a historic high of $1.45/bu in 1918 (Nielsen, 2017) Following the end of World War I, there was a steep decline in commodity prices resulting in a steep decline in acreage dedicated to corn from 1918 to 1919 (Nielsen, 2017).
1920s-1930s
Crop acreage in the 1920s and 1930s decreased due to a number of factors. Economic depression, the dust bowl, and commodity price declines all served to knock farmers from the optimism experienced leading up to World War I Corn commodity prices dropped to $0.46/bu in 1921 and to $0.26 in 1932 (NASS USDA, 2018). The commodity price of $0.26 in 1932 was the lowest commodity price received for corn on record since 1866 (NASS USDA, 2018). Low commodity prices received and other factors all served to decrease corn acreage.
Post World War II
Corn acreage, recovered modestly following the crash of the 20s and 30s leading up to World War II, however following World War II corn acreage decreased. This decrease following World War II however was different than that of the 1920s and 1930s. Unlike during the 1920s and 1930s, even as corn acreage was decreasing, corn production was increasing. Following World War II, war time technology was moved from the battlefront to the fields and used to intensify operations to get greater yields out of the same amount of land. The U.S. was the only nation who left World War II with an intact agricultural sector, and therefore there was a significant demand for American farmers to keep producing, and to increase production to supply world food demand.
1960s-1980s
Despite increased production, American corn acreage declined from World War II until the 1960s. Corn acreage in the 1960s and 1970s increased due to a number of factors.
Corn acreage increased in the 1960s in response to increased commodity prices as well as other factors (Barnett, 2003). However in the early 1970s, a significant shift occurred in which American agriculture experienced significant growth and optimism, followed by a crash which resulted in the Farm Crisis of the 1980s (Barnett, 2003).
Following World War II, the U.S. experienced a trade deficit (Barnett, 2003). In an attempt to increase exports the Nixon administration devalued the dollar, with the goal of making American products more affordable for foreign buyers (Barnett, 2003). By 1973, it was clear this would be ineffective and the U.S. moved to a flexible exchange rate system (Barnett, 2003). The dollar
19 depreciated further and the balance of trade improved primarily due to significant growth in agricultural exports (Barnett, 2003). Simultaneously, overall federal spending was increasing, as well as inflation (Barnett, 2003). In an attempt to curb inflation, wage and price controls were imposed (Barnett, 2003). Unemployment was also increasing, despite the theoretical tradeoff between inflation and unemployment (Barnett, 2003). Politically, contractionary fiscal policy (increasing taxes or decreasing federal spending), was not favorable, and was not used to try to decrease inflation (Barnett, 2003). Therefore in an attempt to bolster the American economy, U.S. tax code was adjusted to incentivise investment (Barnett, 2003). In addition to government tax policy, agricultural policies encouraged farmers to invest and modernize their farms (Barnett, 2003). Price support and supply control programs were used to elevate the price of particular agricultural commodities to artificially high levels (Barnett, 2003). This created an air of optimism for farmers to invest in production of particular commodities (Barnett, 2003). In addition the federal government made loans to farmers at below market interest rates, largely for new acreage acquisition (Barnett, 2003). Therefore during this time, corn acreage increased. In addition opening of world export markets such as China, increased agricultural exports (Barnett, 2003). From 1970 to 1973, U.S. export of feed grains nearly doubled (Barnett, 2003). High returns led to heavy investment and borrowing.
However in the early 1980s, with the Reagan administration, critical changes were enacted which resulted in the appreciation of the U.S. dollar (Barnett, 2003). The rising value of the dollar slowed U.S. agricultural exports, and caused crop commodity prices to fall as well (Barnett, 2003).
Corn creage dipped from 74,524,000 acres in 1981 to 51,479,000 acres in 1983 (Barnett, 2003). Corn prices fell 64% between 1980 and 1986 (Barnett, 2003). Economics were involved in low export demand as well as politics. In 1980 a grain embargo was imposed by the U.S. against the Soviet Union. The U.S. imposed this embargo in response to the Soviet Union invading Afghanistan in 1979 (Barnett, 2003).
Low commodity prices and debts from acreage expansion of the 70s acreage expansion caused economic crisis amongst many small-medium farmers, and led to a record number of foreclosures (Barnett, 2003). This served to decrease corn acreage significantly.
Expansion of corn acreage in recent decades is attributable to a number of factors such as the opening of new trade markets in East Asia which have significantly contributed to increased livestock demand. Recent legislation encouraging the use corn derived ethanol demand as also
20 increased corn acreage, this will be discussed in further detail in the Land Use section.
Figure 1. U.S. Corn Acreage 1866-2015 Data source: NASS USDA, 2018
While acreage peaked in 1917, and declined until the 60s, U.S. corn production has increased since the 1940s. This is due to rapid increases in yield following World War II and a movement towards intensified farming systems.
21
Figure 2. U.S. Total Corn Production 1866-2016 Data source: NASS USDA, 2018
Increased Corn Yield
Pre World War II
In 1866 U.S. corn yield averaged at 26 bu/acre and remained stagnant until 1936 (Nielsen, 2017). Following 1936 corn yield per acre began to increase. This was largely catalyzed by reactionary policy and practices taken on following the Great Depression and Dust bowl (Nielsen, 2017). During this time the government funded agricultural research and implemented agriculture extension programs. These programs funded research on agricultural technology to increase yield. Furthermore USDA extension provided training to farmers on these technologies to encourage their use (Nielsen, 2017). One of the first outcomes of these programs was the rapid adoption of double-cross hybrid corn varieties which steadily increased corn yield. From 1937 to 1955 there was an average increase of 0.8 bu/ac per year (Nielsen, 2017).
Post World War II
Following World War II, corn yield increased significantly due to development of high yield varieties, increased application of agrochemicals (nitrogen fertilizer and synthetic pesticides), as well as advances and increased use of agricultural mechanization (Purdue Historic Yield). Most recently adoption of genetically modified corn varieties since the late 90s has continued to
22 increase corn yield (Nielsen, 2017). The following developments have resulted in a 622% increase in U.S. average corn yield since 1930.
Figure 3. U.S. Average Corn Yield 1866-2016 Data source: NASS USDA, 2018
The following sections have served to discuss the prevalence of corn within U.S. agriculture, as well as historical trends in corn acreage, yield, and production, all of which serves as important context to discuss the resource use and environmental impacts of modern corn production in the U.S. However what has not been discussed is where in the U.S. this is occuring. This will be addressed in the upcoming section, “The Corn Belt”.
The Corn Belt
Importance of Geography
While the prior section has delineated the current amount of U.S. acres dedicated to corn production, the bushels of corn produced, and even the size and characteristics of U.S. farms which tend to produce corn, a key aspect of U.S. corn production is missing, where it occurs. Understanding the physical location of the region in which corn is most intensively cultivated is important to understand the natural characteristics of this system, how those natural
23 characteristics influence production practices, and how corn production itself is connected to negative environmental outcomes in regions where it is grown. While more detailed descriptions of the natural characteristics of the Corn Belt will be given, the next sections will first identify key regions of corn production using a study by Laingen.
Economic Output
The most intensive cultivation of corn is concentrated in what is referred to as the “Corn Belt”. The Corn Belt accounts for approximately a third of global production of corn and soybean (Zhong et al., 2014). There have been varied definitions of the spatial region deemed the Corn Belt, since the term was first coined in The Nation in 1892 (Auch et al., 2013). The Corn Belt region is defined by corn’s dominance in agricultural land use, production, and economy (Auch et al., 2013).
While many studies have used different parameters to define the Corn Belt region, the 2007 study “Delineating the US Corn Belt” by Laigden defined the Corn Belt using six key parameters (Laingen, 2012). The six parameters used were to identify counties within the Corn Belt and within the peripheries of the Corn Belt: bu of harvested corn, acres of corn harvested, corn per square mile, corn per square mile of cropland, total cropland per square mile, value of corn harvested.
Table 8. Laingen’s Parameters Test 1 pt 2 pt 3 pt
Bu of Harvested Corn (million) 10-20 20-30 30+
Acres of Corn Harvested (thousand) 50-100 100-200 200+
Corn per Square Mile (acres) 160-240 240-320 320+
Corn per Square Mile of Cropland (acres) 160-240 240-320 320+
Total Cropland per Square Mile (acres) 320-400 400-480 480+
Value of Corn Harvested (million dollars) 25-50 50-100 100+ Source: Laingen, 2012
Using these parameters, the study classified regions at the county level as either periphery, marginal, corn belt, or core based on the summed scores of counties.
Table 9. Laingen’s County Classification County Classification Total Points
Peripheral 2-4
24 Marginal 5
Corn Belt 6-12
Core 13-18 Source: Laingen, 2012
Figure 4. Map of the Corn Belt Source: Laingen, 2012
Peripheral counties border the Corn Belt along the southern boundaries of Illinois, Iowa, and Minnesota, as well as the Western region of Kansas, the western panhandle of Oklahoma, and areas of northwest Texas (Laingen, 2012). Peripheral counties also border the corn belt to the west from mid-Nebraska, eastern South Dakota, reaching up to occupy more than half of Eastern North Dakota, the north most of the peripheral county range (Laingen, 2012).
Outcomes
The Corn Belt itself includes 10 states: Ohio, Indiana, Illinois, Wisconsin, Iowa, Minnesota, North Dakota, South Dakota, Nebraska, Kansas. Iowa and Illinois are nearly entirely by the Corn Belt.
25 There are three core corn belt regions, one occurring in Illinois, covering most of the state, one in Iowa and Minnesota, and one Nebraska (Laingen, 2012).
From Where to Why
Having identified where corn is cultivated in the U.S. in the greatest concentration, the more complex question of why is raised. What are the natural characteristics of this region which make is suitable for growing corn and how has human intervention further served to increase corn yield and production in this area. These questions will be addressed in the following sections.
Ecoregions and Hydrology of the Corn Belt
Introduction
The previous sections have focused on the scale and location of corn production of corn within the United States. This has given context to the magnitude of corn in U.S. agriculture and how concentrated this production is. However what has not been addressed is the abiotic and biotic characteristics of the regions in which corn cultivation is concentrated. These characteristics will be important for informing the resource use of corn production as well as differences in production practices within the Corn Belt. Furthermore they will ultimately inform our understanding of the environmental impact of corn production within the U.S. Therefore this section will describe the ecoregions of the corn belt, which cover broad patterns in climate, surface hydrology, geology and soil and historic vegetation, as well as a more detailed discussion of the surface and subsurface hydrology.
Three Ecoregions of the Corn Belt
The Corn Belt is divided into three ecoregions based on their biotic and abiotic characteristics (Auch et al., 2013). These three ecoregions are the Eastern Corn Belt Plains, Central Corn Belt Plains, and Western Corn Belt Plains (Auch et al., 2013).
26
Figure 5. Corn Belt Ecoregions Source: Wilkens et al., 2011
Eastern Corn Belt Plains (ECBP) (Region 55, 8.2.4 Wilkens 2011 pg 62) The ECBP region falls within central and eastern Indiana and western Ohio, as well as small portion in southern Michigan (Wilkens et al., 2011).
Climate of the Eastern Corn Belt Plains
The climate of the ECBP is defined by dramatic temperate seasonal temperatures, with an average annual temperature in 9 degrees in the north and 13 degrees in the south. The average annual precipitation is 985 mm, however ranges from 864 mm to 1,143 mm (Wilkens et al., 2011).
Hydrology of the Eastern Corn Belt Plains
The region’s water bodies include some wetlands, lakes, and reservoirs, as well as perennial and intermittent streams as well as abundant groundwater (Wilkens et al., 2011).
Geology and Soil of the Eastern Corn Belt Plains
27 The region has been affected significantly by late Pleistocene epoch glacial events, topographic features of the region consisting primarily of rolling till plains as well as end moraines (Wilkens et al., 2011). The soil composition of the region is primarily glacial till deposits of the Wisconsinan age, of outwash and thin loess (Wilkens et al., 2011). The bedrock composition of the region is dominated by Paleozoic carbonate, shale, and sandstone. The region has primarily Alfisol and Mollisol soils with Mesic temperature and Udic moisture regimes (Wilkens et al., 2011).
Alfisols are moderately leached soils that have high natural nitrogen, phosphorus, and carbon content. These soils primarily have formed under forests and have a subsurface horizon layer of clay (Soil and Land Resource Division University of Idaho, n.d.). These soils are highly productive and contribute significantly to the productivity in the Eastern Corn Belt.
Mollisols are soils of grassland ecosystems, and are thick and dark at their surface horizon. This dark fertile surface or mollic epipedon, occurs from long term addition from organic materials derived from plant roots. Prairie grass root systems are important to this soil’s surface horizon which is rich in nutrients, organic matter, and moisture content (Soil and Land Resource Division University of Idaho, n.d.).
Historic Vegetation of the Eastern Corn Belt Plains
Two main groups of plant communities dominated this region pre-settlement. Beech forests dominated post-glacial Wisconsinan soils, while beech forests and elm-ash swamp forests dominated pre-Wisconsian soils which tended to be wetter (Wilkens et al., 2011).
Central Corn Belt Plains (CCBP) (Region 54, 8.2.3 pg 62) This region consists primarily of northern Illinois and northwestern Indiana as well as some of southeastern Wisconsin (Wilkens et al., 2011).
Climate of the Central Corn Belt Plains Similar to the Eastern Corn Belt Plains, the region experiences a temperate climate with mean annual temperatures ranging from 8 degrees Celsius to 12 degrees Celsius. The mean annual precipitation, lower than that of the Eastern Corn Belt Plains coming to an annual average of 942 mm, and ranging from 863 to 1,040 mm (Wilkens et al., 2011).
Hydrology of the Central Corn Belt Plains
The region contains perennial and intermittent streams and rivers which are low in density. There are a number of areas within the region whose surface waters have been altered by human activity into artificial drainage systems (Wilkens et al., 2011).
Geology and Soil of the Central Corn Belt Plains
28 Just as the Eastern Corn Belt Plains, the Central Corn Belt Plains have been greatly influenced by glaciation events, resulting in glaciated, flat to rolling plains. The region also has areas with sand dunes and lake plains (Wilkens et al., 2011). The bedrock of the region, composed of paleozoic shale, siltstone, and limestone, is deeply buried. Western soils of the region tend to be derived from loess deposits while central and eastern soils are derived from glacial drift. Soils occurring are often Mollisols and Alfisols, and are dark and fertile with mesic temperatures regimes and udic moisture regimes (Wilkens et al., 2011).
Historic Vegetation of the Central Corn Belt Plains
Historically the region’s vegetation was characterized by prairie community regions as well regions of oak-hickory forest (Wilkens et al., 2011). Mesic prairies’ vegetation included big bluestem, indiangrass, prairie dropseed, switchgrass as well as some sugar maple and American Elm. Dry upland prairie vegetation consisted of little bluestem and sideoats grama. Oak-Hickory forest vegetation contained predominantly white oak, black oak, and shagbark hickory (Wilkens et al., 2011).
Western Corn Belt Plains (WCBP) (Region 47, 9.2.3 pg 77 Wilkens 2011)
The WCBP region includes southern Minnesota, large portions of central and western Iowa, eastern South Dakota and Nebraska as well as northwest Missouri and northeast Kansas.
Climate of the Western Corn Belt Plains
The temperature of this region has a greater range than that of the Central and Eastern Corn Belt Plains, ranging from a mean annual temperature of 6 degrees Celcius in the North to 12 degrees Celsius in the south (Wilkens et al., 2011). This region is also drier than the Central and Eastern Corn Belt Plains, with mean annual precipitation occurring as 800 mm, ranging between 610 to 1,000 mm, occurring mainly in the growing season (Wilkens et al., 2011).
Hydrology of the Western Corn Belt Plains
Similar to the perennial and intermittent streams of the Central Corn Belt Plains, streams within this region are frequently channeled (Wilkens et al., 2011). The region also has some areas with natural lakes (Wilkens et al., 2011).
Geology and Soil of the Western Corn Belt Plains
Just as within the Central Corn Belt Plains and Eastern Corn Belt Plains, the region is heavily influenced by glacial events (Wilkens et al., 2011). The region’s topography consists largely of rolling glaciated till plains and hilly loess plains (Wilkens et al., 2011). The bedrock of the region is comprised of Mesozoic and Paleozoic shale, sandstone, and limestone (Wilkens et al., 2011).
29 The soils of the region are primarily Mollisols and Alfisols with mesic temperature and udic moisture soil regimes (Wilkens et al., 2011).
Historic Vegetation of the Western Corn Belt
The vegetation of the region historically was tallgrass prairie covered by little bluestem, big bluestem, Indian Grass, switchgrass, numerous forbs, as well as pockets of bur oak and oak-hickory woodland (Wilkens et al., 2011). The region also contained mixed wetlands of herbaceous marshes and wooded floodplains (Wilkens et al., 2011).
Hydrology of the Corn Belt
Each ecoregion is distinct in its climate as well as its geological history. Both climate and geology directly influence critical the hydraulic features of each region. Both surface and subsurface hydrological features are important to the connectivity of regions, as well as the movement of agrochemical pollutants such as pesticide and fertilizer.
Surface Hydrology of the Corn Belt
Figure 6. United States River Basins Source: Aldridge et al., 2017
The Corn Belt connects to three critical river basins, the Missouri River Basin, the Upper Mississippi River Basin and the Ohio River Basin, which are all key contributors to the North Gulf of Mexico (Panagopoulos et al., 2015).
The Upper Mississippi River Basin and the Ohio River Basin both overlay core areas of the Corn Belt region and connect at the headwaters of the Mississippi/Atchafalaya River basin (Panagopoulos et al., 2015).
30
Figure 7. Upper Mississippi River Basin and Ohio Tennessee River Basin Source: Panagopoulos et al., 2015
The Upper Mississippi River Basin is the headwater to the Mississippi River and reaches from Lake Itasca in Minnesota to north of Cairo Illinois above the Ohio River. It’s made up of large sections of Illinois, Iowa, Minnesota, Missouri and Wisconsin. The region is Region 07 in the USGS “2-digit watershed” system (Panagopoulos et al., 2015). The Upper Mississippi River Basin covers 492,000 km^2 in total (190,000 mi^2), cropland of mainly corn and soybean cover 50% of this total area (Panagopoulos et al., 2015).
The Ohio River Basin begins in Pennsylvania and ends in Illinois and flows to the Mississippi River. Within the Ohio-Tennessee River Basin the Tennessee River joins the Ohio River in Paducah Kentucky upstream of the confluence of the Ohio and Mississippi river (Panagopoulos et al., 2015). The total area of the Ohio-Tennessee River Basin is 528,000 km^2 (204,000 mi^2), with cropland covering 20% of the total area (Panagopoulos et al., 2015).
31 Both the Upper Mississippi River Basin and Ohio River Basin have an important role in contributing to the Mississippi River, and are covered largely by the Corn Belt, the Missouri Drainage Basin is also a critical drainage basin in which corn is intensively cultivated.
Figure 8. Missouri River Basin Source: US Army Corp of Engineers
The Missouri River Basin is the largest watershed within the United States, covering over 804,672 km^2 (500,000 km^2) covering Montana, Wyoming, Colorado, Kansas, North Dakota, South Dakota, Nebraska, and Missouri. The basin encompasses the nearly the entire states of Montana, South Dakota, and Nebraska and in total covers 1/6th of the continental United States land area (Bureau of Reclamation U.S. Department of Interior, 2011).
In addition to surface water, groundwater is a key aspect of the hydrology of the corn belt. Defining the location and characteristics of aquifers within the Corn Belt is critical to understanding the impact of agriculture on the subsurface hydrology of the region in addition to surface hydrology. This impact includes direct influence to the water balance, such as surface water diversion and groundwater depletion, as well as surface and subsurface pollution and degradation.
Aquifers of the Corn Belt
The USGS defines an aquifer as “a geologic formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of
32 water to wells and springs” (Reilly et al., 2008). 94% of U.S. groundwater withdrawals come from 30 principle aquifers (Reilly et al., 2008).
Figure 9. United States Principal Aquifers Source: Reilly et al., 2008
Primary aquifers occuring in the Corn Belt include the Cambrian-Ordovician aquifer system (4), the High Plains aquifer (10), the Lower Cretaceous aquifers (11), the Mississippian aquifers (14), the Silurian-Devonian aquifers (21), and the Glacial sand and gravel aquifers (28).
33
As depicted above, the glacial sand and gravel aquifers which span across the Northern United States are a significant aquifer system which span nearly continuously throughout the Corn Belt. Another significant aquifer system is the High Plains aquifer system which covers key areas of the Western Corn Belt. This aquifer may not cover as much area as the glacial sand and gravel aquifers however its location is of particular interest because the high plains aquifer system is in an area of the Corn Belt which is particularly water scarce and is highly irrigated in comparison to other regions of the corn belt. Other aquifer systems are important as well, however these two systems are of particular interest and will be discussed in further detail later on.
Cambrian-Ordovician Aquifer System (4)
The Cambrian-Ordovician Aquifer system, is a multiaquifer complex made up of three main units bound by the Maquoketa confining unit (Olcott, 1992). This aquifer system is comprised of primarily sandstone in its lower layers and sandstone and shale interbedded with limestone and dolomite in its upper layers (Olcott, 1992). Each unit of the aquifer crops out at the bedrock surface, at these crop outs the aquifer system are hydraulically connected to overlying surface waters (Olcott, 1992). This aquifer system overlays much of Wisconsin and only reaches marginal areas of the Corn Belt in southeast Minnesota and northern Illinois.
Lower Cretaceous Aquifers (11)
The lower cretaceous aquifer is an important groundwater source for northwestern Iowa and southwestern Minnesota, where it is generally the only source of groundwater (Olcott, 1992). The lower cretaceous aquifer was formed as a result of sequences of sandstone, limestone, and shale deposits occurring in the Cretaceous Period from five major transgressive-regressive marine (flooding) cycles (Olcott, 1992). The aquifer as a result is composed of thick to thin discontinuous sandstone beds which are overlain intermittently by limestone and shale beds which confine the aquifer (Olcott, 1992). In other areas glacial deposits overlay the aquifer (Olcott, 1992).
Mississippian Aquifers (14)
The Mississippian Aquifer is an aquifer system which covers a large amount of Iowa, underlying 60% of the state (Olcott, 1992). In Iowa is is comprised primarily of limestone and dolomite, and is overlain by rocks from the Pennsylvanian period or more recent. These rocks serve to confine the aquifer and reduce groundwater circulation (Olcott, 1992). Furthermore in Iowa this aquifer system is overlain by by superficial aquifer systems of fine glacial deposits (Olcott, 1992).
Silurian-Devonian Aquifer System (21)
The Silurian-Devonian Aquifer System is comprised similarly to the Mississippian Aquifer of primarily limestone and dolomite. It underlays a significant amount of Iowa, covering about 90%
34 of the state (Olcott, 1992). This aquifer system also covers parts of Wisconsin, and Michigan (Olcott, 1992). Generally the aquifer is overlain with a surficial aquifer system, largely glacial sands, however there are areas where the aquifer crops out to the surface (Olcott, 1992). Where the aquifer system is unconfined and overlain with a superficial aquifer system, karst features have developed, and the aquifer is susceptible to contamination from the land surface (Olcott, 1992). This contamination can occur directly through outcrop areas, or more frequently, through overlying surficial aquifer systems (Olcott, 1992). This contamination is limited in units of the aquifer which are confined (Olcott, 1992).
