sign in sign up
<<
Home , Cover crop

CCoovveerr CCrrooppss ffoorr OOrrggaanniicc FFaarrmmss

by Keith R. Baldwin and Nancy G. Creamer

• The economics of planting cover over are pivotal parts of every crops. Each farmer must consider the organic farmer’s management cost of establishing the cover and scheme. They are crucial to the main CC its benefits. goals of building health and • Cover crops for the Southeast. We preventing soil . Cover crops are review recommended winter and also important tools for increasing fertility summer cover crops and how they fit and controlling , pathogens, and specific cropping schemes. insects in organic crops. In this publication, • Recommended resources for further we will discuss planting, growing, and study. incorporating cover crops as amendments

into the soil. Our discussion will include

the following topics: • How cover crops affect the soil, including how they impart nitrogen for cash crops and how they can be used to control crop pests and diseases. We will also point out some concerns of grow- ing cover crops, such as the potential for them to rob soil of moisture needed for cash crops and to harbor damaging insects and pathogens.

• Establishing cover crops involves Figure 1. Millet is planted in fields where using a drill and cultipacking the field. cover crops are incorporated into the soil. • Managing cover crop residue. The (Photo courtesy of USDA) residue can be incorporated or left on the surface after using a kill method.

Contents How Cover Crops Affect the Soil—Page 2 The Economics of Planting Cover Crops—Page 12 Establishing Cover Crops—Page 11 Cover Crops for the Southeast—Page 12 Managing Cover Crop Residue—Page 11 Recommended Reading—Page 17

late planting of crops, such as hairy HOW COVER CROPS AFFECT THE vetch, can result in poor stands and small SOIL plants with limited root systems. If the cover crop is established early, however, its vigorous fall growth protects soil and reduces erosion. Because of its rapid growth The best place to begin our discussion of in fall and its continued growth during cover crops is to focus on how they help to winter, cereal (Secale cereale) provides reduce one of the most serious, persistent excellent protection from erosion during threats to long-term productivity and the winter. The use of cover crops in no-till the environment—soil erosion. Cover crops systems provides extended do this partially by keeping the soil covered because residue is left on the surface after during rainy periods when it might the cover crop is killed and the subsequent normally be bare and subject to erosion by is planted. rainfall. Langdale et al. (1991) concluded that cover crops reduced soil erosion by 62 How Cover Crops Improve the Soil percent based on a comparison of bare soil Increase soil through and soil planted with a cover crop in the additions of plant biomass. southeastern United States. Form soil aggregates, which stabilize soil and reduce runoff and erosion. Erosion decreases , and it also causes Increase soil porosity and decrease soil sedimentation of our rivers, reservoirs, and bulk density to promote root growth. estuaries. Sedimentation occurs when Improve soil tilth, which reduces crusting runoff from rainfall carries eroded soil and and increases the rate of water infiltration. deposits it into waterways. In time, Encourage populations of soil microbes, sediment deposits produce losses of aquatic micro- and macro- and habitat that are extremely difficult to earthworms, all of which contribute to reverse. The short-term costs of soil erosion efficient nutrient cycling and include nutrient losses in runoff from farm improvements in . fields and negative impacts to the soil’s physical structure. Soil Moisture

One of the most important considerations Soil scientists have estimated that the United in growing cover crops is their impact on States has lost 30 percent of its topsoil in the soil moisture. During the summer months, past 200 years due to agricultural practices cover crops left on the surface can help to that leave bare, fallow for a significant portion of the year. conserve soil moisture by reducing evaporation from the surface and by (Tyler et al., 1994) increasing water infiltration. However, How well do cover crops help to prevent grown too late in the spring, cover crops soil erosion? During the fall, winter, and can draw moisture down from the soil’s early spring, this depends largely on when upper layer, where it will be needed for seed the cover crop is established. Timing is germination and stand establishment of particularly important in the fall because subsequent cash crops.