Glacial Sands and Gravel Aquifers (28)
This aquifer system overlays all of the aforementioned aquifer systems, and is one of the most significant aquifer systems due to its large area and shallow nature. Glacial sands and gravel aquifers are the result of glacial deposits (USGS Aquifer Basics). While all have been influenced substantially by glaciation events and glacial recession, varying periods of deposition and glaction drainage and outflow in various locations mean this aquifer system is highly variable in its historical formation and origin, and that the aquifer itself is highly varied in depth.
High Plains Aquifer (10)
The High Plains aquifer is one of the largest aquifers in the U.S. covering 174,000 square miles and spanning eight States (Reilly, 2008).
35
Figure 10. High Plains Aquifer Source: USGS
The High Plains aquifer was formed historically as sediments were deposited by streams which flowed east from the Rocky Mountains (Reilly et al., 2008). The High Plains aquifer is highly hydraulically connected, consisting of one or more geological unit (Reilly et al., 2008). The primary formation within the High Plains aquifer is the Ogallala Formation of the Miocene age as well as overlapping Quaternary deposits (Water Availability USGS). This aquifer covers nearly the entire state of Nebraska and spans down to Texas. It is an increasingly important source of water for irrigated corn in Nebraska, and is of particular interest in regards to nitrate pollution. This aquifer and irrigation in Western corn production will be discussed in greater detail in the water use system.
The characteristics and location of aquifers within the Corn Belt region play an important role in shaping water availability, water balance, and the quality of groundwater, as well as surface water hydrology, as well as land use. In formerly glaciated regions, poor drainage poses a substantial barrier to root growth and therefore to high yields, in dry arid regions aquifers serve
36 as a principle source of water for irrigation, and in many regions of intensive corn production aquifers and surface waters are at risk for contamination by agrochemical pollution. The following will be discussed in greater detail in the following sections.
Land Use
Introduction
Conversion of existing ecosystems to cropland often results in altertering an ecosystems’ existing hydrology, soil, and vegetation. This fundamentally changes the way ecosystems function and often results in drastic changes to ecosystems biotic communities, abiotic characteristics, and results in environmental degradation. The two major ecosystems in the U.S. which have been lost due to corn cropland expansion are wetlands and prairies. The degradation and loss of wetlands has resulted from altering hydrology of surface waters through channelization, as well as altering drainage properties through subsurface drainage systems. The loss of prairie ecosystems has resulted from extensive plowing and replacing diverse communities of perennial grasses with annual cropland. The following of which have severe consequences for soil, water, and wildlife.
Wetlands
An ecosystem is characterized as a wetland by having three criteria: hydric soils, standing water or saturated soil for part of the year or more, as well as having vegetation which is adapted for highly saturated soil (Minnesota Board of Water and Soil Resources).
Wetlands provide important ecosystem services such as flood control, groundwater replenishment, sediment and nutrient retention and export, and water purification (RAMSAR). Wetlands also support diverse vegetation and wildlife, serving as critical nesting and nursery habitat for many species (RAMSAR).
Wetland Loss
1780-Present
In 1780 the continental United States contained over 391 million acres of wetland (Dahl, 1990). However by 1980, wetland acreage had declined to 104 million acres, a 53% loss, largely due to cropland conversion (Dahl, 1990). Over this period (1780-1980), Illinois, Indiana, Iowa, Missouri, and Ohio had some of the highest declines in wetland acreage, losing 70% or more of their wetland acreage (Dahl, 1990). Wetland loss in the Corn Belt (Illinois, Indiana, Iowa, Michigan,
37 Minnesota, Ohio, and Wisconsin) accounted for a third (36 million acres) of total wetland loss. (Dahl, 1990).
1810-1860
Technical advances throughout the 1800s expedited wetland conversion and channelization (Dahl et al., 1996). The Erie Canal, built in 1825, encouraged agriculture in the Midwest by decreasing the cost of transporting food goods (Dahl et al., 1996). The steam powered dredge allowed for increased channelization and clearing of small waterways, by lowering the labor requirements to achieve comparable outcomes(Dahl et al., 1996). Mechanical innovation and adoption on farms from 1810 through 1840 also increased wetland loss. The adoption of improved plows, rakes, and cultivators, as well as mechanical reapers increased the acreage able to be cultivated and promoted the drainage of wetlands (Dahl et al., 1996).
Furthermore conversion of some wetlands provided valuable goods such as timber During westward expansion from 1800 to 1860, many wetlands in the Ohio and Mississippi River Valleys were converted to cropland (Dahl et al., 1996). These wetlands in Ohio, Indiana, and Illinois were full of birch, ash, elm, oak, cottonwood, poplar, maple, basswood, and hickory and in addition to providing cropland, provided timber (Dahl et al., 1996). The Black Swamp within the northwest corner of Ohio, 120 miles long and 40 miles wide comprised of Swamp Elm Ash Forested Wetland, was nearly entirely destroyed by the end of the 1800s (Dahl et al., 1996).
In 1850 under the Swamp Lands Act, large amounts of wetland in Illinois, Indiana, Iowa, Michigan, Missouri, Ohio, and Wisconsin were granted to the states for “reclamation”. “Reclamation” of wetlands meant turning these wetlands into “productive” agricultural lands. In 1860 the Swamp Lands Act also granted acreage to Minnesota for conversion to cropland.
Table 10. State Acreage Granted Under the Swamp Lands Act State Acreage Granted (acres)
Illinois 1,460,164
Indiana 1,259,231
Iowa 1,196,391
Michigan 5,680,310
Missouri 3,482,481
Ohio 6,372
Wisconsin 3,360,786
38 Minnesota 4,706,508 Source: Dahl et al., 1996
1880-1930
Westward, beginning in the 1860s, prairie pothole wetlands of western Minnesota, northern Iowa, and North and South Dakota were altered (Dahl et al., 1996). Mid to late 1880s agrarian expansion continued westward along major river systems (Dahl et al., 1996). Steam power allowed for more rapid manufacturing and installation of drainage tiles, a key components of subsurface drainage systems (Dahl et al., 1996).
By 1880, 1,140 factories in Illinois, Indiana, and Ohio were manufacturing drainage tiles for the drainage of wetland to farmland (Dahl et al., 1996). Tile drainage throughout the midwest grew rapidly. 30,000 miles of tile drains were operating in Indiana by 1882, and by 1884 Ohio had over 20,000 miles of ditches draining 11 million acres of land (Dahl et al., 1996).
Drainage continued through the 1900s and by 1930, virtually all prairie wetlands in Iowa, southern Minnesota and the Red River Valley of North Dakota and Minnesota had been drained (Dahl et al., 1996).
1930-1970
Technological advancements created further demands for farmland. The increase in farms in the 1930s and 1940s resulted in the loss of millions of acres of wetlands from the prairie pothole region (Dahl et al., 1996).
Beginning in the 1930s, the Federal government provided free engineering services to farmers to aid in the drainage of wetlands. In the 1940s, the government shared the cost of drainage projects with farmers (Dahl et al., 1996). The Watershed Protection and Flood Prevention Act of 1954 also encouraged land drainage and wetland destruction (Dahl et al., 1996). Under the Agriculture Conservation Program, tile and open-ditch drainage were supported, accounting for an average annual loss of 550,000 wetland acres from the mid 1950s to the mid 1970s (Dahl et al., 1996).
Channel and drainage construction significantly contribute to the degradation of wetlands. These channel and drainage systems, remain prevalent in the Corn Belt today. Drainage and conversion of wetlands to farmlands result in sediment loading of surface waters and decreased water retention, leading to flooding downstream (Pierce et al., 2012)
Channelization and Flood Control
Environmental Impacts of Channelization
39
Channelization and flood control systems degrade aquatic ecosystems by decreasing water quality and changing water system characteristics. This occurs through increased sediment loading and increased water velocity (Pierce et al., 2012). Extensive channelization, also results in stream incision (Pierce et al., 2012). Narrow, directed channels increase water velocity, causing erosion and establishment of riparian vegetation. The loss of riparian vegetation results in destabilization and soil loss (Pierce et al., 2012). Ditches in particular lack the bends and obstacles of a natural stream. These obstacles improve water quality by slowing water speed, and decrease suspended sediment in the water column (Pierce et al., 2012). These barriers also increase habitat complexity and serve as important features for aquatic organisms. Without these objects and barriers water velocity increases, and suspended sediment remains in the water column, decreasing water quality.
Artificial channel systems reduce biodiversity due to lack of habitat complexity, highly variable water speed, and high suspended sediment. These conditions decreases the diversity of fish and mussels within these channelized systems (Pierce et al., 2012). Mussels and other freshwater bivalve filter feeders are keystone species in that they provide the critical ecosystem service of water filtration.
Subsurface Drainage
Subsurface Drainage has been applied to Corn Belt agriculture on a very large scale. Early forms of subsurface drainage involved clay tiles directing water to subsurface pipes which discharged water into running surface water or channels (Pierce et al., 2012). Current subsurface drainage systems are perforated, corrugated plastic pipes, used to lower the water table to allow for increased root growth in aerated soil.
40
Figure 11. Subsurface Drainage System Source: Blann et al., 2009
By 1987, 35% of drained agricultural land in the United States utilized subsurface drainage, with the majority dedicated to corn (Pierce et al., 2012). Subsurface drainage in the United States is most concentrated in the Corn Belt (Pierce et al., 2012).
41
Figure 12. Subsurface Tile Drainage System Cover by County Source: Sugg, 2007
The study “Assessing U.S. Farm Drainage” by Sugg, Z. in 2007, compiled data from the USDA 1987 survey as well as the NRI survey of 1992, the Soil Drainage Class estimates based on GIS analysis. The report found high amounts of subsurface drainage in the Corn Belt region (Sugg, 2007).
Table 11. Total Subsurface Drainage Acreage and Percent Cropland with Subsurface Drainage by State State Total Subsurface Drainage Percent Total Cropland with (Millions of Acres) Subsurface Drainage
Iowa 8.8 32.4%
Illinois 11.6 47.8%
Ohio 5.7 48.3%
42 Indiana 5.6 42.2%
Minnesota 3.4 14.4%
Michigan 2.3 28.7%
Wisconsin 0.7 5.9%
Missouri 0.6 3.4% Source: Sugg, 2007
Environmental Impact of Subsurface Drainage
Subsurface drainage leads to higher concentrations of nutrients and pesticides in agricultural runoff. Higher levels of soluble agrochemical contaminants in subsurface outflow are due to increased rate of percolation in artificially drained soils (Pierce et al., 2012). Frequency, concentration, and seasonal timing of fertilizer and pesticide application also impact subsurface drainage outflow contamination (Pierce et al., 2012) In addition to wetland loss, American acreage expansion has also resulted in the loss of native prairie.
Native Prairie Loss
Historically native prairies were one of the most expansive biomes within North America, spanning across the continental U.S. from Canada to Mexico, and from the eastern edge of the Rocky Mountains to western Indiana and and Wisconsin (Samson et al., 1994).
43
Figure 13. Historic Boundaries of Tallgrass, Mixed-Grass, and Shortgrass Prairie Source: University of Arizona
The three principle prairie ecosystem types are tall grass prairie, mixed grass prairie, and short grass prairie (Samson et al., 1994). These three prairie types covered approximately 405 million acres prior to conversion to cropland and pasture following colonization and westward expansion (Samson et al., 1994). As seen, Tallgrass Prairie historically was prevalent in the areas in which corn is now intensively cultivated today, covering all of Iowa, much of Illinois, Nebraska, as well as substantial portions of Missouri, Kansas, and North and South Dakota (University of Arizona).
44 Table 12. Historic Tallgrass, Mixed-grass, and Shortgrass Prairie Acreage
Source: Samson et al., 1994
Since 1830, it is estimated tallgrass prairie has declined 82-99% (to 1994) (Samson et al., 1994). Declines in mixed-grass prairie, range from 30% in Texas to 77% in Nebraska and 71% in North Dakota (Samson et al., 1994). Short grass declines are also substantial yet varied with an estimated 80% decline in Texas and a 20% decline in Wyoming (Samson et al., 1994).
In the 1930s sodbusting and intensive cultivation in conjunction with drought contributed to the Dust Bowl and Black Blizzards. This was due to loss of native grasses which destabilized prairie soils and abiotic conditions of drought and wind (Samson et al., 1994). Following the dramatic soil erosion events of the 1930s, misguided federal conservation efforts further threatened prairie ecosystems (Samson et al., 1994).
The Civilian Conservation Corps from 1938 to 1941, planted trees which had not been historically present on prairies (Samson et al., 1994). The USDA Soil Conservation Service attempted to rehabilitate prairie rangelands by reintroducing grasses, however the species they used to seed was an exotic and invasive crested wheatgrass from Siberia (Samson et al.,1994).
45 Furthermore following World War II, farming intensified with newly affordable technologies (application of synthetic nitrogen fertilizers, improvements in machinery, and development of synthetic pesticides). The Omnibus Farm Acts of 1985 and 1990, further encouraged intensive farming of North American Prairie (Samson et al., 1994).
Environmental Impact of Prairie Loss
Prairie ecosystems began to develop 8,000 to 10,000 years ago and were the largest continuous ecosystem on the North American continent (National Park Service U.S. Department of the Interior). Prairies are unique in that their vegetation is dominated by a grasses (National Park Service U.S. Department of the Interior).
Tallgrass, mixed-grass, and short grass prairie are distinguished by their vegetation. Species composition of vegetation is influenced by soil type and depth as well as moisture, slope and climate, as well as disruptive events such as grazing and fire (Tallgrass Prairie National Preserve). Shortgrass prairie in the west lies within the rainshadow of the Rocky Mountains, this region is more arid, resulting in dominance of buffalo grass and blue grama (National Park Service U.S. Department of the Interior). In the east, higher levels of precipitation support tallgrass prairie ecosystems with big bluestem, indian grass, and switchgrass (National Park Service U.S. Department of the Interior). Within the mid-plains in mixed-grass prairie side-oats grama and wheatgrass are the dominant vegetation as well as pockets of shortgrass and tallgrass in localized arid and wet regions respectively (National Park Service U.S. Department of the Interior).
Loss of tallgrass, mixed-grass, and shortgrass prairie have had a substantial environmental impact. Tallgrass in particular has been lost largely due to row-crop agriculture, primarily corn. About 80% of foliage of tallgrass prairies is made up of 40-60 species of grasses (National Park Service U.S. Department of the Interior). The other 20% is comprised of over 300 species of forbs or flowers (National Park Service U.S. Department of the Interior). Tallgrass prairie also hosts over 100 species of lichens and liverworts, as well as a number of plant species which grow at wetland edges and riparian zones (National Park Service U.S. Department of the Interior). However tallgrass prairie has been reduced to 1% of its original area (National Park Service U.S. Department of the Interior).
Loss of tallgrass and alteration of plant communities has critical consequences for natural cycles. Prairie ecosystems, located in the middle of the continent, face climate which is greatly varied in its seasonal ranges of precipitation and temperature. Extreme heat and drought in summer are contrasted by freezing winters with heavy snow (National Park Service U.S. Department of the Interior). Fire is also a regular disruptive event (National Park Service U.S. Department of the Interior). Prairie vegetation is able to withstand these conditions due to unique adaptations (National Park Service U.S. Department of the Interior). Fire is critical to adding nutrients to prairie soils and to encouraging the growth of fire-adapted native prairie vegetation. Fire also eliminates competition by preventing non-fire adapted plants from
46 establishing (U.S. Fish & Wildlife Service). Additionally, 75-80% of prairie biomass is underground (National Park Service U.S. Department of the Interior). Underground, prairie grass root systems are protected from frost, drought, fire, grazing, and trampling (National Park Service U.S. Department of the Interior).
Roots of some species can reach 10 to 15 feet in depth (National Park Service U.S. Department of the Interior). Extensive root systems of these prairie grasses creates a thick dense sod (National Park Service U.S. Department of the Interior). These dense root systems contribute to high organic matter content within the soil as well as high nutrient content characteristic of mollisols.
These root systems are also critical for holding topsoil and preventing erosion. The Great Plains region experiences some of the highest average annual wind speeds of the United States. (NRLE)
Figure 14. U.S. Wind Speed Map Source: NRLE
47 Dense root systems of prairie grasses hold topsoil and help prevent wind erosion.
In addition conversion of prairie to cropland results in the loss of year long vegetation cover. While native prairie grasses are perennial, row-crops cultivated in former prairies are annual. This loss of vegetative cover further contributes to erosion.
Prairie Ecosystems and Biodiversity
Loss of prairie vegetation has not only contributed to erosion, but has also dramatically decreased biodiversity. As stated prior plant species diversity in prairie systems is incredible, hosting hundreds of species of grasses, flowers, and forbs (U.S. Fish & Wildlife Service). Loss of native prairie vegetation, has also resulted in mass losses of dependent organisms within the prairie ecosystem.
Figure 15. Prairie dogs Source: Hank Bentlage, World Wildlife Fund
Losses of critical herbivores include loss of the buffalo and loss of prairie dog populations (Black tailed prairie dog, gunnison’s prairie dog, white tailed prairie dog) (Samson et al., 1994). These declines further decrease ecosystem services. Prairie dog species play a role in nutrient cycling, soil aeration, and soil formation. The decline in their populations results in a decrease in ecosystem services they provide (Samson et al., 1994). Species which have also faced threat include black-footed ferret, swift fox, ferruginous hawk, and mountain plover (Samson et al., 1994). 48
The Great Plains prairie not only serves as habitat for many animals but also serves as breeding ground for 76% (330 of the 435) of bird species which breed in the continental United States (Samson et al., 1994). Loss and alteration of habitat have declined grassland bird species within the Great Plains prairie biome (Samson et al., 1994). These declines vary but are highest in regions in which corn cultivation is concentrated, with declines in grassland birds of 24-91% from 1969 to 1991 in Illinois, Minnesota, Wyoming, Nebraska, and Missouri; and declines ranging from 17-48% in Colorado, North and South Dakota, Kansas, New Mexico and Texas (Samson et al., 1994). Loss of grassland habitat for breeding and wintering contributes to a decline in grassland birds, as well as changes to habitat which promote competing species which displace grassland birds. A decrease in fire events which are critical to maintain prairie ecology as well as the introduction and establishment of woody plants favors bird species adapted to forest-edge habitat (Samson et al., 1994). Therefore vegetation changes have not only decreased habitat for grassland species, but also increased competition with an increase in bird species which historically were limited to midwestern oak and eastern deciduous forest (Samson et al., 1994).
Conservation Reserve Programs
These losses in critical wetland and prairie habitat to agriculture gained mounting attention during the environmental reforms of the 1970s and 1980s. In 1985 the federal government created and funded the Conservation Reserve Program to encourage wetland and grassland restoration and conservation (Farm Service Agency). Through this program, farmers receive payments to retire farmland close to or in sensitive ecosystems and to manage the land to restore native vegetation and wildlife (Farm Service Agency). CRP enrollment is a long term contract which ranges from 10-15 years (Farm Service Agency). The goal of CRP is to restore sensitive ecosystems to promote and re-establish ecosystem services such as water filtration and flood regulation of wetlands and erosion control of prairie grasslands (Farm Service Agency).
CRP is the largest agricultural land retirement program in United States history, and has had considerable success in enrollment since its enactment. The USDA FSA report of 2012, estimated a 221 million ton reduction in sediment loss, a 605 million pound reduction in nitrogen loss, and a 121 million pound reduction in phosphorus loss. The Farm Service Agency (FSA) report also estimated 42 million metric tons of annual CO2 sequestration attributed to CRP lands and a 7 million metric ton reduction of annual CO2 emission attributed to CRP lands reduced use of fuel and fertilizer. (Farm Service Agency, 2013)
It was found in the study that Nitrogen and phosphorus runoff from CRP fields are 95 to 86% less than conventional row-crop land. CRP land also reduced sediment loading of surface water and nutrient and agrochemical runoff to surface water with the reintroduction and management of grass filter strips and riparian vegetation (Farm Service Agency, 2013). Wetland restoration improved water quality proximate to CRP lands, facilitating denitrification and reducing excess
49 nitrate in water (Farm Service Agency, 2013). The FSA 2012 Report found that in Iowa’s 75 CRP wetlands, developed to mitigate high nitrate water coming from agriculture subsurface drainage, 900,000 pounds of nitrate was removed from agricultural drainage water (Farm Service Agency, 2013).
CRP lands have been demonstrated to successfully restore some ecosystem services which have been lost in wetland and grassland conversion. These ecosystem services include reducing nitrogen and phosphorus runoff, reducing sediment loading of surface waters, increasing carbon sequestration, improving soil productivity, reducing downstream flooding, improving aquifer recharge, and preventing erosion (Farm Service Agency, 2013).
CRP has also had an important role in re-establishing populations previously threatened by habitat loss. A number of studies have indicated CRP’s direct influence of conservation of the sage grouse, lesser prairie chicken, northern bobwhite quail, ring necked pheasants, duck species which rely on prairie pothole region, and many species of birds reliant on grasslands for breeding (Farm Service Agency, 2013).
CRP lands have provided a number of environmental benefits, and have shown substantial success in recruiting enrollment since their implementation in 1985, however CRP is currently facing a number of challenges (Morefield et al., 2016). From 2007 to 2016 CRP land acreage has declined by 25% with significant amounts being rededicated to cropland (Morefield et al., 2016).
50
Figure 16. Changes in CRP Enrollment by County Source: Farm Service Agency
While the most dramatic losses have occurred in the far northwestern regions of the U.S., in Montana and North Dakota especially, notable losses have occurred in the Corn Belt region in South Dakota, Nebraska, Missouri, Iowa, and Illinois.
CRP land enrollment reached its peak of an estimated 36,818,700 acres in 2007 (Morefield et al., 2016). However in 2008 the new Farm Bill put a cap on enrolled CRP acreage at 31,876,594 acres (Morefield et al., 2016). In the 2014 Farm Bill this cap was reduced again to 23,969,222 acres. Since 2007 CRP enrolled acreage has declined significantly (Morefield et al., 2016)
51
Figure 17. Changes in Total CRP Enrollment Area Source: Farm Service Agency
The USDA estimated that 28% of CRP land enrolled in 2007 was diverted from the CRP by 2012 (Morefield et al., 2016). 60% of converted land was estimated to be rededicated to cropland and 34% estimated to be rededicated to pasture (Morefield et al., 2016).
A study by Morefield, LeDuc, Clark and Iovanna in 2016 studied CRP land use changes in expiring CRP lands in 12 states in the midwest region, found that 30% or 397,500 of 1,325,000 acres of expiring CRP land between 2010 and 2013 was converted back to five major crops (corn, soy, winter and spring wheat, and sorghum) (Morefield et al., 2016). The study found that the corn and soy accounted for the majority of acreage converted back to cropland in the area, with corn accounting for 34% of this cropland acreage. (Morefield et al., 2016). Of the states studied North and South Dakota, Nebraska, and southern Iowa were major centers of loss of CRP enrollment and reenrollment to cropland (Morefield et al., 2016).
Causes of the loss of CRP land since 2007 have been identified by studies as not just caps on CRP enrollment, but also as increase in crop commodity price of corn and soy (Wright et al., 2013). Conversion of CRP land to corn cropland has been prevalent within the Western Corn Belt. High corn commodity prices in response to corn derived ethanol, demand were a large factor in increasing the conversion of CRP lands to corn cropland (Wright et al., 2013). It is not only CRP land which has faced losses due to increase commodity prices but also a number of other grass-dominated land.
Recent Grassland Conversion and Wetland Degradation
Native prairie, managed pasture, hay, and CRP grasslands have faced recent losses to cropland conversion in West Corn Belt States (Wright et al., 2013). Grassland conversion from 2006 to 2011 was found in a 2012 study of Land Use Changes in the Western Corn Belt region by Wright and Wimberly, to be concentrated in North and South Dakota (Wright et al., 2013). The westward expansion of corn, faces challenges of a drier climate and higher risk of drought. However with high commodity price as an incentive for farmers despite costs of irrigation, has to
52 grow corn further west (Wright et al., 2013). In addition, expansion of federal crop insurance programs and disaster relief programs provide further incentive to cultivate corn in Western areas with higher drought risk (Wright et al., 2013). The following further favored the conversion of grasslands to corn cropland and contributed to grassland conversion in the Western Corn Belt (Wright et al., 2013). Within the Western Corn Belt Region between 2006 and 2011 over 1.3 million acres of grassland was lost, largely in South Dakota and Iowa, and largely to cropland conversion (Wright et al., 2013).
Areas of the Western Corn Belt region in which grassland has been converted to cropland are areas with high erosion potential, shallow poorly drained soils, and dry and hot climate which is less suitable for the cultivation of corn (Wright et al., 2013). In addition grassland conversion within the Western Corn Belt region is occurring in close proximity to the Prairie Pothole Region, a series of critical wetlands. In South Dakota, 550,000 acres of grassland converted to cropland was within a 100 meter buffer of wetlands and with more than 80% of converted grassland within a 500 meter buffer of wetlands (Wright et al., 2013). A similar phenomenon was observed in North Dakota.
Conversion of grassland to corn cropland in these marginal areas, has serious implications to the preservation of wetlands of the Prairie Pothole Region. The conversion of land with perennial grass cover to annual crops, increases risk of erosion and sediment loading of sensitive proximate wetlands, as well as decreases the amount of carbon sequestered. Annual cropland also reduces overwintering habitat for many sensitive species, particularly migratory birds.
The recent loss of grasslands and encroachment of cropland on wetlands, has a host of potential environmental impacts including loss of biodiversity and ecosystem services. This conversion also increases potential for agricultural runoff, erosion and sedimentation of surface water, as well as surface water and groundwater contamination. As land is used for cropland inputs of water, fertilizer, and pesticide are often introduced. Within the next section the impacts of water use in areas which have been converted to cropland will be discussed.
Water Use
Irrigated Agriculture in the US
Currently, agriculture accounts for 85% of consumptive water use in the United States, with the majority of this occurring in western states (Schaible et al., 2017). Water use within the United States has increased significantly since 1950, with the largest water demands coming from thermoelectric power and irrigation (Schaible et al., 2017).
53
Figure 18. U.S. Water Withdrawals by Sector Source: Schaible et al., 2017
Total water use peaked in 1980 and plateaued at 482 million acre-feet up until 2005 (Schaible et al., 2017). Water use has decreased since 2005 for a variety of reasons, including increases in water use efficiencies (Schaible et al., 2017).
However irrigated agriculture is the largest source of consumptive water use. About 98% of water used for thermoelectric cooling is returned to its origin source or is directly reused (Schaible et al., 2017). This is not the case for irrigated agriculture and why irrigated agriculture accounts for the majority of consumptive water use (85%).
Western States Irrigated Agriculture vs. Eastern States Irrigated Agriculture
Water use in agriculture is often discussed in terms of “Western States” and “Eastern States” due to significant differences in climate, water resources, and water use. (This is limited to the continental U.S.).
The western states include: Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming.
54 The eastern states include: Alabama, Alaska, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Vermont, Virginia, and West Virginia.
In western states, irrigated agriculture accounted for 64% of total western water withdrawals (Schaible et al., 2017). Western agriculture accounts for 61% of surface water withdrawals and 72% of groundwater withdrawals (Schaible et al., 2017).
Eastern states generally rely on precipitation to water crop. However increasingly, irrigation is practiced in eastern states to supplement rainfall when rainfall is insufficient some years (Schaible et al., 2017). Irrigation in eastern states has increased 42% from 1998 to 2013. This increase in irrigation in Eastern States has been due to increases in periodic droughts, and incentivized due to increased corn commodity prices, and increased corn yields in response to water availability. In addition increased access to groundwater with lower extraction cost has increased water sources for irrigation in many areas in Eastern States (Schaible et al., 2017).
In 2012, irrigated farmland accounted for 55.8 million acres. Of this, 52.1 million acres were dedicated to cropland, and 3.1 million acres were dedicated to pastureland (Schaible et al., 2017). In 2012, 16.5% of US harvested cropland was irrigated. Only 0.8% of pasture land was irrigated. Western states accounted for ¾ of total irrigated acreage (Schaible et al., 2017).Nebraska had the highest irrigated acreage at 8.3 million acres representing 14.9% of total US irrigated acreage.
55
Figure 19. Irrigated Acreage Share by State Source: Schaible et al., 2017
Corn Irrigated Acreage
A significant amount of corn is produced in Western States, particularly Nebraska, Kansas, and South Dakota. In 2016, Nebraska produced 11% of U.S. corn and together Kansas and South Dakota produced about 9% of U.S. corn. Collectively these three states produced 20% of U.S. corn (USDA ERS).
In both western and eastern states, irrigated corn cropland makes up a substantial portion of total irrigated acreage. Corn for grain accounted for 24.5% of all harvested irrigated cropland in western states (Schaible et al., 2017). Corn for grain accounted for 24.3% of irrigated cropland acreage in Eastern states in 2012.
56
Figure 20. Irrigated Acreage by Crop, Western and Eastern States Source: USDA ERS
Irrigation is critical for consistent high yield and total output of corn in the Western Corn Belt region. Within the Western U.S. Corn Belt, irrigated corn acreage accounts for 58% of total corn production (Grassini et al., 2011). Intensity of irrigation is motivated by corn’s water productivity.
57 Water productivity is the relationship between a crop’s yield and water input (Grassini et al., 2011).
A study conducted by Grassini et al. in 2011, aimed to quantify the theoretical water productivity of maize within the Western U.S. Corn Belt, using central Nebraska as a model study site. In central Nebraska 94% of corn production comes from irrigated acreage (Grassini et al., 2011). The study demonstrated a positive nearly linear relationship between corn yield and irrigation.
Figure 21. Corn Yield Response to Irrigation Source: Grassini et al., 2011
The study’s model closely matched prior studies observations of corn’s yield response to irrigation. Both modeled and observed data demonstrated a significant increase in corn yields with increased irrigation.
There is a substantial difference in the yield of irrigated corn acreage in comparison to non-irrigated corn acreage in Nebraska, as well as other regions of the Western Corn Belt
58 (University of Nebraska- Lincoln). In 2016 irrigated corn yield in Nebraska averaged 199.9 bu/ac, in contrast to rain-fed corn yield which averaged 147.2 bu/ac (U.S. average corn yield measured 174.6 bu/ac) (University of Nebraska- Lincoln).
Figure 22. Nebraska and U.S. Irrigated versus Non-Irrigated Corn Yield Source: Kranz et al., 2008
As seen above, irrigated corn yield in Nebraska was substantially more resilient to seasonal fluctuations in precipitation compared to non-irrigated corn. This is abundantly clear in 2012 when Nebraska experienced severe drought.
59
Figure 23. U.S. Drought 2012 Source: National Drought Mitigation Center, 2018
Nationally the 2012 droughts reduced average corn yields. Average yield of rainfed corn in Nebraska was reduced to an even greater extent. However, average yield of irrigated corn in Nebraska remained relatively unchanged.
Water availability and yield, are critically associated, as in certain conditions water becomes a limiting factor for growth and productivity of corn. However the water needs of corn vary substantially under different environmental conditions, growth stages of corn, and due to other factors. It is therefore important to understand the different variables contributing to the water needs of corn.
Water Needs of Corn
The water needs of corn are impacted by various factors including atmospheric conditions, growth stage of corn, and soil characteristics. Plants acquire the majority of their water (70-80%) from soil (Kranz et al., 2008). Water is moved throughout the plant through transpiration. Water is critical to distributing nutrients throughout the plant (Kranz et al., 2008). While water is taken
60 up by the roots, there is water lost through the leaves of plants through their stomata during photosynthesis.
Atmospheric conditions are key to water loss through the stomata; therefore, have an important influence on a crop’s daily water requirements (Kranz et al., 2008). High air temperatures, low humidity, clear skies, and high wind speeds result in high water demand to compensate for increased evapotranspiration (Kranz et al., 2008). In conditions with high humidity, cloud cover, and low wind speeds, there is relatively lower water demand due to lower levels of water loss due to evapotranspiration (Kranz et al., 2008)..
The growth stages of corn also influence water demand (Kranz et al., 2008).. Earlier growth stages have little water demand due to a limited root zone and less leaf surface area (Kranz et al., 2008).
Figure 24. Corn Water Use of Growing Season Source: Kranz et al., 2008
As demonstrated, average corn water use, varies daily based on atmospheric conditions; however overall water demand increases from early planting in May, and peaks in the Tassel stage in late July and early August (in North America).
The variety of corn grown influences water demands (Kranz et al., 2008).. One of the most critical factors influencing corn variety water demand is the maturity range of a variety (Kranz et al., 2008). At the same location a corn hybrid with a 113 day maturity has a greater water requirement than a corn hybrid with a 100 day maturity (Kranz et al., 2008). Longer season corn hybrids use more water but produce greater yields (Kranz et al., 2008).
61
Water Use and Climate
Regional differences in precipitation volume and timing influence the water requirements of corn (Kranz et al., 2008). For example in Nebraska total corn water use ranges from 28 inches per year in the southwest to 24 inches in the east (Kranz et al., 2008). Nebraska under the Koppen-Geiger Climate Classification system has two main climate regions. South west Nebraska is classified under the Koppen-Geiger Climate Classification system as cold semi-arid climate, while eastern Nebraska is classified as hot humid continental climate (Climate-Data.org). These differences in climates account for overall differences in annual atmospheric conditions, and therefore ranges in water needs of corn grown in each region.
Water Needs and Soil
In addition to climate, soil plays an important role in water availability, and therefore the water needs of corn. Soil characteristics determine water holding capacity and irrigation needs (Kranz et al., 2008). Nebraska generally has soils which are classified as fine sands, loamy sands, and fine sandy loam, sands are characterized by larger particle size, and therefore tend to be well drained (Kranz et al., 2008). These soils tend to have less water holdings capacity: 1.5 to 2.7 inches per foot, with soils composed of larger particles (Kranz et al., 2008). In soils which are medium to fine-texture, water capacity can range from 5.4 and 7.6 inches due to finer particle size (Kranz et al., 2008).
Well drained soils, require more frequent rainfall or more frequent irrigation to support high corn yield (Hollinger, 1995). Soils in Kansas, Kentucky, Nebraska, North Dakota, and South Dakota are well drained, and require irrigation (Hollinger, 1995). However issues can arise in irrigated soils which hinder crop root development which limit the crop’s capacity to uptake water. Constraints to root growth in irrigated soils can also include compaction and salinity as well as saturation. Root abundance and depth are important factors in crop nutrient uptake, water uptake, and development. These factors directly impacts crop health, growth, and yield.
Water Use and Subsurface Drainage
In the Eastern Corn Belt, where corn is intensively cultivated, these croplands were largely converted wetlands (Blann et al., 2009). In contrast to sandy well drained irrigated soils of Nebraska and other areas of the Western Corn Belt, soils of this region are comprised largely of glacial till, with poor drainage (Blann et al., 2009). Early on settlers drained floodplains and low-lying wetlands, to reduce soil saturation allowing for reduced water stress on crops and enhanced root development (Blann et al., 2009). Today sub-surface drainage increased root growth and improving crop’s capacity to uptake water and increasing yield (Blann et al., 2009).
In 2009, in the Midwest, subsurface drainage occured on approximately 80% of land (Blann et al., 2009). While subsurface drainage systems pose a number of benefits by allowing for deeper
62 root penetration, subsurface drainage can over lower the water table and as a result over drains soils, resulting in increased water demand by crops.
Environmental Impact of Irrigation
Irrigation fundamentally alters the water balance and hydrology of natural systems. A study by Hatfield J. 2015 describes water balance as the relationship of precipitation contributing to base flow of surface waters, groundwater recharge, surface runoff, and evapotranspiration (Hatfield, 2015). Water balance in irrigated agriculture also includes irrigation supplied from surface or groundwater (Hatfield, 2015). When water is artificially diverted to cropland through mechanical irrigation systems, or when groundwater is mined from aquifer systems or water is diverted from surface water systems a variety of consequences on aquatic and terrestrial ecosystems often follow.
Water is a rather unique input in that it serves as both a resource, limitation, and media. As a resource it is critical for crop development and has a direct relationship with corn yield. Yet water can serve as a limitation, because in oversaturated soils root development can be restricted and nutrient availability can also be limited. Of critical concern however in consideration to the environmental impacts of water use is the ways in which water serves as a dynamic media which can transport a number of agrochemical pollutants.
Irrigation and Surface Water Runoff
Surface runoff is an important aspect of the water balance of natural systems (Hatfield, 2015). Surface runoff occurs when precipitation volume exceeds absorption by soil (Hatfield, 2015). Soils susceptible to surface runoff tend to be soils which are poorly drained (Hatfield, 2015). However in poorly managed irrigation systems, there is also a risk for surface runoff (Hatfield, 2015). Water needs of crops vary on a number of conditions therefore inadvertent over irrigation is a risk in nearly all irrigation systems. Irrigation has been shown to stabilize and improve average corn yield in regions susceptible to drought. Surface water runoff occurs often at times of year when there is minimal surface evaporation or when crop water use is low (Hatfield, 2015). Irrigation can contribute to surface water runoff by contributing to saturated soils.
In natural ecosystems surface water runoff is an important contributor to surface water bodies such as streams and lakes. Land use changes by agriculture in the midwest have redirected surface water to man-made channels thus diverting flows from natural water bodies (Hatfield, 2015). Runoff, depending on the duration, velocity and volume of precipitation, can cause substantial soil erosion, in particular topsoil losses (Hatfield, 2015). Soil erosion by surface water runoff from agriculture leads to sediment loading of surface waters which is disruptive to water quality and wildlife (Hatfield, 2015). In addition to carrying sediment, surface water runoff from agricultural fields carries agrochemicals (pesticides and fertilizers) which a have deleterious impact on aquatic life.
63 Groundwater Depletion
In irrigated agriculture, the water balance is further altered by surface water and groundwater withdrawals. In 2010, irrigation accounted for 65% of groundwater withdrawals and 29% of surface water withdrawals (Maupin et al., 2014).
Figure 25. U.S. Freshwater Withdrawals 2010 Source: Maupin et al., 2014
Historically, surface water was the primary source of water for irrigation, accounting for 77% of water used in 1950 for irrigation. Increasingly groundwater is being used as an irrigation source (Maupin et al., 2014). Water use by irrigated agriculture is consumptive. Irrigation causes a net loss of water as water is lost to evaporation during application and evapotranspiration by crops, as well as any water lost to runoff or subsurface drainage. The increased withdrawal of groundwater for irrigation further contributes to a net loss of water balance. This is especially apparent in the Midwest with the rapid depletion of the Ogallala Aquifer.
The Ogallala Aquifer is the largest aquifer in the Great Plains region, spanning eight states and covering 174,000 square miles (USDA, 2016). The Ogallala Aquifer is the main geological formation of the High Plains Aquifer System. 97% of water pumped from the High Plains aquifer is used for irrigation and accounts for 30% of groundwater withdrawn for irrigation in the United States (Groundwater Resources Program USGS, 2010). The High Plains aquifer area covers 15% of corn acreage and is the primary source of water for irrigation (Groundwater Resources Program USGS, 2010). In addition this region accounts for 18% of total cattle production within the United States further contributing to water demand (Groundwater Resources Program USGS, 2010). In some areas water levels have declined more than 100 feet, and in some areas water saturation has been reduced by a half (Bartolino et al., 2003).
Fertilizer Use
64 Synthetic Fertilizer Use
The three main primary nutrients for plants are nitrogen, phosphorus, and potassium. The availability of these three nutrients is critical to plant growth and yield. Prior to the mid 20th century the use of synthetic chemical fertilizers in agriculture was uncommon; and the primary sources of fertilizer were manure and plowed down legumes (Herget et al., 2015).
World War II
During World War II, the U.S. government increasingly invested in the manufacturing of ammonia for explosives. By the end of World War II, U.S. factories were producing 730,000 tons of ammonia each year with the capacity to produce 1.6 million tons (Herget et al., 2015).Following World War II, this ammonia was redirected from ammunition production to chemical fertilizer production. The application of this technology to agriculture rapidly increased the use of synthetic fertilizers and increased corn yield.
1960-Present
Since 1960 nitrogen fertilizer use has increased from 17.0 lbs/ac/year (1960) to 82.5 lbs/acre/year (2007). Nitrogen fertilizer use has grown at a faster rate than that of phosphate or potash. While In 1960 synthetic nitrogen fertilizer accounted for just 37% of fertilizer applied, in 2014 nitrogen fertilizer accounted for 57% of synthetic fertilizer applied (EPA).
Phosphate and potash use has also grown since 1960 along with increases in corn acreage. However rates of application of phosphate and potash fertilizer have remained around 25 and 36 lbs/acre/year since the late 1960s (EPA).
Corn Synthetic Fertilizer Use
Corn is the crop with the highest synthetic fertilizer use, accounting for 40% of total commercial fertilizer use in the United States.
65
Figure 26. Synthetic Fertilizer Use by Four Major Crops 1964-2010 Data source: ERS USDA, 2018
In 2016 corn used 6,123,000 tons of nitrogen fertilizer, 2,081,000 tons of phosphate fertilizer, and 2,249,000 tons of potash fertilizer (ERS USDA, 2018). Totalling at 10,453,000 tons of fertilizer applied over 94.1 million acres (USDA NASS, 2017).
Synthetic Nitrogen Fertilizer Use and Corn Yield
Synthetic fertilizer has become increasingly important in maximizing crop yield. A 2005 study estimated the average percent of crop yield attributable to the application chemical fertilizers ranges anywhere from 40% to 60% (EPA, 2012). A number of factors influence availability of nutrients for uptake including microbial communities of soil, soil pH, and soil drainage properties as well as the timing of fertilizer application.
Natural Nitrogen Content of Soil and Microbes
Nitrogen content of the mollisoils of the Midwest soils tends to be naturally high with the highest levels in Western Ohio, Indiana, Illinois, Iowa, and Northern Missouri, and Southern Minnesota (Hangrove et al., 1998).
66
*green values indicate high nitrogen, red values indicate low nitrogen Figure 27. U.S. Soil Nitrogen Source: Hangrove et al., 1998
Nitrogen Requirements for High Yield Varieties of Corn
Nitrogen content of the Corn Belt soils is naturally high, however the application of synthetic nitrogen fertilizer is necessary to meet the nitrogen requirements of high yield corn varieties grown today. In high yield varieties 140 to 210 lbs of Nitrogen/acre are needed to support grain development (Debruin).
For conventional modern corn hybrids, with yields ranging between 80 and 150 bu/acre, their with a nitrogen demand tends to be less than 65 lbs N/acre (Debruin). However when yields exceed 200 bu/acre, grain nitrogen removal averages 139 lbs N/acre (Debruin).
Nitrogen Requirements and Corn Growth Stages
Nitrogen use differs at various stages of corn growth and development. The majority of nitrogen demand occurs during the pre flowering stage of corn, largely dedicated to leaf blade development, and stalk and leaf sheath growth (Debruin). During the reproductive stage, most nitrogen use is dedicated to grain while some is also dedicated to shanks, husk, and cob development (Debruin).
67
Figure 28. Corn Nitrogen Use and Uptake Source: Debruin
Pre-flowering nitrogen requirements are higher than post-flower nitrogen requirements, however increasingly nitrogen fertilizer is being applied post-flowering to increase yield (Debruin). New hybrids use greater amounts of nitrogen post-flowering in comparison to old hybrid varieties. New hybrids on average have a 7lb greater nitrogen demand over the season with 29% more Nitrogen demand post flowering (Debruin).
68
Figure 29. Seasonal Nitrogen Use Source: Debruin
Corn-Soybean Rotations
Corn’s high nitrogen requirements deplete soil of nitrogen quickly, Corn-soybean rotations are the most prevalent practice for conserving soil nitrogen content on large scale farms while conservation crop rotation and cover crop rotation is uncommon. Only 3 to 7% of corn farms utilized crop rotation or cover crop rotation, accounting for 1% of farmland (Wallander, 2103). However only 18% of cropland planted to corn in 2010 was in corn continuous cultivation from the previous 3 years, with most non-continuous corn cropland in 2 year soy-corn rotations (Wallander, 2013).
While soybean-corn rotations improve nitrogen content of soil, they are not enough to meet the high nitrogen requirements of corn currently cultivated (Sawyer, 2008). In a study by Sawyer J. E. It was found if nitrogen fertilizer was not applied to continuous corn cropland, yield fell by the next season to 50 to 60 bu/ac and in soybean corn rotations, yield would fall to 100 to 110 bu/ac in soybean corn rotation cropland (Sawyer, 2008). Meaning without the application of synthetic nitrogen fertilizer corn yield in soybean corn rotation falls by 30%, and in corn continuous cultivation falls by 45% (Sawyer, 2008).
Therefore even within soybean corn rotations, supplemental nitrogen fertilizer is needed to reach economically viable yield (Sawyer, 2008).
Nitrogen Efficiency of Corn
69
Corn as stated above, has high nitrogen demands, and intensive cultivation of corn can quickly deplete the soil of available nitrogen. However the average national nitrogen efficiency of corn is only 40%; meaning only 40% of nitrogen fertilizer applied is converted to crop biomass (Sawyer, 2008). This means 60% of nitrogen applied goes elsewhere. Given that in 2014 in the U.S. 13,295,000 tons of nitrogen material short were applied to corn cropland, nitrogen going “elsewhere” is of serious consequence.
Environmental Impact of Nitrogen Fertilizer Use
Nitrogen is an important element in biotic systems and is one of the most important limiting factors for primary production in terrestrial and aquatic ecosystems (Compton et al., 2011). Nitrogen accounts for 78% of chemical composition of air in the troposphere, and is a critical in living systems making up about 5% of mass in living organisms (Helesicova).
Given the abundance of nitrogen in the atmosphere and in living organisms, introduction of nitrogen to aquatic and terrestrial ecosystems is common and critical to supporting biotic communities. However concentrated anthropogenic increases in nitrogen availability in aquatic and terrestrial systems is disruptive. Much of this concentrated anthropogenic introduction is due to fertilizer use in commercial agriculture.
Eutrophication
The anthropogenic introduction of nitrogen is significantly disruptive to aquatic ecosystems due to nitrogen’s role as a limiting factor for primary production. The introduction of nitrogen to aquatic systems stimulates rapid population growth of photosynthetic organisms. As these organisms overpopulate the top of the water column light is blocked. Without access to light these photosynthetic organisms switch from photosynthesis to cellular respiration, resulting in increased dissolved oxygen demand, lowering the oxygen content of the water. Furthermore, as these photosynthetic organisms die off microbes decompose these organisms. This further increases dissolved oxygen demand and leads to hypoxia. Hypoxia results in mass die offs of oxygen dependent organisms such as higher level fish, resulting in mass fish kills.
70
Figure 30. Fish Kill Photo by P.J. Hahn, National Geographics
Corn is intensively cultivated in close proximity to a number of critical watersheds. The proximity of corn cultivation to these aquatic ecosystems increases the risk of nitrate loading due to surface water runoff contaminated with fertilizer. In a USGS report fertilizer application and use efficiency of corn, cotton, barley, soybean, sorghum and was related to watershed proximity to visualize the risk of watershed nitrogen contamination (Kellogg et al., 1997).