Organic Production—Cover Crops for Organic 2

and by monitoring soil moisture and ob- Thus, knowing when to kill and incor- taining rainfall predictions, a farmer can porate cover crops in the spring is a decide on the best possible times for killing balancing act. The goal is to produce the and incorporating a cover crop. greatest possible amount of biomass, or living matter, with the cover crop, which maximizes values, while not depleting soil moisture. Timing is every- Management thing. Cover crops and surface crop residues can be used to control or inhibit weeds in In early spring, while cover crops are still subsequent cash crops in three basic ways: actively growing, farmers should begin monitoring soil moisture. During years • By smothering and shading them so with normal to dry weather patterns, the they don’t receive adequate air and best time to kill cover crops is usually two light. weeks before planting cash crops • By outcompeting them for nutrients. (depending on weather forecasts). Biomass • By producing an effect known as yield and nitrogen (N) production by , the toxic effect on weed seed legume cover crops may not be at their germination and seedling growth that maximum levels at this point. However, in occurs as residues of some cover crops most seasons, sufficient rainfall for ade- decompose. quate crop emergence will occur during the The primary way to suppress weed seed two-week preplant period or within the germination and growth is to have a week immediately following planting. In vigorous cover crop stand. Such a stand will wet years or when a rainy period is forecast, simply out-compete weed seeds for light the cover crop can be killed immediately and nutrients (Teasdale and Daughtry, before soil preparation and planting of 1993). When the cover crop is killed, its spring crops. thick residues remain on the surface and hinder weed growth by physically With these weather windows in mind, a modifying the amount of natural light, soil farmer can create a plan to produce the temperature, and soil moisture that is highest possible cover crop biomass and necessary for weed seed germination. biomass N yields. Studies show that when cover crop kill is delayed from early April to It’s important to note that suppressing early May, the yields of hairy vetch, cereal weeds by smothering them becomes less rye, and mixtures of both increase by an effective as cover crop residues decompose. average of 160 percent in the Maryland How fast residues decompose depends on piedmont and by 83 percent in the coastal several variables. For instance, warm plain (Clark et al., 1994). In Clark’s study, temperatures, rainfall, and field can the N contents of hairy vetch and hairy speed up the decomposition rate. Another vetch-rye mixtures were 1.6 to 2 times important factor is the C:N ratio, the greater at the late kill date: They ranged carbon-to-nitrogen ratio of different kinds from 65 to 100 pounds N per acre for early of crop residues. Residues with a high C:N kill, and from 135 to 200 pounds N per acre ratio, such as mature small grain cover for late kill. Based on those considerations crops like rye (which has a C:N ratio of

Organic Production—Cover Crops for Organic Farms 3 around 50), have a much slower decom- environmental conditions, insects, or position rate than like hairy vetch disease pressure, can speed up the (which has a C:N ratio of around 12). Mix- detrimental effects of allelochemicals on tures of legumes and small grains have an weeds. intermediate rate of decomposition (a C:N ratio of around 25). In one study, researchers found that cereal rye residues on the soil surface suppressed Cover crop residues also interfere with weed most common annual broadleaf and grassy emergence through the allelopathic effect weeds for four to eight weeks (Smeda and (Creamer at al., 1996a). Scientists are still Weller, 1996). Thus, using a rye cover crop researching the many (and sometimes could eliminate the need for a soil-applied mysterious) allelopathic effects that one herbicide at transplanting without plant has on another through its allelo- depressing yield. The authors indicated, chemicals, the chemicals a plant releases however, that post-emergence weed control into the environment that can be toxic to of escaped weeds might be necessary in other plants. Some scientists believe that some years. the specific allelopathic effects of certain plants are enhanced by chemicals produced Researchers have reported that the cover by actinomycetes, algae, fungi, or other crops listed in Table 1 have shown microbes associated with particular plant allelopathic effects on certain weeds. We root systems in the upper soil layers should note that the allelopathic effects of (Putnam, 1988). Where and how these crimson clover and hairy vetch are more allelochemicals originate is often hard to apparent if the cover crop is incorporated discern. Each chemical’s biological activity rather than left on the surface in no-till may be reduced or enhanced by other management (Teasdale and Daughtry, factors, such as microbe action in the soil 1993). and oxidation. Other factors, such as

Table 1. A summary of research on the allelopathic effects of cover crops Investigator and Cover Crop Weeds Suppressed Publication Date Hairy vetch Lambsquarters, yellow foxtail, Teasdale et al., 1993 yellow nutsedge, White et al., 1989 pitted morningglory Crimson clover Pitted morningglory, wild mustard, Teasdale et al., 1993 Italian ryegrass White et al., 1989 Cereal rye Lambsquarters, redroot pigweed, Barnes and Putnam, 1986 common ragweed Schilling et al., 1985 Masiunas, 1995 Wheat Morningglory, prickly sida Liebl and Worsham, 1983 Velvetbean Yellow nutsedge, chickweed Hepperly et al., 1992 Fujii et al., 1992 Sorghum sudangrass Annual ryegrass Forney and Foy, 1985