71
Figure 31. Nitrogen Loading from Corn Belt Source: Kellogg et al., 1997
Risk of surface water nitrogen loading was found to be highly concentrated within the Corn Belt and the Mississippi River (Kellogg et al., 1997). Given the high connectivity of critical watersheds within the Corn Belt (Missouri River Basin, the Upper Mississippi River Basin, and the Ohio River Basin) and the chemical properties of nitrates, nitrogen introduced to surface waters is highly mobile. As a result the introduction of nitrate pollutants to highly connected headwaters, can quickly contaminate entire watersheds. In fact the corn production in the Upper Mississippi River Basin has been identified as the major source of nitrate pollution contributing to hypoxia in the Gulf of Mexico (Goolsby et al., 1997).
Hypoxia in the Gulf of Mexico
Within the Northern Gulf of Mexico, due to high levels of nitrate, each summer a massive hypoxic zone forms also referred to as a “dead zone”. In 2017 the “Dead Zone” measured
72 22,720 square kilometers (Mississippi River/Gulf of Mexico Hypoxia Task Force, 2017). This area was above the five year average of 15,032 square kilometers (Mississippi River/Gulf of Mexico Hypoxia Task Force, 2017).
Figure 32. Hypoxia in the Gulf of Mexico Dead Zone Source: Mississippi River/Gulf of Mexico Hypoxia Task Force, 2017
The seasonal growth of the hypoxic zone in the Gulf of Mexico is largely dependent on discharge from the Mississippi River. While this hypoxic zone is present year round, it peaks in summer. Spring rainfall in regions in the Upper Mississippi River Basin has been identified as an important variable in seasonal fluctuations of hypoxic zone area (David et al., 2010). Nitrate loading occurs in close relation to peak precipitation events occurring during a matter of days or weeks in winter and spring (Kellog et al., 1997). Due to the high volume of nitrate movement in a short period, little nitrate is denitrified allowing for the movement of nitrate from the field, to the headwater, and into the Gulf (Kellog et al., 1997).
Upper Mississippi River Basin Contribution to Nitrogen Loading of the Gulf
90-95% of the land use proximate to watersheds in the Upper Mississippi River Basin, is corn cropland which receive high nitrogen inputs (David et al., 2010).
73
Figure 33. Nitrogen Inputs by County within Mississippi River Basin Source: David et al., 2010
As seen, nitrogen inputs within the Corn Belt are high, overlapping in close proximity to the Upper Mississippi River Basin, as well as the Ohio River Basin.
In addition to high nitrogen inputs, subsurface drainage is concentrated within this region which resulting in high nitrate concentrations of subsurface drainage outflow.
Subsurface Drainage and Nitrogen Loss
In areas of the Corn Belt where there is subsurface drainage, there is baseline nitrate loss (Sawyer, 2008). This nitrate loss varies based on climate and soil but ranges from 3 to 10 mg L(-1) or 8 to 20 lbs ac (-1) (Sawyer, 2008). As the amount of nitrogen fertilizer applied increases to compensate for baseline loss, nitrate concentration of subsurface drainage outflow increases (Sawyer, 2008).
The U.S. maximum permissible level of nitrate is 10 mg of nitrate per L(-1) (Sawyer, 2008). Yet at and below optimal economic rates of fertilizer use in areas with subsurface drainage, nitrogen concentrations in subsurface drainage outflow and proximate watersheds are often found above 10 mg N L(-1) (Sawyer, 2008).
74 This further contributes to the to nitrogen loading of the Upper Mississippi River Basin, and in turn the greater Mississippi drainage basin.
Figure 34. Tile Drainage Concentration by County in Mississippi River Basin Source: David et al., 2010
The study by David et al., used these input variables (subsurface drainage and fertilizer application) to predict the nitrate loading of 153 sub-watersheds which contribute to the Mississippi River Basin, the main tributary to the Gulf of Mexico (Nitrate Loadings from Commercial Fertilizer).
75
Figure 35. Predicted Nitrogen Loading of Mississippi River Basin by County Source: David et al., 2010
This model shows the highest nitrate inputs to the Mississippi River Basin coming from the Corn Belt. The high applications of nitrogen fertilizer concentrated in this region contributes directly to hypoxia in the Gulf of Mexico.
Nitrate Contamination of Groundwater
As stated earlier, nitrate is a highly soluble, mobile anion. It not only has high potential to contaminate both surface water, it also frequently is a contaminant in groundwater (Exner et al., 2014). Nitrate occurs naturally in groundwater at an average concentration of <2 mg N/L. However levels of 3-4 mg N/L or above reflect influence of human activity. Use of nitrogen fertilizer in intensive row-crop agriculture is a common anthropogenic sources of groundwater nitrate (Exner et al., 2014). Many regions in which corn cultivation is concentrated, and where nitrogen fertilizer is applied extensively, have shallow groundwater and have limited water treatment. Within the Upper Midwest approximately 70% of residents rely on groundwater for drinking water (Masarik et al., 2014).
76 Aquifer vulnerability to nitrate contamination is influenced by soil drainage and water table depth. Based on fertilizer use, soil drainage characteristics, and water table level, the USGS modeled risk of nitrate contamination of groundwater.
Figure 36. Groundwater Nitrate Contamination Risk Source: USGS, 2017
High risk of groundwater contamination was found throughout the Corn Belt with high risk in parts of Iowa, parts of Illinois, eastern North Dakota and eastern South Dakota, and particularly in Nebraska (USGS, 2017). In addition to soil drainage and water table depth, irrigation also can significantly increase risks of nitrate groundwater contamination. In Nebraska this is particularly evident as the region has the highest concentration of irrigated corn cropland.
Irrigation and Groundwater Nitrate: Nebraska
Groundwater contamination by nitrate occurs frequently in densely irrigated areas (Exner et al., 2014). Intensively irrigated cropland tends to require higher applications of nitrogen fertilizer and tends to have well-drained soils. Both contribute to high levels of nitrate in the groundwater (Exner et al., 2014).
77 Within Nebraska, irrigated corn cropland receives the highest applications of nitrogen fertilizer relative to other irrigated cropland (Exner et al., 2014). Furthermore irrigated corn cropland receives higher levels of nitrogen fertilizer than non-irrigated corn cropland (Exner et al., 2014). Over 70% of Nebraska’s irrigated cropland acreage is dedicated to corn, and 80% of Nebraska’s irrigated corn acreage is concentrated in central and Eastern Nebraska (Exner et al., 2014).
Central and Eastern Nebraska are the regions with the highest levels of groundwater nitrate contamination within the state (Exner et al., 2014). It was found by a study by Exner et al., over the past 30 years, in central and eastern Nebraska, the area underlain by groundwater with nitrate levels over or equal to 10 mg/L in had doubled (Exner et al., 2014).
78
Figure 37. Nebraska Groundwater Nitrate Contamination Source: Exner et al., 2014
Groundwater nitrate contamination is a significant threat to human health. Much of the Midwest relies on groundwater for drinking water. Nitrate consumption through contaminated well-water has been associated with methemoglobinemia in infants and children, a condition in which blood hemoglobin is altered, starving tissues of oxygen (Exner et al., 2014).
High instances of surface water contamination and groundwater contamination are related to high use rates of nitrogen fertilizer, however corn’s poor nitrogen use efficiency itself is largely connected to nitrates prevalence as a pollutant.
Vegetation Cover and Nitrogen Leaching
Historically much of the Corn Belt was covered by Long Grass and Mixed Grass Prairie. Replacement of long grass prairie with intensive corn cultivation has fundamentally increased nitrogen movement. A study by Masarik, Sherman and Brye compared nitrate leaching of continuous corn in a conventional chisel plow tillage field and a no tillage field, as well as a nitrate leaching of a prairie restoration site (Masarik et al., 2014). Conventional levels of fertilizer were applied to all three systems (10 kg N ha-1, 180 kg N ha-1 NH4NO3), and equilibrium tension lysimeters were used to measure year round nitrate leaching for 8 years (1996-2003) (Masarik et al., 2014).
Chisel-plow tilled corn had the highest level of drainage at 52% of cumulative precipitation, followed by no-tillage corn at 37% (Masarik et al., 2014). The prairie restoration site had the lowest level of drainage from precipitation at 16% of cumulative precipitation (Masarik et al., 2014). 79
Despite higher volume of drainage, chisel-plow tilled corn and no-till corn had similar levels of total N input leached measured at 18% and 19% respectively (Masarik et al., 2014). Over the 8 year period average nitrate-N concentration of drainage measured 9.5 mg L-1 for chisel-plow tilled corn, 12.2 mg L-1 for no-till corn, and <0.1 mg L-1 for the prairie restoration site (Masarik et al., 2014).
These levels of leachate for chisel-plow tilled corn and no-till corn, varied throughout the study however were observed to exceed the national standard of 10 mg L-1 (Masarik et al., 2014). Three quarters of leaching occurs in a short window of April 1st to June 30th, during peak precipitation (Masarik et al., 2014). Seasonal nitrate loading further concentrates levels of nitrate and contamination risk to surface and groundwater.
Nitrogen Fertilizer Impact on Soil Properties
Nitrogen Fertilizer Impact on pH
The application of nitrogen fertilizer containing ammonium decreases soil pH (Mosaic Crop Nutrition). When ammonium is converted to nitrate during nitrification, hydrogen ions are released, decreasing soil pH (Mosaic Crop Nutrition). For each pound of nitrogen as ammonium applied, it takes 1.8 pounds of pure calcium carbonate to neutralize soil (Mosaic Crop Nutrition). Highly acidic soils can reduce crop yield, and can increase heavy metal availability, reducing soil health and productivity.
Nitrogen Fertilizer Impact on Soil Microbes
A study done by the Liebeg, Varel, Doran and Wienhold in Nebraska on changes in soil pH with long term fertilizer application in cropping systems found that nitrogen fertilizer application had a significant impact on soil pH (Liebig et al., 2002). The study manipulated other variables (crops, crop rotation, crop root properties, and tillage), however found nitrogen fertilizer application to be the agricultural activity which had the most significant impact on soil properties (Liebig et al., 2002). Increased rates of nitrogen fertilizer application resulted in higher organic C, total N, and POM, however also resulted in lower microbial biomass and soil pH (Liebig et al., 2002). Microbial communities are critical for nutrient availability, soil cycling, soil organic content as well as a host of other soil properties which improve crop productivity. Increasing reliance on nitrogen fertilizer and decreasing health of microbial communities is of concern for long term soil fertility.
Nitrous Oxide Emissions and Nitrogen Fertilizer
Excessive applications of nitrogen fertilizer in the Corn Belt also contributes to greenhouse gas emissions. Nitrous Oxide or N2O is a greenhouse gas, with heating capacity 265x that of Carbon Dioxide (Fu et al., 2017).
80
U.S. intensive agriculture is a significant contributor to N2O, accounting for an annual emission of ~500 Gg from cropland in 2007 due to nitrogen based fertilizers (Millar et al., 2010). Within the agricultural sector 80% of anthropogenic N2O emissions are attributed to soil management practices, and most associated with field crops such as corn (Millar et al., 2010).
The fixation of atmospheric nitrogen (N2) occurs naturally within microbes as well as some leguminous plants, however increased use of nitrogen fertilizer is contributing significantly to increased levels of nitrous oxide in the atmosphere (Millar et al., 2010). Biological and industrial fixation of N2 are currently outpacing nitrification and denitrification, resulting in a net increase in N2O (Millar et al., 2010). N2O is produced in soil by microbial nitrification (ammonia oxidation) and denitrification (nitrate reduction) (Millar et al., 2010).
Inorganic nitrogen availability is a limiting factor to the metabolic activities of nitrification and denitrification by microbes. Therefore increased inorganic nitrogen in soils increases nitrous oxide emissions (Millar et al., 2010). Highly productive soils have greater background emissions of N2O, moreover nitrogen fertilizer application adds inorganic nitrogen at a more rapid rate than natural processes. This causes nitrous oxide to be emitted at even greater quantities over a shorter period of time.
Denitrification and nitrification are regulated in soil by available carbon and oxygen, as well as other nutrients in addition to inorganic nitrogen (Millar et al., 2010). Carbon, inorganic nitrogen, and oxygen levels are influenced by soil moisture, porosity, and composition. Intensive agriculture has a substantial impact on these soil characteristics as well as microbial community composition, N2O emissions (Millar et al., 2010) The timing and quantity of fertilizer applied, crop type, tillage practices, and irrigation all influence N2O emissions (Millar et al., 2010).
Nitrous Oxide Emissions and Corn
Within the Midwest (Iowa, Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin), nitrous oxide is the primary greenhouse gas emitted by the agricultural sector, due to intensive fertilizer application to corn (Millar et al., 2010).
A study undertaken by Decock on N2O emissions the study found in intensive corn cultivation in which yield is 12-14 Mg ha -1, fertilizer application can take over 220 N kg ha-1 yr-1. Surplus nitrogen leads to higher rates of N2O emissions. A study by the USDA ERS suggested 31% of U.S. corn acres may receive surplus nitrogen (Decock, 2014).
Environmental Impact of Phosphorus Fertilizer
Phosphorus, like nitrogen, is another important nutrient frequently applied along with nitrogen to corn cropland. It is an important limiting factor for primary productivity and phosphate runoff, has
81 similar consequences to nitrogen runoff in aquatic systems which are phosphate limited (Penuelas et al., 2013).
In addition like nitrogen, phosphorus fertilizer use is an important input in intensive cultivation of corn. Corn cultivation is the largest consumer of phosphate fertilizer in the U.S. Phosphate, unlike nitrogen fertilizer, is derived primarily from mining (Penuelas et al., 2013).
Cadmium in Phosphate Fertilizers
Phosphate rock used to make phosphate fertilizers can contain varied amounts and types of impurities depending on the source of the rock. These impurities include heavy metals (Minnesota Department of Health, 2016). Heavy metals which have been found in association with inorganic fertilizer application such as phosphate and potash are lead, cadmium, arsenic, and mercury.
Cadmium is naturally occuring in phosphate rock and a common impurity (Roberts, 2014). Levels of cadmium in phosphate range greatly based on the region of the phosphate deposit. A survey by the International Fertilizer Development Center assessed the cadmium content of 35 phosphate deposits in 20 countries and observed substantial variation amongst regions as well as within regions. Average cadmium content for sedimentary deposits was 21 mg/kg, ranging between 1 and 50 mg/kg (Roberts, 2014). In comparison to other sedimentary rocks, sedimentary phosphate rocks have 69x more enriched Cadmium (Roberts, 2014). Igneous phosphate rocks had a lower cadmium content averaging 2 mg/kg, 7.5 times that of other igneous rocks (Roberts, 2014).
82 Table 13. Cadmium Content of Phosphate Rocks
Source: Roberts, 2014
Sedimentary phosphate rocks account for 85% of world phosphate production (Roberts, 2014). Cadmium contamination of fertilizer is influenced by the original cadmium content of the phosphate source rock, as well as the production process (Roberts, 2014).
In the production of superphosphate (SSP), in which phosphate rock is treated with sulfuric acid, nearly all cadmium (Cd) content of the phosphate rock is transferred to the fertilizer (Roberts, 2014). Similarly in the production of triple superphosphate phosphate (TSP), phosphate rock is treated with phosphoric acid and nearly all Cd within the phosphate rock is transferred to the refined fertilizer (Roberts, 2014). SSP contains anywhere from 2-40< mg/kg Cd, and TSP’s cadmium content can range from <10-100< mg/kg Cd (Roberts, 2014).
This is in contrast to phosphate fertilizer produced by wet process phosphoric acid (WPA), used to produce Ammonium phosphates (monoammonium phosphate and diammonium phosphate). During WPA, 55-90% of Cd is transferred out of the rock into the acid-gypsum byproduct (Roberts, 2014). However the source rock’s Cd content is still an important determinant and Cd content still ranges from <1-100< mg/kg (Roberts, 2014). Cadmium is of significant concern due
83 to its toxicity and its persistent bioaccumulative properties (Minnesota Department of Agriculture, 2007).
Heavy Metals in Fertilizers
Testing of other fertilizer mixes has indicated presence of arsenic, lead, and mercury, in addition to cadmium. A heavy metal analysis summary compiled from 2002-2007 by the Minnesota Department of Agriculture, Pesticide and Fertilizer Management Division, analyzed commonly used fertilizer’s content and often found arsenic, cadmium, lead, and mercury (Minnesota Department of Agriculture, Pesticide, and Fertilizer Management Division, 2007).
Table 14. Claimed and Found Contents of NK Fertilizer Mix
Source: Minnesota Department of Agriculture, Pesticide, and Fertilizer Management Division, 2007
Table 15. Claimed and Found Contents of Iron Amendment
Source: Minnesota Department of Agriculture, Pesticide, and Fertilizer Management Division, 2007
84 Table 16. Claimed and Found Contents of Zinc and Zinc Sulfate
Source: Minnesota Department of Agriculture, Pesticide, and Fertilizer Management Division, 2007
Heavy metal availability in soil increases as the pH of soil decreases (Minnesota Department of Health, 2016). Heavy metal accumulation and availability is concerning due to its impact on plant uptake and accumulation of heavy metals, heavy metal leaching, and health of microorganisms in soil, as well as its contamination of surface and groundwater (Minnesota Department of Health, 2016).
Cadmium Accumulation in Corn and Soil pH
An experiment by the Institute of Sustainable Agrotechnology in Malaysia tested the influence of pH and cadmium content of soil on cadmium uptake and accumulation in corn. The study manipulated pH levels of soil and added 5 levels of cadmium (Shareef et al., 2016).
Soil was adjusted to pH 4.0, 5.2, 6.0, 7.0, and 8.0 by adding 2 ml solution of 0.15 M HCl, distilled H2O, and 0.06 M, 0.15 M, and 0.6 NaOH respectively to 20 g of soil aerobically dried (Shareef et al., 2016).
Altering levels of Cd were then added to the soil of each sample(0 mg, 2 mg, 4 mg, 6 mg, and 8 mg) and all plants were treated with recommended amounts of NPK fertilizer (60 N kg ha-1, 60 P kg ha-1 and 40 K kg,ha-1) (Shareef et al., 2016).
85 The study then evaluated Cd levels within the root, stem and grain of corn at different pH levels (4.0, 5.2, 6.0, 7.0, 8.0) and different levels of Cd applied (Cd 1= 2 mg. kg-1, Cd 2= 4 mg. kg-1, Cd 3= 6 mg. kg-1, Cd 4= 8 mg. kg-1) and used Cd 0 as a control (Shareef et al., 2016).
86
Figure 37. Cadmium Concentrations in Grains with Manipulations of Soil pH and Soil Cadmium Source: Shareef et al., 2016
Within grain, the highest levels of cadmium were found when the highest levels of Cd were applied at Cd 4 (8 mg. kg-1) and Cd 3 (6 mg. kg -1), and at pH 5.2 and pH 6 (Shareef et al., 2016). pH was a significant factor in Cd availability and uptake. The pH of soil, plays an important role in nutrient availability as well as heavy metal accumulation.
While phosphorus and nitrogen are often the most discussed fertilizer components due their role as limiting factors in most environments and the consequence of their introduction e.g. eutrophication. However potassium, or potash is critical to corn growth and development and is applied in greater quantities than phosphate fertilizer (Monsanto). Unlike nitrogen which is primarily taken up during corn’s pre-flowering period, potassium is used most from flowering to early pod development (Monsanto). Potassium is important to the movement of water, nutrient, and carbohydrates in a plant, with insufficient potassium decreasing crop’s capacity to uptake water and nitrogen (Monsanto).
Phosphate and potassium levels of cultivated soils decrease significantly after corn harvest, for every bushel of corn harvested per acre 0.4 pounds per acre of P2O5, and 0.29 pounds per acre of K2O are removed (Monsanto). While mitigation strategies can be used, in areas with intensive cultivation, applications of phosphate and potash fertilizer the next growing season are often required to achieve high-yields.
Pesticides Use
Corn Pesticides Use
87 Following World War II, use of synthetic organic pesticides became widespread and by 1960 were a key input in industrial agriculture.
Figure 39. U.S. Annual Pesticide Use Data source: Fernandez-Cornejo et al., 2014
From 1960 to 1972, cotton had the highest pesticide use of American field crops, however corn’s pesticide use surpassed cotton in 1972, and is currently the crop with the highest pesticide use.
88
Figure 40. Pesticide Use By Crop Source: Fernandez-Cornejo et al., 2014
Most pesticides applied to corn are herbicides (Fernandez-Cornejo et al., 2014). Herbicide use increased rapidly from the 1950s to the 1970s. While only 10% of U.S. corn acreage in 1952 was treated with herbicide, 90% of U.S. corn acreage was treated with herbicide in 1976 Fernandez-Cornejo et al., 2014). Currently 98% of U.S. corn acreage is treated with herbicide Fernandez-Cornejo et al., 2014).
89
Figure 41. Corn Acreage Treated with Pesticides Source: Fernandez-Cornejo et al., 2014
The percent of corn acreage treated with insecticides has fallen since the mid-1980s, while herbicide treatment of acres has remained close to 100% since the early 80s. Herbicide use has increased primarily due to its price in relation to alternative weed management practices which require higher inputs costs such as fuel, labor and machinery (Fernandez-Cornejo et al., 2014). Herbicide use has also increased due to the adoption of minimal tillage practices for soil conservation, as well as the rapid adoption of HT crops since the late 90s (Fernandez-Cornejo et al., 2014).
Corn Herbicide Use
In 2016, 97% of corn acreage was treated with herbicide, while only 12% of corn acreage was treated with insecticide and fungicide (NASS USDA, 2017). The herbicides used most by growers in 2016 were: atrazine, glyphosate potassium salt, and glyphosate isopropylamine salt. (NASS USDA, 2017).
In 2016 atrazine was applied to 60% of corn acreage at an average of 1.082 lbs/acre, with a total application of 56.0 million pounds applied (NASS USDA, 2017). Glyphosate potassium salt was applied to 33% of corn acreage, at an average of 1.323 lbs/acre, totalling 37.6 million pounds applied (NASS USDA, 2017). Glyphosate isopropylamine salt was applied to 32% of corn acreage, at an average rate of 1.011 lbs/acre, with a total of 28.5 million pounds applied (NASS USDA, 2017).
Glyphosate Use 90 Glyphosate is a glycine derivative, with the molecular formula C3H8NO5P (PubChem Compound Database)
Figure 42. Glyphosate Structure Source: PubChem Compound Database
Glyphosate is a non-selective systemic herbicide, which is applied to plant foliage. As a systemic herbicide, glyphosate is translocated throughout the plant accelerating death. When absorbed by the plant, glyphosate blocks EPSP Synthase which produces EPSP. EPSP is critical to plant production of aromatic amino acids (National Pesticide Information Center, 2011).
Glyphosate was first registered for use in the United States in 1974 by Monsanto and is currently the most commonly applied herbicide in the United States. In 2015 an estimated 300 million pounds of glyphosate were applied to U.S. acreage. (USGS, 2017). Corn and soybean cultivation accounts for the highest use of glyphosate (USGS, 2017).
91
Figure 43. Glyphosate Use by Crop Source: USGS, 2017
The Midwest, bordering regions of the Mississippi River, South East coast, North and South Plains and central California are the areas in which glyphosate use is currently concentrated.(USGS, 2017). In these areas glyphosate application is estimated to exceed an average of 88.06 pounds per square mile (USGS Pesticide Use Map Glyphosate) The USGS provides an estimated use map for various pesticides with high and low estimates for glyphosate use, beginning in 1992.
92
Figure 44. Estimated Agriculture Use of Glyphosate 1992, Low Use Estimate Source: USGS, 2017
93
Figure 45. Estimated Agriculture Use of Glyphosate 1992, High Use Estimate Source: USGS, 2017
Genetically Modified Corn and Glyphosate Use
Glyphosate use has increased rapidly since the introduction of glyphosate resistant or “Round-Up Ready” corn in 1998 (Sidhu et al., 2000). GA21, or glyphosate resistant corn was developed by inserting a gene that modifies corn 5-enolpyruvylshikimate-3-phosphate synthase, resulting in “modified” EPSPS or mEPSPS (Sidhu et al., 2000). When this inserted gene is expressed, the modified corn variety is resistant to glyphosate. In mEPSPS, mEPSPS is fused to a transit peptide, allowing the enzyme to function in the presence of glyphosate and produce EPSP, unlike in normal plants and other organisms where the enzyme is inhibited by glyphosate (Sidhu et al., 2000).
Glyphosate resistant corn varieties have been rapidly adopted in the U.S. Rapid adoption of herbicide tolerant corn varieties has led to a dramatic increase in glyphosate applied to corn in relation to other herbicides.
94
Figure 46. Herbicide Acreage in Relation to HT Corn Acreage Data source: Fernandez-Cornejo et al., 2014
Glyphosate is the herbicide which has become most widely used on corn. The USGS Pesticide Use Map for 2015, both high and low estimates show the high volume and concentration of glyphosate use in regions where corn is cultivated intensively (Fernandez-Cornejo et al., 2014).
95
Figure 47. Estimated Agricultural Use for Glyphosate 2015, Low Use Estimate Source: USGS, 2017
96
Figure 48. Estimated Agricultural Use for Glyphosate 2015, High Use Estimate Source: USGS, 2017
Glyphosate Mobility, Persistence, and Degradation
Glyphosate Uptake by Plants
Glyphosate is absorbed across leaf surfaces and the stem of plants, it is then spread throughout the plant, concentrating in meristem tissue (National Pesticide Information Center, 2011). Weeds exposed to glyphosate display stunted growth, loss of chlorophyll (yellowing/browning of leaves), leaf wrinkling or malformation, and tissue death (National Pesticide Information Center, 2011).
Plants exposed to sodium salt of glyphosate have disrupted growth regulation and exhibit accelerated fruit ripening (National Pesticide Information Center, 2011). Since glyphosate targets the shikimic acid pathway which is only present in plants and some microorganisms, non-target organisms such as vertebrates experience low toxicity of glyphosate in its applied form (National Pesticide Information Center, 2011).
Glyphosate Degradation
97 Glyphosate does not have high volatility, therefore is not found or transported in atmosphere, but is stable in air (National Pesticide Information Center, 2011). Glyphosate is relatively stable to photo-oxidation and chemical decomposition.The primary pathway of glyphosate degradation is soil microbial activity (National Pesticide Information Center, 2011). Glyphosate’s primary metabolites are AMPA and glyoxylic acid, both of which can undergo further reactions and form carbon dioxide (National Pesticide Information Center, 2011).
Glyphosate Persistence in Water
The median half life of glyphosate in water varies from a few days to 91 days (National Pesticide Information Center, 2011). Glyphosate was not observed to undergo hydrolysis in buffered solutions of pH 3,6, and 9, at 35 degrees C (National Pesticide Information Center, 2011). Glyphosate in the solutions was also not observed to photodegrade significantly in natural light in pH 5,7, or 9 (National Pesticide Information Center, 2011).
When glyphosate was applied to aquatic plants in fresh and brackish waters, glyphosate concentrations in both waters declined rapidly, with some binding of glyphosate to bottom sediments occuring, depending on metals in the sediments (National Pesticide Information Center, 2011). The sediment half-life of glyphosate was increased by presence of chelating cations (National Pesticide Information Center, 2011). Glyphosate has little degradation in water however is more likely to contaminate soil due to it’s high binding affinity (Aparicio et al., 2013).
Glyphosate Persistence in Soil
Glyphosate is moderately persistent in soil with a half life ranging from 3 to 130 days (Aparicio et al., 2013). Glyphosate is an amphoteric compound (able to act as an acid and a base) with both pKa and pKb values, accounting for high soil binding affinity (Wauchope et al., 2001).
Glyphosate’s adsorption to soil particles, is largely influenced by soil phosphate levels (Aparicio et al., 2013). Glyphosate competes with inorganic phosphate for soil binding sites. Therefore levels of phosphate in soil directly impact the availability of these binding sites, and glyphosate adsorption to soil particles (Aparicio et al., 2013).
Glyphosate Interactions with Microorganisms
Glyphosate is primarily broken down in soil through microbial bacteria and fungi metabolic activity (Aparicio et al., 2013). The breakdown of glyphosate is highly dependent on the microbial community within soil and is primarily facilitated by microbial enzymes which cleave the C-P bond of the glyphosate molecule (Gill et al., 2017).
Glyphosate degradation by many different microorganisms is largely due to the presence of phosphorus in glyphosate (Gill et al., 2017). Pseudomonas PG2982, Rhizobium meliloti,
98 Arthrobacter GLP-1 strain, Agrobacterium radiobacter and Rhizobium strains, all have been observed to degrade glyphosate for phosphorus (Gill et al., 2017).
Some microorganisms such as Nit-1 utilize glyphosate for nitrogen, and others such as Streptomyces spp. utilize glyphosate for phosphorus and nitrogen (Gill et al., 2017). Nitrogen and phosphorus are both critical for metabolic functions of living organisms and are often limiting factors to growth in different environments, particularly phosphorus. Microbial degradation of glyphosate allows for increased availability of phosphorus and nitrogen (Gill et al., 2017).
Glyphosate Interactions with Mycorrhizal Fungi
Glyphosate can reduce the lateral root growth of mycorrhizal fungal species. At concentrations of over 10 ppm, Glyphosate was observed to reduce the growth of common mycorrhizal fungal species, Hebeloma crustuliniforme, Laccaria laccata, Thelephora americana, T. terrestris, and Suillus tomentosus (Pesticide Action Network Asian Pacific, 2009).
Glyphosate Interactions with Annelids and Insects
Glyphosate has been found to have severe negative impacts on earthworm reproduction and development at a range of concentrations; with morphological abnormalities observed at high concentrations (Pesticide Action Network Asian Pacific, 2009).
Glyphosate has also been found to negatively impact important insect predators, which are important controls to herbivorous insect populations (Pesticide Action Network Asian Pacific, 2009). These include: predatory mites, carabid beetles, ladybugs, and green lacewing (Pesticide Action Network Asian Pacific, 2009).
Glyphosate also has been found to negatively impact species which are important for ecosystem function such as Collembola, which are important in decomposition and formation of soil microstructure (Rusek, 1998).
99
Figure 49. Collembola Source: Maddison, 2002
Glyphosate also negatively impacts Oniscidea, which are important for terrestrial ecosystems in their mechanical and chemical breakdown of plant litter, contributing to microbial activity (Zimmer, 2002).
Figure 50. Oniscidea Source: Van Walkt, 2015
Glyphosate has also been observed to harm field spiders which are an important predator, feeding on many different herbivorous insect species which are considered pests (Pesticide Action Network Asian Pacific, 2009).
Glyphosate Resistance
100 An increasing concern with glyphosate being the primary herbicide used is the emergence of resistant weeds. The first reporting of a glyphosate resistant strain was in rigid ryegrass in 1996 in Australia. In 2000, glyphosate resistant horseweed was discovered in Delaware, the first case of a glyphosate resistant weed in the U.S.. (DuPont-Pioneer, 2016).
Currently there are at least 16 weeds in the United States with glyphosate resistance, and 35 weeds in the world with glyphosate resistance (DuPont-Pioneer, 2016). Glyphosate resistant weeds have been observed in 38 states in the United States (DuPont-Pioneer, 2016).
Figure 51. Glyphosate Resistant Species by State Source: DuPont-Pioneer, 2016
Within the Corn Belt, glyphosate resistance (GR) is highest in Missouri, Nebraska, and Kansas, as well as within Ohio and Indiana.
Table 17. Glyphosate Resistant Weeds in the Corn Belt State Number of Glyphosate Species Present Resistant Species Present
101 Missouri 6 Annual bluegrass Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth
Nebraska 6 Common ragweed Common waterhemp Giant ragweed Horseweed Kochia Palmer amaranth
Kansas 6 Common ragweed Common waterhemp Giant ragweed Horseweed Kochia Palmer amaranth
Ohio 5 Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth
Indiana 5 Common ragweed Common waterhemp Giant ragweed Horseweed Palmer amaranth
North 3 Common ragweed Dakota Common waterhemp Kochia
South 4 Common ragweed Dakota Common waterhemp Horseweed Kochia
Iowa 3 Common waterhemp Giant ragweed Horseweed
Illinois 3 Common waterhemp Horseweed Palmer amaranth Source: DuPont-Pioneer, 2016
102 Glyphosate resistant occurs rapidly, as resistant weed populations grow at a logarithmic progression (DuPont-Pioneer, 2016).
Figure 52. Glyphosate Resistance in Populations Source: DuPont-Pioneer, 2016
The prevalence of glyphosate resistant individuals in a weed species is impacted by a number of genetic variables: the mutation rate for resistance genotype, number of genes required to express resistance phenotype, dominance of the resistance allele(s), inheritance of resistance trait, and how the expression of resistance gene(s) impact the overall fitness of resistant individuals (survival and reproduction) (DuPont-Pioneer, 2016).
Many weeds have high reproductive capacities, particularly horseweed and waterhemp (DuPont-Pioneer, 2016). This allows for resistant individuals to spread rapidly.
While glyphosate is the most commonly used herbicide in corn cultivation, atrazine is also a significant herbicide used in corn cultivation.
Atrazine Use
Atrazine is a selective triazine herbicide, which is primarily used as a pre-emergent herbicide for broadleaf plants (PubChem Compound Database). It’s chemical formula is C8H14ClN5 (PubChem Compound Database).
103
Figure 53. Atrazine Structure Source: PubChem Database
Atrazine binds with a protein complex of Photosystem II in the photosynthetic membranes of chloroplasts, and inhibits the transfer of electrons (Becker et al., 2016).
Atrazine was first registered for use in the U.S. by the USDA in 1958 (Becker et al., 2016). Since its initial registration, atrazine has become one of the most common herbicides used in the U.S., and is primarily applied to corn (Becker et al., 2016). Corn accounts for a significant amount of atrazine used. 90% of atrazine is applied to corn (Becker et al., 2016).
104
Figure 54. Atrazine Use by Crop Source: USGS, 2017
Regions in which atrazine use is concentrated are areas in which corn is intensively cultivated. The USGS Pesticide Use map for 2015 shows both a high and low estimate of spatial distribution of atrazine use, with both maps showing high concentration of atrazine use in the Corn Belt (USGS, 2017)
105
Figure 55. Estimated Agricultural Use for Atrazine 2015, Low Use Estimate Source: USGS, 2017
106
Figure 56. Estimated Agricultural Use for Atrazine 2015, High Use Estimate Source: USGS, 2017
Atrazine is applied at rates greater than 64.23 pounds per square mile, nearly continuously throughout the Corn Belt.
Atrazine Mobility, Persistence, and Degradation
Atrazine Persistence in Soil
In soil atrazine has a half-life of 14 to 109 days (Dorsev, 2003). Atrazine in soil is generally degraded by soil microorganisms to form metabolites hydroxyatrazine, deethylatrazine, or deisopropylatrazine (Dorsev, 2003). Aerobic conditions are important for timely biodegradation of atrazine, as anaerobic biodegradation of atrazine occurs at a much slower rate (Dorsev, 2003). Oxygen availability in soil is therefore an important influence in the rate of biodegradation of atrazine, as well as the composition and population of the soil microbial community (Dorsev, 2003). However under strongly reducing conditions, atrazine has been observed to biodegrade more rapidly in anaerobic soils (Dorsev, 2003). Adsorption of atrazine to soil is influenced by soil moisture regimes (Dorsev, 2003). Overall, although varying depending on soil type, atrazine has been observed to have high mobility in soils (Dorsev, 2003).
Atrazine Persistence in Air 107
An estimated 14% of atrazine is volatilized and exists in a particulate form or as vapor (Dorsev, 2003). In the atmosphere atrazine undergoes little photo degradation, however in its vapor form atrazine can be degraded photochemically by hydroxyl radicals (Dorsev, 2003). In its particulate form, atrazine can be removed from the atmosphere via wet or dry deposition and is commonly found in rainwater in areas near its original application during peak use seasons (Dorsev, 2003).
Atrazine Persistence in Water
Atrazine is very persistent in aquatic systems, and can persist with a half life longer than 6 months (Dorsev, 2003). Atrazine can be introduced to surface water through rainwater, dry deposition, wet deposition, soil drainage systems or surface water runoff (Dorsev, 2003).
Atrazine can also be introduced to groundwater through leaching through the soil profile (Dorsev, 2003). Atrazine tends to have slow or no biodegradation in surface water or groundwater (Dorsev, 2003). Abiotic factors influencing degradation of atrazine in aquatic systems include sunlight and oxygen (Dorsev, 2003). Given the lack of significant biodegradation in atrazine in aquatic systems, abiotic factors are important in influencing the persistence of atrazine in aquatic systems (Dorsev, 2003).
Since atrazine has a high affinity for water and is highly mobile and persistent in aquatic systems, therefore groundwater and surface water contamination are one of the primary environmental impacts of atrazine use (Dorsev, 2003).
Atrazine Contamination of Surface Water
A study by the USGS on the presence of pesticides in streams and groundwater analyzed water quality data from 1992 to 2006 and found atrazine present in over 75% of streams surveyed (Gilliom, 2007). Under the Atrazine Reregistration Eligibility Decision of 2003, attempts were made to mitigate atrazine contamination of surface and groundwaters by mandating monitoring programs in areas vulnerable to atrazine contamination (EPA, 2018).
The Atrazine Ecological Exposure Monitoring Program uses a threshold concentration equivalent level of concern (CE-LOC) of 10 ppb at a 60 day average concentration (EPA, 2018). This standard was set by the EPA, based on levels of atrazine application that change aquatic plant community composition, function, and productivity significantly (EPA, 2018).
If a watershed shows atrazine concentrations above this CE-LOC for any two years of monitoring, atrazine registrants are required to “initiate watershed-based mitigation activities” (EPA, 2018). Mitigation activities include education, stewardship and outreach programs for growers and distributors (EPA, 2018). A watershed can be exempt from the water monitoring program if its 60 day running average falls below the CE-LOC for two consecutive years (EPA,
108 2018). While the program had monitored up to 33 watersheds, the program as of 2015 only monitors 9 watersheds (EPA, 2018).
Concentrations of atrazine have seasonal peaks in water bodies. This is due to precipitation patterns as well as cultivation practices. The study of the 2017 Ecological Exposure Monitoring Program, composed by Syngenta, the primary manufacturer of atrazine, demonstrated the seasonal fluctuations in atrazine concentrations (EPA, 2018).
The study, compared the 9 watersheds enrolled in the Atrazine Ecological Exposure Monitoring Program: West Branch Sugar Creek (Iowa), Little Sugar Creek (Iowa), Branch of Boeuf River (Louisiana), South Fabius River (Missouri), Youngs Creek (Missouri), Honey Creek (Missouri), West Fork Cuivre River (Missouri), Big Blue River, Upper Gage (Nebraska), and Chocolate Bayou (Texas).
Table 18. Atrazine Ecological Exposure Monitoring Program State Watershed Name Sample ID Atrazine Max Atrazine Min Atrazine (ppb) (ppb) Average (ppb)
Iowa West Branch IA-04 36.51 0.08 3.35 Sugar Creek
Iowa Little Sugar IA-05 36.64 0.09 3.77 Creek
Louisiana Branch of Boeuf LA-04 36.99 0.09 4.28 River
Missouri South Fabius MO-01 49.83 0.03 6.42 River
Missouri Young Creek MO-02 49.07 0.09 7.35
Missouri Honey Creek MO-07N 95.02 0.08 11.05
Missouri West Fork Cuivre MO-08 54.01 0.36 7.92 River
Nebraska Big Blue River, NE-04 64.93 0.06 9.19 Upper Gage
Texas Chocolate Bayou TX-01 22.81 0.06 3.15 Data source: Syngenta Ecological Monitoring Program
Data was collected primarily March to October with the exception of some watersheds (TX-01, LA-04) in which data was collected earlier. The following graphs were created from this data.
109
Figure 57. Atrazine Levels in West Branch Sugar Creek
Figure 58. Atrazine Levels in Little Sugar Creek
110
Figure 59. Atrazine Levels in Branch of Boeuf River
Figure 60. Atrazine Levels in South Fabius River
111
Figure 61. Atrazine Levels in Young Creek
Figure 62. Atrazine Levels in Honey Creek
112
Figure 63. Atrazine Levels in West Fork Cuivre River
Figure 64. Atrazine Levels in Big Blue River, Upper Gage
113
Figure 65. Atrazine Levels in Chocolate Bayou
All sites had multiple periods of dramatic peaks, as well as extended periods in which the LOEC was exceeded. Sites varied in the timing, duration, and value of peak concentrations of atrazine, which may reflect a number of differences.
Impact of Atrazine on Aquatic Plants
In the latest Ecological Risk Assessment by the EPA in 2016, aquatic plants were concluded to be severely impacted by the use of atrazine. For aquatic vascular and non-vascular plants, levels of concern were exceeded in watershed testing (EPA, 2016). The study concluded that even following current restrictions for atrazine use, ELOC (ecological level of concern) was exceeded for aquatic plants, and even with lower application rates risk to aquatic plant communities was still predicted (EPA, 2016).
The influence of atrazine on aquatic plant communities is particularly concerning, due to the important of primary producers to higher trophic orders. The potential impacts of loss of aquatic plant species include habitat loss for other aquatic species, loss of habitat complexity, biodiversity loss, reduced populations of higher consumers, reduction in spawning and nursery habitat, as well as alteration in water chemistry and velocity (EPA, 2016).
Impact of Atrazine on Aquatic Vertebrates and Invertebrates
114 The latest Ecological Risk Assessment published in 2016 by the EPA concluded that Atrazine is moderately toxic to freshwater and estuarine/marine fish, highly toxic to freshwater aquatic invertebrates and very highly toxic to estuarine/marine aquatic invertebrates on an acute exposure basis (EPA, 2016).
Given that atrazine has been observed to be introduced to the environment at high volumes during specific periods, acute exposure is of concern. In addition, given the prevalence of atrazine in an overwhelming number of streams, as well as its persistence in aquatic systems, chronic toxicity is also of significant concern (EPA, 2016).
The study conducted by the Ecological Risk Assessment found that chronic exposure to atrazine for freshwater and estuarine/marine fish, aquatic phase amphibians and aquatic invertebrates resulted in significant decrease in survival rates, alterations of growth, as well as reproductive consequences (EPA, 2016). Freshwater fish were observed to have the most sensitive chronic endpoint, due to their susceptibility to reproductive effects (EPA, 2016). The Ecological Risk Assessment concluded the EPA’s level of concern are exceeded for freshwater and estuarine marine fish based on chronic exposure to atrazine through runoff.
The Ecological Risk Assessment found substantial areas, in which atrazine use exceeded the CELOC and posed substantial risk to amphibians and fish (EPA, 2016).
115 Figure 66. Atrazine Ecological Risk Map Source: EPA, 2016
Areas which were of the highest level of concern for the health of fish and amphibian species were concentrated in the Corn Belt where Atrazine use is concentrated. Atrazine poses a threat to ecological health through surface waters, however it also poses a risk to human health through its contamination of groundwater.
Atrazine Contamination of Groundwater
Atrazine is a common contaminant in surface water as well as groundwater. Many rural communities rely on groundwater as a drinking water source (Wu et al., 2010). The Natural Resource Defense Council (NRDC) analyzed water samples from the Atrazine Monitoring Program mandated by IRED, and found atrazine present in 80% of more than 35,000 samples (Wu et al., 2010). Of the samples of raw water, 100 water systems had concentrations above 3 ppb (Wu et al., 2010). For finished water, 67 water systems had concentrations above 3 ppb which is the maximum allowable concentration under the Clean Drinking Water Act (Wu et al., 2010).
Within these systems there were a number of peaks in municipalities which exceeded this level (Wu et al., 2010). In the Piqua City Public Water System in Ohio, there was a peak concentration of atrazine of 59.57 ppb in finished water, the highest finish water observed throughout the study(Wu et al., 2010). There were several other water systems with averages higher than the level permissible under the Clean Water Act (Wu et al., 2010).
116 Table 19. Water Systems with Annual Averages above 3 ppb Atrazine
Source: Wu et al., 2010
This figure indicate highest running averages of raw and finished water for communities which exceed 3 ppb. In addition to high averages of atrazine concentration, many communities experienced exposure to water with high atrazine contamination during seasons of peak application and peak precipitation. These spikes represent critical threats to human health, in consideration to vulnerable populations (children, elderly), as well as pregnant women (Wu et al., 2010).
117 Table 20. Water Systems with Highest Peak Atrazine Concentrations in Raw Water
Source: Wu et al., 2010
On days with peak concentration of atrazine in raw water, there were large disparities between levels of atrazine in finished water amongst communities. While Mt. Orab Village Public Water System, had the highest raw water concentration of 227.00 ppb, they had the lowest finished water concentration of 0.00 ppb. This greatly contrasts with atrazine concentrations of the finished water of McClure Water Treatment Plant, which were 33.83 ppb, despite having a much lower raw water concentrations of 42.89 ppb.
In addition, it is important to note the duration of time which atrazine concentration of community water exceeded 3 ppb. The community with the highest finished water levels and the longest duration of water which exceeded 3 ppb was Piqua City Public Water System, which had a finished water concentration of 59.57 ppb and whose water concentration exceeded 3 ppb for 12 weeks. Prolonged rates of high finished water are particularly concerning for human health, as finished water is directly consumed by residents.
118 Table 21. Water Systems with Highest Peak Atrazine Concentrations in Finished Water
Source: Wu et al., 2010
While a number of these water systems did not have water concentrations that exceeded 3 ppb for over a week, Versailles Water Works in Indiana had a peak finished water concentration of 30.48 ppb, and finished water which exceeded 3 ppb for 7 weeks. Evansville experienced a peak finished water of 25.75 ppb, and finished water which exceeded 3 ppb for 3 weeks.
Groundwater is most commonly contaminated in midwestern communities in areas in which atrazine is most heavily applied, mostly on corn. Groundwater contamination by atrazine has also been linked to shallow groundwater. Despite the attention atrazine has gotten from being a common pollutant in groundwater the issue persists.
119
Figure 67. Atrazine and Groundwater Contamination Risk Source: USGS, 2017
Areas with the greatest probability of drinking water containing atrazine levels which exceed the permissible standard are most concentrated in the Corn Belt. This is due to the hydrology of the region as well as the heavy use of atrazine. It is important to note as well that the area with the highest likelihood of groundwater contamination is concentrated in Eastern Nebraska, where high atrazine use is accompanied by high irrigation, increasing the risk of atrazine leaching.
Insecticides and Bt Corn
Unlike herbicides use, insecticide use on corn has decreased since peaking through the 80s and early 90s (Fernandez-Cornejo et al., 2014). One of the main causes of decreased insecticide use, is the development of Bt Maize varieties.
Plant varieties which are “Bt varieties”, are modified to express insecticidal proteins proteins which naturally produced by the species Bacillus thuringiensis (Betz et al., 2014). Bacillus thuringiensis (Bt) is a ubiquitous gram positive bacterium. Bt has two lifecycle phases: 120 vegetative cell division and sporulation (Betz et al., 2014). During sporulation Bt forms crystalline (Cry) protein inclusions.
Figure 68. Cry Proteins Source: Betz et al., 2014
These inclusions are formulated from Cry proteins, which act as an insecticide (Betz et al., 2014). Cry proteins are “prototoxins”. Upon ingestion by insects they are proteolytically activated (activated by peptide cleavage) by digestion (Betz et al., 2014). Cleaved Cry proteins bind to target sites in the midgut cells of certain insects, as well as ion-selective channels in the cell membrane ((Betz et al., 2014). Upon interactions with activated Cry proteins, cells experience swelling to cell lysis and mass cell death, leading to insect mortality (Betz et al., 2014).
Bt strains contain mixtures of six to eight different Cry proteins, providing pest control for pests such as the European corn borer, southwest corn borer, tobacco budworm, cotton bollworm, pink bollworm, and Colorado potato beetle (Betz et al., 2014)..
121 Table 22. Bt-Crops, Cry Proteins and Target Pests
The primary pests which impact corn are the European corn borer and southwestern corn borer. Bt corn is modified to express Cry1Ab and Cry1Ac, and prevents crop damage from the European corn borer and southwestern corn borer (Betz et al., 2014)..The use of Bt crop varieties has decreased crop loss to these pests significantly, and has also contributed to the decreased use of insecticides.
122 Environmental Impacts Summary
Table 23. Summary of Intensive Agriculture Activities Activity Methods Goals
Land Use -Cropland conversion -Increase production by -Surface water channelization increasing land available for -Subsurface drainage systems cultivation by altering -Tillage ecosystems -Increase viability of crops by altering subsurface hydrology which limits root growth
Water Use -Irrigation -Increase crop yields by increasing water availability -Mitigate risk of crop yield loss due to drought
Fertilizer Use -Nitrogen use -Increase crop yield by -Phosphate Use increasing nutrient availability
Pesticide Use -Herbicide use (Atrazine use, glyphosate -Mitigate risk of crop yield loss use) due to competing weeds -Insecticide use -Mitigate risk of crop yield loss due to herbivorous pests
Table 24. Summary of Environmental Impacts of Intensive Agriculture Activities Biotic Impacts Soil/Land Impacts Aquatic Impacts Atmospheric Impacts
Eutrophication Erosion Groundwater Depletion Nitrous oxide emissions
Biodiversity Loss Soil Pesticide Surface Water Particulate matter Contamination Sediment Loading
Primary Productivity Heavy Metal Surface Water Airborne/volatilized Loss in Natural Accumulation Agrochemical Pollution forms of agrochemicals Ecosystems
Habitat Soil Acidification Groundwater -Carbon dioxide Loss/Degradation Agrochemical Pollution
123 Climate Change
The following sections on land, water, fertilizer and pesticide use in corn cultivation have described resource inputs in the Corn Belt, as well as their environmental consequences. These resource inputs are affected by a number of different abiotic factors including climate. Climate plays a significant role in influencing the resource needs of crops and therefore often shapes farming practices. Climates globally over the past century have changed significantly due to increased concentrations of greenhouse gases in the atmosphere increasing. It is projected that these concentrations of greenhouse gases will increase in the future, further altering global climates. This will have a substantial impact on agriculture, and will likely increase resource use of corn cultivation within the United States.
Climate Change Intro
Over the past century, average global temperature has risen by about 1.5 degrees, and is predicted to rise another 2 to 11.5 degrees within the next century (U.S. Global Change Research Program, 2009). Increased atmospheric concentrations of compounds such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have increased average global temperatures.
Atmospheric composition has an important influence on how electromagnetic radiation enters and leaves the earth’s atmosphere
High concentrations of ozone (O3) in the earth’s stratosphere, also known as the ozone layer, absorb some waves on the UV spectrum. Some of the incoming radiation is directly reflected back into space. As radiation enters the earth’s atmosphere and interacts with the earth’s surface, this energy is converted to infrared radiation and are re-reradiated to space.
While some of this re-radiated infrared radiation is lost to space, a larger quantity is absorbed by “greenhouse gases” (CO2, CH4, N2O, H2O). These gases are critical to the earth maintaining a temperature viable for life. However, rapid anthropogenic production of CO2 emissions through activities such as fossil fuel combustion (coal, petroleum, natural gas) for utilities and transportation, CH4 emissions from livestock and fossil fuel extraction, N2O emissions from cropland and fertilizer use, have increased concentrations of greenhouse gases in the atmosphere at unprecedented rates (EPA, 2018).
Agriculture within the U.S. contributes to 8.6% of total U.S. greenhouse gas emissions (Climate Change Impacts on U.S.). However, agriculture contributes to 80% of U.S. anthropogenic nitrous oxide emissions and 31% of anthropogenic methane emissions (Climate Change Impacts on U.S.).
124
Figure 69. Carbon Dioxide Atmospheric Concentrations Source: EPA, 2018
Carbon dioxide concentrations have fluctuated naturally between 300 and 200 ppb, within time periods of about 50,000 years. However in the past 300 years, CO2 concentrations have risen from 280 ppm in the late 1700s to 408.35 ppm in 2018 (EPA, 2018) (co2.earth, 2018).
125
Figure 70. Methane Atmospheric Concentrations Source: EPA, 2018
Concentration of methane in the atmosphere has reached 1,800 ppb, due to agriculture and fossil fuel use (EPA, 2018).
126
Figure 71. Nitrous Oxide Atmospheric Concentrations Source: EPA, 2018
Concentrations of nitrous oxide in the atmosphere have risen substantially. While concentrations rarely exceeded 280 ppb over the past 800,000 years, since the 1920s levels have risen dramatically, reaching a high of 328 ppb in 2015. This increase is primarily due to higher applications of nitrogen fertilizer in agriculture (EPA, 2018).
Global Warming Potential
Greenhouse gases have varying impacts based on their concentrations in the atmosphere as well as their radiating capacities. The metric of Global Warming Potential (GWP) was developed to evaluate differences in how greenhouse gas compounds absorb and reemit solar radiation (EPA, 2017). This is based on compound’s radiative efficiency (their capacity to absorb and reemit energy), as well as their persistence in the atmosphere (EPA, 2017). The GWP metric is an index which relates the radiative efficiency and persistence of greenhouse gases to carbon dioxide (EPA, 2017). Specifically how much energy the emission of 1 ton of a gas will absorb over a given time period (usually 100 years), relative to the emission of 1 ton of carbon dioxide (EPA, 2017).
127 Carbon dioxide as the baseline of the GWP metric has a GWP of 1 regardless of the time period (EPA, 2017). CO2 is persistent in the atmosphere, with emissions resulting in net increase in CO2 concentrations which last thousands of years (EPA, 2017).
Methane (CH4) has a GWP of 28 to 36 over 100 years. Methane absorbs a far greater amount of energy than carbon dioxide, however has a shorter lifetime in the atmosphere. Therefore its GWP value ranges between 28 to 36 over a time period of 100 years (EPA, 2017)
Nitrous oxide (N2O) has a GWP 265-298 times that of CO2 over 100 years (EPA, 2017). N2O emitted remains in the atmosphere for over 100 years on average, and has a far higher capacity to absorb energy. This is reflected in its GWP which is almost 300x that of carbon dioxide, and is about 10x that of methane (EPA, 2017).
Carbon dioxide is by far the most common greenhouse gas present in the atmosphere. However high GWP values of methane, and nitrous oxide, mean that these compounds can impact global temperatures to the same or a similar extent at lesser concentrations. The radiating potential and lifetime of these gases is important to consider when assessing the impact of their concentrations on global temperatures.
Natural Sources and Sinks
Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have been amplified by anthropogenic activities. However CO2, CH4, and N2O are all naturally occurring compounds and are involved in a variety of biotic and abiotic cycles. Concentrations of these gases in the atmosphere is determined not only by direct emission, but also by rate of sequestration.
Living organisms are a source of CO2, emitting CO2 during respiration. Likewise dead organisms are also a source of CO2 as they are decomposed (The Environmental Literacy Council, 2015). During photosynthesis, terrestrial plants and aquatic plants, as well as phytoplankton absorb CO2 from the air and water. These photosynthetic organisms sequester carbon in biomass.The ocean also represents a significant carbon sink, as carbon dioxide diffuses into water and is utilized for primary production of phytoplankton, algae, and other photosynthetic organisms. Carbon is also sequestered in the ocean in the form of hard cell structures of different organisms which is formed from calcium carbonate. These structures represent important long term sequestration of carbon. After an organism with a calcium carbonate structure dies, these structures can be deposited in the form of sedimentary rock acting as long term carbon storage (The Environmental Literacy Council, 2015).
Methane is emitted from wetlands, gas hydrates, permafrost, termites, oceans, freshwater bodies, non-wetland soils, as well as sources such as wildfires (The Environmental Literacy Council, 2015). However, methane in many soils can be transformed through oxidation to
128 carbon dioxide by methanotrophs (methane-oxidizing bacteria) (The Environmental Literacy Council, 2015).
Nitrous oxide is emitted through oxidation of ammonia in the atmosphere, and from nitrogen metabolism in soils (American Chemistry Council, 2013). However the major sink of atmospheric N2O is in the stratosphere (American Chemistry Council, 2013).
Anthropogenic activities such as fossil fuel combustion contribute to directly to greenhouse gas concentrations in the atmosphere through direct emissions. Anthropogenic activities such as deforestation also contribute indirectly to greenhouse gas concentrations by activities which increase natural greenhouse gas sources and decrease greenhouse gas sinks.
The following results in a net increase in average global temperature. This increase in average global temperature, means regional shifts in timing and extremes of precipitation and temperatures. This has a dramatic influence on agriculture: impacting crop’s viable cultivation range, yield, and resource requirements. The high volume of corn production within the United States as the result of intensified farming systems and high resource input (NPK, pesticides, etc.) has become integral to the food system of the United States and the world. The potential impacts of climate change on precipitation and temperature may increase abiotic and biotic stress to corn and result in decreased yield. If this occurs required resource inputs to achieve yield comparable to current U.S. corn yield.
Impact of Climate Change on Cultivation
The impacts of climate change may vary throughout the U.S., even within the Corn Belt. Abiotic changes due to global warming, are projected to include to be rising winter temperatures, increased droughts, and altered patterns of precipitation (U.S. Global Change Research Program, 2009). These abiotic changes will impact biotic communities in various ways in different regions.
Temperature and Precipitation Changes
High temperature events will increase water needs of plants to compensate for higher water loss through evapotranspiration (U.S. Global Change Research Program, 2009). Temperature is critically connected to water availability. Climate change therefore acts directly on water resources through altered patterns of precipitation (U.S. Global Change Research Program, 2009).
Climate change is predicted to cause an increase in the frequency and intensity of heavy downpours in the U.S.(U.S. Global Change Research Program, 2009). Precipitation has become less frequent but more intense and sporadic throughout the U.S. (U.S. Global Change Research Program, 2009). Heavy precipitation events can cause field flooding during the growing season which starves soil of oxygen, increase susceptibility of roots to disease, as well
129 as increase surface water runoff. Increased surface water runoff has the potential to increase agrochemical (pesticides, fertilizers) and sediment pollution of surface water, as well as increase soil erosion on the field.
Extremes in weather events such as droughts, floods, unseasonable high or low temperatures have substantial potential to decrease crop yield. (U.S. Global Change Research Program, 2009)
The Midwest
Recently there have been average increases in Midwest average annual temperatures with the largest increase occurring in winter, extending the growing season due to earlier occurrence of the last spring frost (U.S. Global Change Research Program, 2009). Heavy downpours in the Midwest have increased, with summer and winter precipitation increasing (U.S. Global Change Research Program, 2009). Within the midwest there have also been severe heat waves (U.S. Global Change Research Program, 2009).
The Great Plains
The great plains, similar to the midwest experience strong seasonal climate variations, with periods of extended drought and high precipitation. Throughout the Great Plains, semi-arid conditions in the west transition to higher moisture in the eastern region (U.S. Global Change Research Program, 2009). The eastern region of the Great Plains, including eastern South Dakota and Nebraska, are areas in which corn is intensively cultivated and in which corn is regularly irrigated.
Precipitation over Western cropland in the United States is projected to decline (Schaible et al., 2017). This is particularly during warmer times of year (Schaible et al., 2017). Overall temperature increase forecasted for this region will decrease winter snowpack, making it less viable as a freshwater source (Schaible et al., 2017). Spring rain is predicted to increase in frequency and intensity, early in the season (Schaible et al., 2017). The following shifts change the seasonality of streamflow and reservoir recharge, causing heavier flow early in the season, and reduced reserves of water within reservoir storage (Schaible et al., 2017). Alterations of seasonality of stream flow and reservoir recharge put a severe strain on irrigation supplies (Schaible et al., 2017). The decrease in available water associated with the timing of peak irrigation demand, in the summer and fall, will put a strain on irrigation systems (Schaible et al., 2017). In addition temperature increase anticipated during summer months, will likely also increase demand for irrigation (Schaible et al., 2017).
These changes in precipitation and temperature have a substantial impact on water availability. As mentioned prior there is high reliance on groundwater withdrawal. Alterations in precipitation events and increased drought events decrease groundwater recharge and therefore water availability.
130
Figure 72. High Plains Aquifer Groundwater Withdrawals for Irrigation
These shifts in precipitation are predicted to be most severe in the West, however overall moderate warming likely will have a substantial impact on water use for irrigation (Schaible et al., 2017). Temperature increases water stress for crops, as well as increase presence of weeds and pests, both of which will decrease corn yield and increase resource demand (U.S. Global Change Research Program, 2009).
Impact of Climate Change on Weeds and Pests
Changes of temperature, precipitation, and CO2 have important influences on species fitness and therefore have an impact on biotic community composition. Species within a community compete for the same limited resources. Rapid changes to normal abiotic conditions of an ecosystem impact different species ability to compete for those limited resources. This impacts species fitness.
131 In an agricultural context, the impact of climate change on the fitness of weeds and pests in relation to corn is concerning. Changes in temperature allow for species to survive in regions in which they previously could not survive. A concern of increased temperature in Northern regions of the United States is the expansion of southern weed species (U.S. Global Change Research Program, 2009). Cold northern winter temperatures serve as an important barrier to the spread of Invasive weed species of the South which are not tolerant to winter drops in temperature. Kudzu, Pueraria montana, which is pervasive throughout the South cannot withstand (U.S. Global Change Research Program, 2009). As temperature rises in northern regions, particularly in winter, expansion of weed species not previously present is likely to have a significant impact on crop yield.
Invasion of southern weeds is further complicated in considering the prevalence of Glyphosate resistant weed varieties in the South (DuPont-Pioneer, 2016). Mississippi has the highest number of glyphosate resistant weed species in the United States having 9 species of weed which are resistant to glyphosate. This is followed by Tennessee and Arkansas who have 7 varieties of glyphosate resistant weed species. Increases in temperature in northern states may not only increase weeds, but increase weed populations which are resistant to Glyphosate.
Similarly, insects and pathogens are able to expand their range northward under warmer conditions (U.S. Global Change Research Program, 2009). In addition warmer winter temperatures allow more insects to survive winter, therefore decreasing the ability of winter to act as a population control (U.S. Global Change Research Program, 2009). Furthermore longer growing seasons (earlier last frost), allow some insect species to produce more generations in a single growing season leading to rapid increase in insect populations (U.S. Global Change Research Program, 2009). These insects may also carry disease which damage crops (U.S. Global Change Research Program, 2009).
Increased populations of insects, weeds, and disease will increase demand for pest control measures which given current cultivation practices and prices indices of fuel, labor, and chemicals will result in demand for more insecticides and herbicides.
Climate Change Summary
Agricultural practices represent important sources of greenhouse gases; and land use practices associated with agriculture represent losses of greenhouse gas sinks.Furthermore changes in seasonal temperature, precipitation, and carbon dioxide caused by climate change may decrease current levels of crop productivity, particularly corn. These changes include increased water demands, decreased water resource availability, decreased grain production, and increases in weeds, insect pest, and disease. To mitigate these impacts, increased inputs of water and chemical pesticides may be required, which will further contribute to the environmental impact of intensified corn production.
132
Demand
As demonstrated by prior sections, corn plays a significant role in U.S. agriculture. Corn has the second highest acreage dedication of any crop, uses the most herbicides and fertilizers, and accounts for a significant amount of irrigated acreage within the United States. Considering its substantial resource use and environmental impact the natural question to follow, is where does this grain go and what role does it play in the domestic and global food system.
Production Volume and Trade
In 2017 the U.S. produced 14.6 billion bushels of corn and reached a record national average yield of 176.7 (USDA NASS, 2018). Domestic use accounts for over 85% of U.S. corn trade, and exports respectively account for less than 15% of corn trade (USDA ERS, 2017).
Domestic Demand
Corn’s primary domestic uses are livestock feed, ethanol, and food, seed, and industrial uses (FSI) (USDA ERS, 2017).
Figure 73. U.S. Corn Domestic Use Source: USDA ERS, 2017
133 In 2017, feed and residual use of corn was 5,375 million bushels, accounting for 37.6% of domestic use. Dried distiller grains (DDGs) accounting for another 1,243 million bushels or 8.7% of corn used for livestock feed (National Corn Growers Association, 2018).
Ethanol accounted for 4,282 million bushels (30.1%) of domestic use in 2017 (National Corn Growers Association, 2018). Together feed, DDGs, and ethanol accounted for 76.4% of domestic corn use in 2017, and 13% was allocated for export (National Corn Growers Association, 2018). FSI (high-fructose corn syrup, sweeteners, starch, cereal/other, beverage/alcohol, seed), accounted for 10.6% of domestic corn use, with 3.2% (of total) going to high fructose corn syrup production. (National Corn Growers Association, 2018).
Feed and Residual Use
Corn is the primary feed grain in U.S. livestock feed, accounting for over 95% of total feed grain production and use. (USDA ERS, 2017)
Figure 74. U.S. Feed Grain Production Source: USDA ERS, 2017
134 Livestock feed is comprised mostly of feed grains. accounting for 70% of most feed (Livestock and Poultry Feed Use and Availability). The high starch content of feed grains is the main source of caloric content in feed (Schnepf, 2011).
Corn is high in fats and carbohydrates, and low in fiber content, making it an ideal feed component for livestock weight gain (Schnepf, 2011). However, corn is insufficient in several essential amino acids, calcium, and most vitamins. Corn-based feed must be supplemented by other crops higher in these nutrient components, however, the primary component of most livestock feed is corn (Schnepf, 2011).
Demand for meat, dairy, and eggs, has a significant impact on livestock feed demand (USDA ERS, 2017). Proportions of corn in feed vary based on corn’s supply and price (USDA ERS, 2017). Inversely, the price of other supplemental feed grain ingredients (Sorghum, Barley, Oats) also influence the amount of corn in feed grain (USDA ERS, 2017). Feed cost represent 60-70% of livestock production cost, rises in feed cost have a significant impact on livestock production (Lawrence et al., 2008).
Livestock have different feed conversion ratios (FCR), meaning different efficiencies in converting feed mass to body mass (Schnepf, 2011). FCR is the weight of feed needed to produce one pound of meat (Schnepf, 2011).
Table 25. Feed Conversion Ratio
Source: Schnepf, 2011
Broilers (Chicken) and turkeys have the lowest FCR, meaning the highest feed use efficiency; while cattle had the highest FRC indicating low feed use efficiency (Schnepf, 2011). Lower feed use efficiency, is indicative of greater resource use for relatively comparable nutritional gain. Of
135 U.S. livestock, beef and poultry had the highest use of corn feed. Corn produced within the U.S., is resource intensive, and therefore feed efficiency is important in considering how the resources allocated to corn as opposed to other crops are translated to human nutrition.
Figure 75. Corn Use by Livestock Species Source: Shepon et al., 2016
A study by Shepon et. al in 2016, found in the U.S. <10% of feed calories or protein is transformed to available calories for consumption in the form of meat, milk, or egg calories (Shepon et al., 2016). This study demonstrates the impact of trophic cascade inefficiencies, consumer preference (bias against organ meat etc.), and inefficiencies in production (which plant-based food production is also subject to) on caloric and protein efficiency. (Shepon et al., 2016). Beef was found to have the lowest caloric efficiency (3%), followed by pork (9%), and poultry (13%) (Energy and protein feed-to-feed-conversion in U.S.). While eggs (17%) and dairy (17%) had the highest caloric feed efficiency (Shepon et al., 2016).
136
Figure 76. Livestock Feed Efficiency Source: Shepon et al., 2016
The is same study also calculated protein efficiencies of various corn-fed livestock, and found similar results. Beef had the lowest protein conversion efficiency at 3%, followed by pork with a protein conversion efficiency of 9%, and dairy at 14% (Shepon et al., 2016). The highest protein conversion efficiency was observed in eggs at (31%), and poultry at (21%) (Shepon et al., 2016).
Admittedly the prior information may not account for concentrated micronutrients and fat which may be received in “protective” foods such as livestock, egg, and dairy. However currently the overwhelming availability of these foods, which has been significantly contributed to through availability of corn for feed grain as well as other factors, has also led to negative public health outcomes such as high prevalence of cardiovascular disease. It is not to say that increased availability of corn has been the only factor in increasing the availability of animal products, however it has been a significant factor.
137
In addition to direct feed grain input, corn contributes to livestock feed through the by-products of ethanol production. Dry-milling and wet-milling of ethanol produce distiller dried grains with solubles (DDGS) (USDA ERS, 2017). DDGS is a valuable coproduct of ethanol production, which can be used as a feed ingredient for livestock (USDA ERS, 2017). For every bushel of corn (56 lbs) use in dry-mill ethanol production, 17.4 lbs of DDGS are produced (USDA ERS, 2017). The primary user of DDGS for feed use is for cattle (dairy and beef), and increasingly hog and poultry(USDA ERS, 2017).
Figure 77. Corn Feed Grain and DDG/CGF Source: National Corn Growers Association, 2018
Ethanol
Energy Policy Act of 2005
Between 2004 and 2014, U.S. ethanol production increased from 3.4 billion gallons (2004) to 14.3 billion gallons (2014), increasing 320% over 10 years (Life Cycle Analysis, Corn Based Ethanol). Two key pieces of legislation which encouraged ethanol production were the Energy Policy Act of 2005 and the Energy and Security Act of 2007. The Energy Policy Act of 2005 established the Renewable Fuel Standard (RFS), which set a mandatory minimum of biofuel use of 4 billion gallons starting in 2006 as well as scheduled rises to 7.5 billion gallons by 2012 (Life Cycle Analysis, Corn-Based Ethanol).
Energy Independence and Security Act of 2007 138
Two years later the Energy Independence and Security Act of 2007 replaced the RFS with the Revise Renewable Fuel Standard (RFS2). The RFS2 included a new schedule which required biofuel use to increase to 9 billion gallons in 2008, and increase to 15 billion gallons in 2015 and held constant through 2022 (IFC, 2017l). 90% of ethanol used in the United States is derived from corn (Condon et al., 2013).
Figure 78. Corn Prices and Ethanol Use Source: Condon et al., 2013
Ethanol and Corn Commodity Price
Rapid increased ethanol demand encouraged by the Energy Policy Act of 2005, resulted in an increase in food crop prices for the first time after 30 years of declining food crop prices (Condon et al., 2013). The FAO food index reached a historical high in the summer of 2008, which was then surpassed by a greater high in late 2010 (Condon et al., 2013). The U.S. market for corn, as the main exporter and user of corn, has some of the strongest influence on corn commodity prices.Increased commodity prices of corn brought by ethanol expansion has increased corn acreage (ICF, 2017). 16% of corn and soybean farms in 2008 brought in acreage into production between 2006-2008, this was during the dramatic rise in commodity price of corn (Wallander et al., 2011).
Ethanol and Increased Corn Acreage
139 A study conducted by the USDA in 2011, which analyzed increased corn production in response to increased biofuel demand from 2000-2009, found that the increase in corn production in response to biofuel policy was dissimilar to previous increases in corn production. Unlike previous recent expansions in corn production, which had responded to increased demand with increased yield, corn production during this period increased largely due to increased harvested acreage, in contrast to increased yield per acre. This acreage was taken out of unharvested land, hay forage, pasture, and CRP programs (Wallander et al., 2011). In other instances, this land was diverted from other, less profitable crops. Environmental impacts of increasing acreage dedicated to intensive corn cultivation, are those which have been characterized in Section 2. Biofuel diverts resources away from food production, to the energy sector. This is a highly contested issue both domestically, and globally.
Global Demand
Production, Consumption, and Trade by Country
The United States is world’s largest producer of corn, producing 14,604 million bushels in 2017; followed by China who produced 8,499 million bushels, Brazil who produced 3,740 bushels, and the European Union (EU-27) who produced 2,366 million bushels of corn (National Corn Growers Association, 2018)
140
Figure 79. World Corn Production Source: National Corn Growers Association, 2018
The world’s top consumers of corn are the United States (12,545 million bushels), China (9,448 million bushels), the EU-27 (2,945 million bushels), and Brazil (2,421 million bushels).
Despite only exporting 10-20% of its corn production, the United States is the world’s largest exporter accounting for 32% of the corn export market, followed by Brazil at 23.5%, Argentina at 17.8%, and the Ukraine at 13.5%.
141
Figure 80. World Corn Exports and Imports Source: Corn Growers Association, 2018
Future of Global Demand
The world’s total consumption of corn from October 1st, 2017 to September 20th, 2018 was 41,994 million bushels. World corn trade is projected to continue to increase (Ag Outlook 2017). Globally, the a significant portion of corn goes to livestock feed. Increased livestock demand is a key factor in global corn demand (USDA ERS, 2017). Global pork and poultry are projected to increase significantly, and will contribute to increased demand for corn production (USDA ERS, 2017)
142
Figure 81. Global Pork and Poultry Production Projections Source: Hansen, 2017
Poultry and pork in particular, as grain dependent livestock. Overall strong income and population growth in many developing countries and urbanization will lead to increased import demand for corn for feed (Hansen, 2017).
Economic and population growth in developing countries is likely to be a strong factor in increasing corn demand for livestock feed (USDA ERS, 2017). Increasingly, these countries are adopting a more western diet which means higher consumption of animal products and processed food high in high-fructose corn syrup.
In addition, increasingly countries are committing to “green energy” are making commitments to supplement fossil fuels with ethanol. This increased ethanol demand, primarily in the transportation sector, will further contribute to increased demand for corn production. The next section will attempt to make a calculation of the acreage, water, fertilizers, and pesticides which would be needed to supply this future demand of our increasing global population.
Calculation
143 The world’s population is estimated to reach 9.8 billion by 2050 (United Nations Department of Economic and Social Affairs, 2017). As the world’s population nears 10 billion, and as nations continue to develop their economies and their populations gain wealth and technology, it is likely the demand for livestock and biofuel will increase substantially. Given these conditions, corn production will have to increase substantially on an international scale. If intensified production systems of corn production, resembling those of the United States, are established or escalated in developed and developing nations, this will require substantial increased inputs of acreage, water, fertilizer, and pesticide.
Together the Organisation for Economic Co-operation and Development (OECD) and the Food and Agriculture Organization (FAO) of the United Nations created a baseline projection to forecast patterns of global production and consumption of a number of agricultural commodities (OECD-FAO, 2017). This forecast was based on projected increases in consumption, economic development, and global population, as well as on current macroeconomic conditions, weather, agriculture and trade policy, weather conditions, long term productivity trends, and international market development (OECD-FAO, 2017). Using these projections as a foundation, projections of corn production, acreage, and yield will be expanded to the year 2050. From this projection, global water, fertilizer, and pesticide use will be extrapolated, assuming intensive agriculture schemes are adopted.
OECD-FAO Projections 2015-2025
The following tables indicate the FAO and OECD projections for the corn production, acreage harvested, and yield for 2015-2025 (OECD-FAO Stat Database, 2018).
Table 26. Projected Corn Production (bushels) 2015 2025 Increase 2015-2025
World 39,220,038,582.7 45,119,855,511.8 5,899,816,929.1
Developed 18,830,553,149.6 21,319,078,740.2 2,488,525,590.6 Countries
Developing 20,507,595,669.3 23,800,616,929.1 3,293,021,259.8 Countries
Table 27. Projected Corn Harvested Acreage (acres) 2015 2025 Increase 2015-2025
World 449,216,625 462,048,625 12,832,000
Developed 140,671,225 145,074,225 4,403,000
144 Countries
Developing 308,545,400 316,972,925 8,427,525 Countries
Table 28. Corn Yield Projections (bu/ac) 2015 2025 Increase 2015-2025
World 87.2 97.64 10.44
Developed 133.07 146.92 13.85 Countries
Developing 66.46 75.12 8.66 Countries
Using the FAO-OECD projections and assuming similar increases in global population growth, global consumption patterns, and economic development, projections for the year 2050 were extrapolated.
2050 Projections of Corn Production, Acreage, and Yield
Table 29. Corn Production, Acreage, and Yield Production for 2050 Year 2050 World Developed Developing Countries Countries
Production (bu) 59,869,397,834.6 27,540,392,716.7 32,033,170,078.6 Acreage (acres) 494,128,625 156,081,725 338,041,737.5 Yield (bu/ac) 123.74 181.545 96.77
Assuming that yield gains in both developed and developing nations, will be achieved through similar methods of intensive corn cultivation practiced in the United States; global use of water, pesticides, and fertilizers will increase substantially.
The following calculations use this projection of corn production, acreage, and yield for the year 2050, and predict potential scenarios of water use, fertilizer use, and pesticide use in developing and developed nations.
While there may be differences in climate, precipitation, soil characteristics, and pests present in these regions impacting yield and required resource inputs, throughout the Corn Belt there are
145 also varied abiotic and biotic factors which influence yield’s relation to various inputs (water, pesticides, fertilizer).
Therefore using intensive agriculture in the U.S. to model future scenarios maybe an effective means of demonstrating the potential increased resources necessary to support a world population of 9.8 billion.
2050 Corn Water Requirements
Calculating water requirement or future corn yield is difficult given the influence on climate and soil on crop water requirements. In addition, the impact of climate change on water availability and precipitation by 2050 are likely to influence future corn water requirements. Infrastructure, availability of water sources, irrigation technology, and water competition all also play important roles. Therefore the following scenario is highly theoretical.
However, given that over a growing season a corn yield of 100 bu/ac typically requires 14 acre inches and a corn yield of 200 bu/ac requires 20 acre inches, in a scenario in which projected yields would be achieved, the future water requirements of corn in developed and developing nations for 2050 can be predicted as follows (Licht et al., 2017).
Table 30. 2050 Water Requirements 2050 Water Required to Water Required to Total Projected Yield Achieve Yield over Achieve Yield over Water (bu/ac) Growing Season Growing Season Requirements (acre inches) (gallons/acre) (million gallons)
Developed 96.77 13.58 368,755.2 57,555,947.7 Nations
Developing 181.545 18.2 494,208 167,062,931 Nations
World - - - 224,618,878.7
While these water requirements may be partially satisfied by natural precipitation, it is likely they will be met with additional applications of surface water and groundwater will be employed via irrigation. Increasing global consumptive water use.
2050 Corn Fertilizer Use
In addition to increased water requirements with increased yield, high yield varieties of corn will require greater fertilizer applications. Although natural nutrient content of soil varies, high yield varieties of corn use high levels of nitrogen and rapidly depletes soil of nitrogen. These varieties
146 also require higher levels of phosphorus and potash. Fertilizer requirements can be extrapolated from current U.S. fertilizer use in relation to yield.
In 2016 U.S. corn yield was approximately 175 bu/ac. The average rate of nitrogen fertilizer applied to corn acreage was at 145 lbs/acre and was applied to 97% of planted acres (NASS USDA, 2017). Phosphate fertilizer was applied to at a rate of 61 lbs/acre and applied to 79% of acreage (NASS USDA, 2017). Potash was applied at a rate of 80 lbs/acre and applied to 65% of acreage (NASS USDA, 2017).
Assuming that by 2050 based on patterns of fertilizer use in the U.S.:
-Nitrogen fertilizer is applied to 100% of corn acreage in developed nations at 150 lbs/acre and to 80% of acreage in developing nations at 65 lbs/ac
-Phosphorus fertilizer is applied to 85% of corn acreage in developed nations at 70 lbs/acre and to 60% of acreage in developing nations at 30 lbs/ac
-Potash fertilizer is applied to 85% of corn acreage in developed nations at 80 lbs/acre and to 60% of acreage in developing nations at 35 lbs/ac
Total fertilizer use for corn by the year 2050 would be as follows:
Table 31. 2050 Fertilizer Use Nitrogen Fertilizer Phosphorus Fertilizer Potash Fertilizer (million lbs) (million lbs) (million lbs)
Developing Nations 17,578.17 6,084.75 7,098.88
Developed Nations 23,412.26 9,286.86 10,613.56
World Total 40,990.43 15,371.61 17,712.44
2050 Pesticides Use
Unlike fertilizer and water inputs which increase availability of factors limiting growth and development, pesticides contribute to yield by mitigating crop loss. Pesticides use is difficult to project given regional differences in pest species and populations. Furthermore genetic engineering of corn varieties further complicates predicting the use of pesticides.
Herbicide tolerant varieties of corn have increased the use of herbicides such as glyphosate, while the adoption of Bt corn has decreased insecticide use. The emergence of glyphosate resistant weeds further complicates predictions of pesticide use, as well as the likely development of new genetically modified corn varieties. Divided political opinion of the safety
147 and transparency of genetically engineered crop use is another factor which may influence pesticides use.
However assuming a scenario where both developed countries and developing countries undergo a similar adoption of stacked HT and Bt corn varieties, and similar atrazine and glyphosate use, global atrazine and glyphosate use by 2050 would be as follows:
Table 32. 2050 Pesticides Use Prediction Atrazine Applied* Glyphosate Glyphosate (lbs/year) Potassium Salt Isopropylamine (lbs/year) (lbs/year)
World Corn Acreage 320,788,303.35 215,731,616.39 159,860,492.76
*Within the EU, atrazine use has been banned due to its high capacity to contaminate surface water and groundwater. This calculation does not account for this when considering acreage in the EU
While this projection high adoption of glyphosate and atrazine use may seem high, it is a scenario which attempts to account for the likely possibility of the development and adoption of more potent herbicides by 2050. This predictions accounts for this by using a high-use scenario of glyphosate and atrazine, use which is equivalent to a mid to low-use scenario of more potent herbicides which may replace them in the future.
Calculation Summary
If the world is to produce 60 billion bushels of corn, this will require over 500 million acres of cropland, with about 150 million acres in developed countries, and 330 million acres in developing countries. Total water needed to achieve 60 billion bushels of production is estimated to be over 224 million million gallons or 224,000,000,000,000 per growing season. An olympic swimming pool contains approximately 660,430 gallons of water. This means the amount of water required would equate to the amount of water which could fill nearly 34 million olympic swimming pools. Fertilizer use would equate to over 40,000 million lbs of nitrogen fertilizer, 15,000 million lbs of phosphorus fertilizer, and 17,000 million lbs of potash fertilizer. Pesticide use would total nearly 700 million lbs of herbicide per year.
These projections are not meant to be exact, but to demonstrate that water, fertilizer, and pesticide use will grow substantially, and that this will likely result in environmental consequences paralleling those of intensified corn production of the United States. Intensification of corn production in developing nations will account for a significant amount of the growth of corn production, and an even greater significant amount of acreage in corn production. Competition for resource use, such as water and land may have important implications not just for the environment, but may also have social consequences such as 148 displacement of people or disruption of political stability. These forecasts are further troubling when considering the impact of climate change on corn production which will pose several challenges to conventional regional cultivation practices and may push further farmers to adopt intensive practices to mitigate yield loss. Overall, this calculation serves to say the scenario of developed nations escalating their intensive production schemes, and developing nations adopting intensive production schemes will require resource inputs which will likely result in serious environmental degradation.
Conclusion
Agriculture is currently one of the most significant contributors to global environmental degradation (Foley et al., 2011). Agriculture has converted over 70% of the world’s grasslands, 50% of the world’s savanna, 45% of the world’s temperate deciduous forest, and 27% of the world’s tropical forests (Foley et al., 2011). Global cropland currently covers over 3.8 billion acres and is expanding rapidly, often into sensitive ecosystems (Foley et al., 2011). Cultivation practices fundamentally alter the ecology, nutrient cycles, and hydrology of ecosystems. Increases in resource requirements of global corn cultivation will have expansive impacts directly on the environment and on various stakeholders.
Impacts of Increased Acreage
While some increased acreage may be reallocated from other cropland dedicated to other crops or pasture, it is likely that much will be converted from sensitive ecosystems such as grasslands, forests, or wetlands. This will decrease habitat complexity, biodiversity, and important ecosystem services such as carbon sequestration, flood control, and potential storm barriers.
Furthermore expanded acreage will likely fundamentally change the hydrology and soil of whatever ecosystem cropland replaces. This may increase erosion or runoff, and may lead to conflicts with those who may rely on water downstream from agricultural activity. Similarly, expanded acreage may be associated with expanded use of fertilizers or pesticides which may degrade water quality leading to consequences for those relying on water in the region, or those relying on aquatic ecosystems which may be negatively impacted by proximate agricultural activity.
Impacts of Increased Fertilizer
The increased use of fertilizer will contribute to eutrophication, soil acidification, heavy metal contamination of soil and water, as well as NO2 production. There is particular concern for nitrate contamination of groundwater. Over 50% of the world’s population relies on groundwater for drinking water (Conor, 2015). The expanded use of nitrogen fertilizer substantially increases the risk of nitrate contamination of wells, especially in well drained areas with high water tables. Aquatic ecosystems may be at risk for nitrate and phosphorus contamination, and people relying on those ecosystems for fish may face severe hardship as nitrate and phosphorus
149 contamination lead to mass fish die offs. Lastly, increased use of nitrogen fertilizer may contribute substantially to cropland NO2 emissions, further contributing to the issue of climate change.
Impacts of Increased Pesticide
Increased herbicide use poses a significant concern to human and ecosystem health. Groundwater contamination due to high atrazine use is currently a public health hazard throughout the Corn Belt. Expanded use of atrazine, or use of other pesticides with high water affinity increases the risk of global groundwater contamination. The increased use of pesticides will also increase surface water contamination and soil contamination; and threaten the health of a wide range of organisms.
Impacts of Increased Water Use
World water resources will be threatened by the increased water requirements of high yield corn cultivation. This increased demand and increased water use will fundamentally alter the water balance of many natural ecosystems. Furthermore, this high demand will be compounded by the impacts of climate change on precipitation, and the impacts of land use changes on water movement. This will occur while other sectors will experience increased water demands due to population growth, and will add addition pressure to global freshwater resources. The following will all contribute to competition for water resources and intensify and expand areas facing water stress.
Resource Allocation and Efficiency
The allocation of land, water, fertilizer, and pesticide does not occur in isolation. As these resources are concentrated in their application to corn production, increasingly there will be competition for these resources, as other industry and agriculture are likely to develop as the global population increases and economic development occurs.
What has been striking about the environmental impacts of corn production within the U.S., is the clear consequences of the throughput systems industrial agriculture has created. Irrigation efficiency by limiting evaporation however irrigation still represents the greatest consumptive water use. Nitrogen fertilizer has been concentrated by the the use of synthetic nitrogen fertilizers such as anhydrous ammonia however corn remains largely inefficient in its nitrogen use efficiency and large amounts of nitrogen fertilizer enter the environment following application represents significant waste, in addition to the environmental consequences which follow.
A Hungry Production System
150 While staggering yields have been achieved in the U.S. with this system, soil nutrient content is quickly exhausted after a seasonal planting of high-yield corn varieties, much herbicide is lost to soil or waterways, water from irrigation is lost, and what has resulted is a “hungry production system”. One which represents the constant need for inputs to achieve outputs. Furthermore this “hungry production system” is often degraded in its ability to achieve outputs by its own inputs. As synthetic fertilizer is applied soil health declines as soil becomes increasingly acidic, irrigation can lead to soil becoming increasingly compacted and can reduce crop’s ability to absorb water. As glyphosate is applied consistently, populations of resistant weed varieties increase and greater applications are required for comparable outcomes. These technological approaches which are aimed often at one aspect of the production system such as pest or nutrient management, often fail to account for the system as a whole or consider the problems which their use may create. This leads to new problems, which are often addressed by similar throughput approaches. This idea of a “throughput” production system has not come from a vacuum. It has been built up by U.S. agricultural policy and through economic conditions which have favored its use.
Agroecology
Long term adoption of agro-ecological farming methods is crucial for sustainability. Agroecology is defined as “the ecology of the food system” (Synergies, 2015). This outlook involves consideration of environmental health, economic viability, and social justice (Synergies, 2015). Agroecology fundamentally shifts resource management on farms from a linear model to a cyclical model, intending to minimize environmental degradation by mimicking natural systems. This is achieved by altering a number of currently used crop management strategies. For example shifting from heavy applications of NPK fertilizer, to a integrated management of nutrient inputs and increased dependence of microbial nitrogen fixation and mycorrhizal systems (Synergies, 2015). Other strategies include integrated pest management as well as elimination of monocropping systems (Synergies, 2015).
State Intervention
In regard to making cultivation practices more sustainable, a number of changes need to occur. Nations with well developed infrastructure and economy have a substantial obligation to facilitate these changes. Policy and legislation have a large role in shaping the state of agriculture. Increasing standards for evaluating human health risk and ecological impacts must be enacted. While the E.U. leads the world in some of the most stringent evaluations of chemical safety, the U.S., and many other nations must adopt a similar approach.
There must be a separation of state and corporate interest. In the case of atrazine, Syngenta, the primary manufacturer of atrazine, was involved in conducting most research which the EPA used to evaluate the safety of atrazine. This pressure of accountability within the agribusiness sector is key in creating pressure for the development of new technology which is green and
151 facilitates a cyclical production cycle. Preventing vertical integration within the agribusiness sector will be an important in controlling corporate influence in policy.
Investing in the development of sustainable agriculture in developing nations is key. Particularly in regions which are susceptible to climate change. Discussed briefly, the volatility of global agricultural commodity markets can have substantial consequences. The increased distance of markets leaves room for risk of natural and political interference, and can leave food systems vulnerable. Developing food systems which are both sustainable environmentally as well as economically and politically will mean increasing investment in food sovereignty for nations and communities.
Change within the U.S.
Within the U.S. movement towards a farming system which uses principles of agro-ecology will be key. Investment in technology and research which prioritizes a life cycle approach to farming practices will be key, and programming which increases farmers access and education to these technologies and practices will as well. This could be achieved by a priority shift from the USDA. USDA policies have favored consolidated agriculture, farms decreasing in number and increasing in size. Programs such as CRP have been vastly successful in achieving gains in conservation and in mitigating the environmental impacts of intensive agriculture. If further efforts were taken on to create incentives which move away from agribusiness towards agro-ecology, tremendous progress could be achieved.
This shift is not likely to come from within but will need to come from public outcry. Empowerment of farmers and farm workers, education and mobilization of the public, and strong pressure by NGOs (non government organization) will all be key in vocalizing demand for sustainability. The agricultural sector is one which we all participate in, yet is one which we are increasingly removed from. Reconnecting to what goes into our food, who grows our food and the land it comes from will be essential for our future.
Acknowledgements
I would like to thank my senior thesis advisor and mentor Dr. Dwight Peavy for the time you dedicated in guiding me through this process. Though I am sure it took patience at times, this has been one of the most rewarding aspects of my Brandeis career.
I would also like to thank Dr. Colleen Hitchcock for serving on my committee as well as inspiring my interest in citizen science and phenology. I have greatly enjoyed taking classes with you, and have greatly appreciated your advice and support during my time here at Brandeis. I would like to thank Dr. Brian Donahue for serving on my committee and for further inspiring my interest in agriculture and food systems through your class Food and Farming, and through your work on New England food systems.
152
I would also like to acknowledge my role model and friend Luke Dorney who has passed down a great deal of wisdom to me. Finally, I would like to thank my family and friends for their support and my reviewers for braving the length of this thesis.
Sources
Corn in U.S. Agriculture
CONKIN, P. (2008). American Agriculture before 1930. In A Revolution Down on the Farm: The Transformation of American Agriculture since 1929 (pp. 1-30). University Press of Kentucky. Retrieved from http://www.jstor.org.ezproxy.library.tufts.edu/stable/j.ctt2jckzc.6
NASS USDA (February 2017). Farms and Lands in Farms 2016 Summary. Retrieved from http://usda.mannlib.cornell.edu/usda/nass/FarmLandIn//2010s/2017/FarmLandIn-02-17-2017.pdf
Economic Research Service, United States Department of Agriculture (Thursday, April 19, 2018). Corn & Other Feedgrains. Retrieved from https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/
MacDonald, J.M., Hoppe, R.A., Newton, D. (March 2018). Three Decades of Consolidation in U.S. Agriculture. Economic Information Bulletin Number 189, Economic Research Service, United States Department of Agriculture
Roser, M, Ritchie, H (2018). Yields and Land Use in Agriculture. Retrieved from https://ourworldindata.org/yields-and-land-use-in-agriculture
Hilliard, S. (1972). The Dynamics of Power: Recent Trends in Mechanization on the American Farm. Technology and Culture,13(1), 1-24. doi:10.2307/3102653
National Park Service, U.S. Department of Interior (April 10, 2015). About the Homestead Act. Retrieved from https://www.nps.gov/home/learn/historyculture/abouthomesteadactlaw.htm
Wallace, H. (1956). Corn and the Midwestern Farmer. Proceedings of the American Philosophical Society, 100(5), 455-466. Retrieved from http://www.jstor.org.resources.library.brandeis.edu/stable/3143679
National Agricultural Statistics Service, USDA (January 29, 2018). Quick Stats. Retrieved from https://quickstats.nass.usda.gov/
Barnett, B. (2003). The U.S. Farm Financial Crisis of the 1980s. In Adams J. (Ed.), Fighting for the Farm: Rural America Transformed(pp. 160-172). University of Pennsylvania Press. Retrieved from http://www.jstor.org.resources.library.brandeis.edu/stable/j.ctt3fhfpk.11
153 Nielsen, R.L. (May 2017). Historic Corn Grain Yields for the U.S. Retrieved from https://www.agry.purdue.edu/ext/corn/news/timeless/yieldtrends.html
Zhong, L., Gong, P., & Biging, G. S. (2014). Efficient corn and soybean mapping with temporal extendability: A multi-year experiment using Landsat imagery. Remote Sensing of Environment, 140, 1-13.
Auch, R. F., Laingen, C., Drummond, M. A., Sayler, K. L., Reker, R. R., Bouchard, M. A., & Danielson, J. J. (2013). Land Use and Land Cover Change in Three Corn Belt Ecoregions: Similarities and - - Differences. FOCUS on Geography, 56(4), 135-143.
Laingen, C. (2012). Delineating the 2007 Corn Belt region. In Applied Geography Conference (Vol. 35, pp. 173-181).
Ecoregions and Hydrology of the Corn Belt
Soil and Land Resource Division, University of Idaho (n.d.) Mollisols retrieved from https://www.cals.uidaho.edu/soilorders/mollisols.htm
Soil and Land Resource Division, University of Idaho (n.d.) Alfisols retrieved from https://www.cals.uidaho.edu/soilorders/alfisols.htm
Olcott, P. G. (1992). Ground Water Atlas of the United States—Segment 9: Iowa. Michigan, Minnesota, Wisconsin: US Geological Survey Hydrologic Investigations Atlas HA–730–J.
Olcott, P. G. (1992). Ground Water Atlas of the United States—Segment 9: Iowa. Michigan, Minnesota, Wisconsin: US Geological Survey Hydrologic Investigations Atlas HA–730–J.
Auch, R. F., Laingen, C., Drummond, M. A., Sayler, K. L., Reker, R. R., Bouchard, M. A., & Danielson, J. J. (2013). Land Use and Land Cover Change in Three Corn Belt Ecoregions: Similarities and - - Differences. FOCUS on Geography, 56(4), 135-143.
Wilken, E, Jimenez Nava, F., Griffin, G., (April 2011). North American Terrestrial Ecoregions-Level III. Commision for Environmental Cooperation. Retrieved from http://www3.cec.org/islandora/en/item/10415-north-american-terrestrial-ecoregionslevel-iii
Auch, R. F. (2014). Western Corn Belt Plains Ecoregion Summary. US Geological Survey (USGS) Land Cover Trends Project. USGS Center for Earth Resources Observation and Science, Sioux Falls, South Dakota. Available online at: http://landcovertrends. usgs. gov/gp/eco47Report. html# _ftn1.
Aldridge, C., Baker, B. (2017). Watersheds: Role, Importance,& Stewardship. Mississippi State University. Retrieved From http://extension.msstate.edu/publications/publications/watersheds-role-importance-stewardship
Panagopoulos, Y., Gassman, P. W., Jha, M. K., Kling, C. L., Campbell, T., Srinivasan, R., ... & Arnold, J. G. (2015). A refined regional modeling approach for the Corn Belt–Experiences and recommendations for large-scale integrated modeling. Journal of Hydrology, 524, 348-366.
154
Bureau of Reclamation, U.S. Department of Interior. (2011) Basin Report: Missouri River. Retrieved from https://www.usbr.gov/climate/secure/docs/2016secure/factsheet/MissouriRiverBasinFactSheet.pdf
Reilly, T. E., Dennehy, K. F., Alley, W. M., & Cunningham, W. L. (2008). Ground-water availability in the United States (No. 1323). Geological Survey (US).
USGS. (n.d.). HA 730-J Mississippian aquifer text. Retrieved from https://pubs.usgs.gov/ha/ha730/ch_j/J-text5.html
USGS. (n.d.). HA 730-J Silurian-Devonian aquifer text. Retrieved from https://pubs.usgs.gov/ha/ha730/ch_j/J-text6.html
USGS, & U.S. Geological Survey Office of Groundwater. (n.d.). Sand and gravel aquifers. Retrieved from https://water.usgs.gov/ogw/aquiferbasics/sandgravel.html
Land Use
Minnesota Board of Water and Soil Resources. (n.d.). Wetland Regulation in Minnesota Retrieved from http://www.bwsr.state.mn.us/wetlands/publications/wetlandregulation2.html
RAMSAR. (n.d.). Wetland and Ecosystem Services- an introduction Retrieved from https://www.ramsar.org/sites/default/files/documents/library/services_00_e.pdf
Dahl, T. E. (1990). Wetlands losses in the United States, 1780's to 1980's. Report to the Congress (No. PB-91-169284/XAB). National Wetlands Inventory, St. Petersburg, FL (USA).
Dahl, T. E., & Allord, G. J. (1996). History of wetlands in the conterminous United States. National summary on wetland resources. USGS, Springfield, 19-26.
Pierce, S. C., Kröger, R., & Pezeshki, R. (2012). Managing artificially drained low-gradient agricultural headwaters for enhanced ecosystem functions. Biology, 1(3), 794-856.
Blann, K. L., Anderson, J. L., Sands, G. R., & Vondracek, B. (2009). Effects of agricultural drainage on aquatic ecosystems: a review. Critical reviews in environmental science and technology, 39(11), 909-1001.
Sugg, Z. (2007). Assessing US farm drainage: Can GIS lead to better estimates of subsurface drainage extent. World Resources Institute, Washington, DC, 20002.
Samson, F., & Knopf, F. (1994). Prairie conservation in north america. BioScience, 44(6), 418-421.
University of Arizona (n.d.). North American Short Grass Prairie. Retrieved from https://wrangle.org/ecotype/north-american-short-grass-prairie
155 National Park Service U.S. Department of the Interior (n.d.). Last Stand of Tallgrass Prairie Retrieved from https://www.nps.gov/tapr/index.htm
U.S. Fish & Wildlife Service (n.d.) Tallgrass Prairie Retrieved from https://www.fws.gov/refuge/Neal_Smith/wildlife_and_habitat/tallgrass_prairie.html
Farm Service Agency United States Department of Agriculture (n.d). Conservation Reserve Program Retrieved from https://www.fsa.usda.gov/programs-and-services/conservation-programs/conservation-reserve-program/
Farm Service Agency United States Department of Agriculture (2013). Conservation Reserve Program Annual Summary and Enrollment Statistics FY 2012 Retrieved from https://www.fsa.usda.gov/Internet/FSA_File/summary12.pdf
Morefield, P. E., LeDuc, S. D., Clark, C. M., & Iovanna, R. (2016). Grasslands, wetlands, and agriculture: the fate of land expiring from the Conservation Reserve Program in the Midwestern United States. Environmental Research Letters, 11(9), 094005.
Farm Service Agency United States Department of Agriculture (n.d.) Conservation Reserve Program Statistics Retrieved from https://www.fsa.usda.gov/programs-and-services/conservation-programs/reports-and-statistics/conservati on-reserve-program-statistics/index
Wright, C. K., & Wimberly, M. C. (2013). Recent land use change in the Western Corn Belt threatens grasslands and wetlands. Proceedings of the National Academy of Sciences, 110(10), 4134-4139.
NRLE. (n.d.). Wind Maps Retrieved from https://www.nrel.gov/gis/wind.html
Water Use
Schaible, G. D., & Aillery, M. P. (2017). Challenges for US Irrigated Agriculture in the Face of Emerging Demands and Climate Change. In Competition for Water Resources (pp. 44-79).
Economic Research Service United States Department of Agriculture (2017). Irrigated Agriculture in the United States. Retrieved from https://www.ers.usda.gov/data-products/irrigated-agriculture-in-the-united-states/
Grassini, P., Thorburn, J., Burr, C., & Cassman, K. G. (2011). High-yield irrigated maize in the Western US Corn Belt: I. On-farm yield, yield potential, and impact of agronomic practices. Field Crops Research, 120(1), 142-150.
Kranz W.L., Irmak S., van Donk S.J., et al. (2008). Irrigation Management for Corn. University of Nebraska- Lincoln. Retrieved from http://extensionpublications.unl.edu/assets/html/g1850/build/g1850.htm
156 University of Nebraska-Lincoln. (n.d.). Irrigated and Rainfed Corn Yield Trends. Retrieved from https://cropwatch.unl.edu/corn/yieldtrends
National Drought Mitigation Center. (2018). United States Drought Monitor. Retrieved from http://droughtmonitor.unl.edu/Maps/MapArchive.aspx
Climate-Data.org (n.d.). Nebraska. Retrieved from https://en.climate-data.org/region/930/
Hollinger, S. E. (1995). Midwestern Climate Center soils atlas and database. Circular no. 179.
Blann, K. L., Anderson, J. L., Sands, G. R., & Vondracek, B. (2009). Effects of agricultural drainage on aquatic ecosystems: a review. Critical reviews in environmental science and technology, 39(11), 909-1001.
Hatfield, J. L. (2015). Environmental impact of water use in agriculture. Agronomy Journal, 107(4), 1554-1556.
Maupin, M. A., Kenny, J. F., Hutson, S. S., Lovelace, J. K., Barber, N. L., & Linsey, K. S. (2014). Estimated use of water in the United States in 2010 (No. 1405). US Geological Survey.
United States Department of Agriculture. (2016). Ogallala Aquifer Initiative 2016 Progress Report. Retrieved from https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/programs/initiatives/?cid=stelprdb1048809
Groundwater Resources Program USGS. (2010). Assessing Groundwater Availability in the High Plains Aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming. Retrieved from https://pubs.usgs.gov/fs/2010/3008/pdf/fs2010-3008.pdf
Bartolino, J. R., & Cunningham, W. L. (2003). Ground-water depletion across the nation (No. 103-03).
Fertilizer Use
Herget G., Nielsen R., Margheim J. (2015). WWII Nitrogen Production Issues in Age of Modern Fertilizers Retrieved from https://cropwatch.unl.edu/fertilizer-history-p3
EPA. (n.d.), Agriculture: Nutrient Management and Fertilizer. Retrieved from https://www.epa.gov/agriculture/agriculture-nutrient-management-and-fertilizer
ERS USDA. (2018). Fertilizer Use and Price. Retrieved from https://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx
USDA NASS. (2017). Acreage. Retrieved from http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1000
157
EPA. (2012). Agricultural Fertilizer. Retrieved from http://www.epa.gov/roe/
Hargrove W.W., Luxmoore R.L. (1998). A New High-Resolution National Map of Vegetation Ecoregions Produced Empirically Using Multivariate Spatial Clustering Retrieved from https://www.geobabble.org/~hnw/esri98/
Debruin, J. (n.d.). Nitrogen Uptake in Corn. Retrieved from https://www.pioneer.com/home/site/us/agronomy/library/n-uptake-corn/#
Wallander, S. (2013). While Crop Rotations Are Common, Cover Crops Remain Rare. Retrieved from https://www.ers.usda.gov/amber-waves/2013/march/while-crop-rotations-are-common-cover-crops-remai n-rare/
Sawyer, J. E. (2008). 5. Nitrogen Rates.
Compton, J. E., Harrison, J. A., Dennis, R. L., Greaver, T. L., Hill, B. H., Jordan, S. J., ... & Campbell, H. V. (2011). Ecosystem services altered by human changes in the nitrogen cycle: a new perspective for US decision making. Ecology letters, 14(8), 804-815.
Helesicova M. (n.d.). Chemical Composition of Living Organisms.
Photo by P.J. Hahn
Kellogg R.L., Wallace S., Alt K. et al. (1997). Potential Priority Watershed for Protection of Water Quality. Retrieved from https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/technical/nra/nri/results/?cid=nrcs143_014067
Goolsby, D. A., Battaglin, W. A., & Hooper, R. P. (1997, July). Sources and transport of nitrogen in the Mississippi River Basin. In From the Corn Belt to the Gulf: Agriculture and Hypoxia in the Mississippi River Watershed, Proc. Conf., St. Louis, MO (pp. 14-15).
Mississippi River/Gulf of Mexico Hypoxia Task Force. (2017). Northern Gulf of Mexico Hypoxic Zone. Retrieved from https://www.epa.gov/ms-htf/northern-gulf-mexico-hypoxic-zone
David, M. B., Drinkwater, L. E., & McIsaac, G. F. (2010). Sources of Nitrate Yields in the Mississippi River Basin Journal of Environmental Quality, 39(5), 1657-1667.
Exner, M. E., Hirsh, A. J., & Spalding, R. F. (2014). Nebraska's groundwater legacy: Nitrate contamination beneath irrigated cropland. Water resources research, 50(5), 4474-4489.
Masarik, K. C., Norman, J. M., & Brye, K. R. (2014). Long-term drainage and nitrate leaching below well-drained continuous corn agroecosystems and a prairie. Journal of Environmental Protection, 5(04), 240.
USGS. (2017). Nitrogen and Water. Retrieved from 158 https://water.usgs.gov/edu/nitrogen.html
Mosaic Crop Nutrition (n.d.). Soil pH. Retrieved from http://www.cropnutrition.com/efu-soil-ph
Liebig, M. A., Varvel, G. E., Doran, J. W., & Wienhold, B. J. (2002). Crop sequence and nitrogen fertilization effects on soil properties in the western corn belt. Soil Science Society of America Journal, 66(2), 596-601.
Fu, C., Lee, X., Griffis, T. J., Dlugokencky, E. J., & Andrews, A. E. (2017). Investigation of the N 2 O emission strength in the US Corn Belt. Atmospheric Research, 194, 66-77.
Millar, N., Robertson, G. P., Grace, P. R., Gehl, R. J., & Hoben, J. P. (2010). Nitrogen fertilizer management for nitrous oxide (N 2 O) mitigation in intensive corn (Maize) production: an emissions reduction protocol for US Midwest agriculture. Mitigation and Adaptation Strategies for Global Change, 15(2), 185-204.
Decock, C. (2014). Mitigating nitrous oxide emissions from corn cropping systems in the midwestern US: potential and data gaps. Environmental science & technology, 48(8), 4247-4256.
Penuelas, J., Poulter, B., Sardans, J., Ciais, P., Van Der Velde, M., Bopp, L., ... & Nardin, E. (2013). Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe. Nature communications, 4, 2934.
Minnesota Department of Health. (2016). Heavy Metals in Fertilizers Retrieved from http://www.health.state.mn.us/divs/eh/risk/studies/metals.html
Roberts, T. L. (2014). Cadmium and Phosphorous Fertilizers: the Issues and the Science. Procedia Engineering, 83, 52-59.
Minnesota Department of Agriculture, Pesticide, and Fertilizer Management Division. (2007). Fertilizer Heavy Metal Analysis Summary Retrieved from http://www.mda.state.mn.us/Global/MDADocs/chemfert/others/heavymetals2002.aspx
Shareef, R. S., Mamat, A. S., Wahab, Z., & Rukunudin et al. (2016). The Accumulation Of Cadmium In Corn (Zea Mays L.) At Different Levels Of Soil Ph.
Monsanto (n.d.). Agronomic Spotlight, Importance of P and K in Corn and Soybean Development
Pesticide Use
Van Walt. (2015). Remediating Soil-The Natural Way Retrieved from https://www.vanwalt.com/news/2015/04/09/remediating-soil-the-natural-way/
Fernandez-Cornejo, J., Nehring, R. F., Osteen, C., Wechsler, S., Martin, A., & Vialou, A. (2014). Pesticide use in US agriculture: 21 selected crops, 1960-2008.
159 NASS USDA. (2017). 2016 Agricultural Chemical Use Survey Corn Retrieved from https://www.nass.usda.gov/Surveys/Guide_to_NASS_Surveys/Chemical_Use/2016_Corn_Potatoes/Che mUseHighlights_Corn_2016
National Center for Biotechnology Information. PubChem Compound Database; CID=3496, https://pubchem.ncbi.nlm.nih.gov/compound/3496 (accessed Apr. 30, 2018).
National Pesticide Information Center. (2011). Glyphosate Technical Factsheet Retrieved from http://npic.orst.edu/factsheets/archive/glyphotech.html
USGS. (2017). Estimated Annual Agricultural Pesticide Use Retrieved from https://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=2015&map=GLYPHOSATE&hilo= L
Sidhu, R. S., Hammond, B. G., Fuchs, R. L., Mutz, J. N., Holden, L. R., George, B., & Olson, T. (2000). Glyphosate-tolerant corn: The composition and feeding value of grain from glyphosate-tolerant corn is equivalent to that of conventional corn (Zea mays L.). Journal of agricultural and food chemistry, 48(6), 2305-2312.
Aparicio, V. C., De Gerónimo, E., Marino, D., Primost, J., Carriquiriborde, P., & Costa, J. L. (2013). Environmental fate of glyphosate and aminomethylphosphonic acid in surface waters and soil of agricultural basins. Chemosphere, 93(9), 1866-1873.
Wauchope, R. D., Estes, T. L., Allen, R., Baker, J. L., Hornsby, A. G., Jones, R. L., ... & Gustafson, D. I. (2002). Predicted impact of transgenic, herbicide tolerant corn on drinking water quality in vulnerable - watersheds of the mid western USA. Pest management science, 58(2), 146-160. -
Gill, J. P. K., Sethi, N., Mohan, A., Datta, S., & Girdhar, M. (2017). Glyphosate toxicity for animals. Environmental Chemistry Letters, 1-26.
Pesticide Action Network Asian Pacific (2009) Monograph on Glyphosate.
Rusek, J. (1998). Biodiversity of Collembola and their functional role in the ecosystem. Biodiversity & Conservation, 7(9), 1207-1219.
Maddison, David R. 2002. Collembola. Springtails. Version 01 January 2002 (under construction).http://tolweb.org/Collembola/8202/2002.01.01 in The Tree of Life Web Project, http://tolweb.org/
Zimmer M. (2002). Nutrition in terrestrial isopods (Isopoda: Oniscidea): an evolutionary-ecological approach. Biological Reviews, 77(4), 455-493.
DuPont-Pioneer. (2016). Glyphosate Resistance in Weeds Retrieved from https://www.pioneer.com/home/site/us/agronomy/library/glyphosate-resistance-in-weeds/
National Center for Biotechnology Information. PubChem Compound Database; CID=2256, https://pubchem.ncbi.nlm.nih.gov/compound/2256 (accessed Apr. 30, 2018).
160 Becker J., Erickson R., Flynn K., et al. (2016). Refined Ecological Risk Assessment for Atrazine Retrieved from https://www.regulations.gov/document?D=EPA-HQ-OPP-2013-0266-0343
USGS. (2017). Estimated Annual Agricultural Pesticide Use. Retrieved from https://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=2015&map=ATRAZINE&hilo=L
Dorsey, A. (2003). Toxicological Profile for Atrazine. Agency for Toxic Substances and Disease Registry.
Gilliom, R. J. (2007). Pesticides in U.S. Streams and Groundwater. USGS
EPA. (2018). Atrazine Background and Updates Retrieved from https://www.epa.gov/ingredients-used-pesticide-products/atrazine-background-and-updates
EPA. (2018). Atrazine Ecological Exposure Monitoring Program Data and Results. Retrieved from https://www.epa.gov/ingredients-used-pesticide-products/atrazine-ecological-exposure-monitoring-progra m-data-and-results
Wu, M., Sass, J., & Wetzler, A. (2010). Natural Resources Defense Council (NRDC) Report: Still Poisoning the Well: Atrazine Continues to Contaminate Surface Water and Drinking Water in the United States.
Betz, F. S., Hammond, B. G., & Fuchs, R. L. (2000). Safety and advantages of Bacillus thuringiensis-protected plants to control insect pests. Regulatory Toxicology and Pharmacology, 32(2), 156-173.
Climate Change
Co2Earth. (2018). Daily CO2 Retrieved from https://www.co2.earth/
Betz, F. S., Hammond, B. G., & Fuchs, R. L. (2000). Safety and advantages of Bacillus thuringiensis-protected plants to control insect pests. Regulatory Toxicology and Pharmacology, 32(2), 156-173.
US Global Change Research Program. (2009). Global climate change impacts in the United States. Cambridge University Press.
EPA. (2018). Overview of Greenhouse Gas Emissions Retrieved from https://www.epa.gov/ghgemissions/overview-greenhouse-gases
EPA. (2018). Climate Change Indicators in the United States Retrieved from https://www.epa.gov/climate-indicators
EPA. (2017). Understanding Global Warming Potentials Retrieved from https://www.epa.gov/ghgemissions/understanding-global-warming-potentials
161 The Environmental Literacy Council. (2015). Sources & Sinks Retrieved from https://enviroliteracy.org/air-climate-weather/climate/sources-sinks/
American Chemistry Council. (2013). Greenhouse Gas Sources and Sinks Retrieved from https://www.acs.org/content/acs/en/climatescience/greenhousegases/sourcesandsinks.html
Schaible, G. D., & Aillery, M. P. (2017). Challenges for US Irrigated Agriculture in the Face of Emerging Demands and Climate Change. In Competition for Water Resources (pp. 44-79).
Demand
USDA NASS. (2018). Corn Yield and Soybean Production Up in 2017 Retrieved from https://www.nass.usda.gov/Newsroom/2018/01_12_2018.php
USDA ERS. (2017). Trade Retrieved from https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/trade/
National Corn Growers Association. (2018). World of Corn Retrieved from http://www.worldofcorn.com/#/
USDA ERS. (2017). Background Retrieved from https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/background/
Schnepf, R. (2011). US livestock and poultry feed use and availability: Background and emerging issues. Congressional Research Service. http://www. nationalaglawcenter. org/assets/crs, 41956.
Lawrence, J., Mintert, J., Anderson, J. D., & Anderson, D. P. (2008). Feed grains and livestock: Impacts on meat supplies and prices. Choices, 23(2), 11.
Shepon, A., Eshel, G., Noor, E., & Milo, R. (2016). Energy and protein feed-to-food conversion efficiencies in the US and potential food security gains from dietary changes. Environmental Research Letters, 11(10), 105002.
ICF. (2017). A Life-Cycle Analysis of the Greenhouse Gas Emissions of Corn-Based Ethanol Retrieved from https://www.usda.gov/oce/climate_change/mitigation_technologies/USDAEthanolReport_20170107.pdf
Condon N., Klemick H., Wolverton A., (2013). Impacts of Ethanol Policy on Corn Prices: A Review and Meta-Analysis of Recent Evidence Retrieved from https://www.epa.gov/sites/production/files/2014-12/documents/impacts_of_ethanol_policy_on_corn_price s.pdf
Wallander S., Claassen R., Nickerson C. (2011). The Ethanol Decade An Expansion of U.S Corn Production, 2000-09. USDA NASS
162 OECD/FAO (2017), OECD-FAO Agricultural Outlook 2017-2026, OECD Publishing, Paris. http://dx.doi.org/10.1787/agr_outlook-2017-en
Hansen J. (2017), Emerging Global Trade Patterns: USDA’s Long-term Agricultural Projections. USDA ERS
Calculation
United Nations Department of Economics and Social Affairs. (2017). World’s Population to reach 9.8 billion in 2050, and 11.2 billion in 2100 Retrieved from https://www.un.org/development/desa/en/news/population/world-population-prospects-2017.html
OECD-FAO. (2018). OECD-FAO Agricultural Outlook 2016-2025. Retrieved from https://stats.oecd.org/Index.aspx?DataSetCode=HIGH_AGLINK_2016
OECD/FAO (2017), OECD-FAO Agricultural Outlook 2017-2026, OECD Publishing, Paris. http://dx.doi.org/10.1787/agr_outlook-2017-en
Licht M., Archontoulis S. (2017). Corn Water Use and Evapotranspiration. Iowa State University Extension and Outreach Retrieved from https://crops.extension.iastate.edu/cropnews/2017/06/corn-water-use-and-evapotranspiration
Conclusion
Foley, J. A., Ramankutty, N., Brauman, K. A., Cassidy, E. S., Gerber, J. S., Johnston, M., ... & Balzer, C. (2011). Solutions for a cultivated planet. Nature, 478(7369), 337.
Connor, R. (2015). The United Nations world water development report 2015: water for a sustainable world (Vol. 1). UNESCO Publishing.
Synergies, S. K. B. (2015). Agroecology for Food Security and Nutrition. In Proceedings of the FAO International Symposium.
USDA, (n.d.). About the U.S. Department of Agriculture Retrieved from https://www.usda.gov/our-agency/about-usda
163