Organic Production—Cover Crops for Organic Farms 4 Disease Management same family of plants (taxonomically related) to the subsequent cash crop can The impact of cover crops on pathogens— also influence whether or not disease cycles agents in the soil, such as bacteria or are interrupted or prolonged. viruses, that cause disease—can be good, bad, or nonexistent. This impact varies Management broadly depending on individual circum- stances and situations. A cover crop can act as a host for soilborne pathogens, or it can are enough of a concern in the serve as an effective form of biological sandy soils of the southeastern U. S. to give control for other plant pathogens. them individual attention when consider- Incorporating cover crop residues can, in ing disease management. The root-knot some cases, provide an organic food base nematode (Meloidogyne spp.) is particularly that encourages pathogen growth (Phillips troublesome in the Southeast. Agricultural et al., 1971). On the other hand, some scientists have more questions than cover crops, such as brassicas (cabbage and answers concerning how to reduce pop- mustard), can actually decrease soil ulations of nematodes with cover crops. pathogen populations (Lewis and Papa- They are struggling to find a selection of vizas, 1971; Subbarao and Hubbard, 1996). crop rotations with cover crops that can address a wide variety of nematodes that The impact of a cover crop on a pathogen have a very diverse host range (Reddy et al., involves many variables. Principally, it 1986). They are also unclear, at this point, depends upon the pathogen’s nature and as to how some cover crops reduce the life cycle requirements. For example, if the population levels of certain nematode pathogen survives best on surface residue species. and the cover crop residue is left on the soil surface as , then the pathogen may Examples of Nematode-Control Success survive until the next crop is planted and with Warm-Season Legumes the level of disease may increase (Fawcett, Warm season legume cover crops are effective 1987). Many plant diseases are associated in reducing populations of some plant- with surface residue, for example, fungal parasitic nematodes: and bacterial leaf blights (Boosalis and Rhoades and Forbes (1986) reported that Cook, 1973). hairy indigo and joint vetch cover crops (coupled by mulching with clippings of At the same time, the increases in soil cowpea) were highly effective for organic matter provided by cover crops can maintaining low populations of B. enhance biological control of soilborne longicaudatus and M. incognita nematodes. plant pathogens. This comes about both through direct antagonism and by Rodriguez-Kabana et al. (1992) reported competition for available energy, water, that velvetbean was effective in lowering and nutrients (Sumner et al., 1986). population densities of several root-knot species (present simultaneously) in Organisms that cause disease can also be greenhouse and field tests. Unfortunately, affected by changes in temperature, this is not always the case in field tests. moisture, soil compaction, and bulk density, as well as nutrient dynamics. Whether or not the cover crop is in the

Organic Production—Cover Crops for Organic Farms 5 For instance, some or cover Insect Management crops placed in a rotation can reduce damage by one nematode species but not Cover crops can be both a blessing and a others. In a study in Florida, the warm- drawback because they attract both season legumes, which included pigeonpea, beneficial and harmful insects to farm fields crotalaria, hairy indigo, velvetbean, and (Altieri and Letourneau, 1982; Andow, joint vetch, reduced root-knot nematode 1988). When a cover crop matures or dies, damage in a subsequent snapbean crop both the beneficial and insects may when the crop was compared to one move to cash crops. The resulting effect on produced in fallow. These same cover crops, insect pest populations on the farm (an however, were no more effective than effect that also depends on several fallow in reducing damage from sting environmental factors) can present (Belonolaimus longicaudatus) and lesion frustrating dilemmas for a farmer. For (Pratylenchus brachyurus) nematodes. example, in a study in 1991, researchers found that a rye cover crop helped to In some cases, a cover crop can reduce reduce fruitworm populations in the populations of one parasitic nematode but tomato crop that followed it. But the rye serve as a host that increases populations of cover also led to increased stinkbug damage other nematodes. While two researchers (Roberts and Cartwright, 1991). (McSorley and Gallaher) reported in 1991 that sorghum-sudangrass cover crops To create the best situation, a farmer grows reduced levels of root- knot nematodes, a cover crop to attract beneficial insects Rhoades and Forbes (1986) found that a before the damaging insects arrive. The sorghum-sudangrass cover crop increased beneficial insects are attracted by the populations of B. longicaudatus and M. moisture, shelter, pollen, honeydew, nectar, incognita nematodes. Farmers attempting to and potential insect prey associated with use crop rotations for controlling one the cover crop. These beneficial insects nematode species must be aware that these subsist in the cover crop and then move rotations could benefit other damaging into the vegetable crop to attack arriving nematodes present in the field (McSorley pest insects. Several studies show that this and Dickson, 1995). Potential rotation approach is often successful. Researchers in crops should be evaluated for their effects Georgia reported high densities of big-eyed on as many different damaging nematodes bugs, lady beetles, and other beneficial as possible. insects in vetches and clovers that moved into ensuing tomato crops (Bugg et al.,

1990). In a more recent study, a researcher Cover Crop Tip reported that assassin bugs destroyed Organic growers commonly plant , Colorado potato beetles feeding on mustard, and other brassicas as rotation crops eggplant that had been planted into strip- to “clean-up” soil during winter months. tilled crimson clover (Phatak, 1998). These plants have been shown to suppress a wide range of parasitic nematodes. (Bending and Lincoln, 1999)

Organic Production—Cover Crops for Organic Farms 6

decompose plant residues. The biomass nitrogen is mineralized and converted first

to ammonium (NH4) and then to nitrate One of the most significant contributions compounds (NO3) that plant roots can take that legume cover crops make to the soil is up and use. The rate of this mineralization the nitrogen (N) they contain. Legume process depends largely on the chemical cover crops fix atmospheric N in their plant composition of the plant residues that are tissues in a symbiotic or mutually beneficial involved (Clement et al., 1995), and on relationship with rhizobium bacteria. In climatic conditions. association with legume roots, the bacteria convert atmospheric N into a form that Determining the ratio of carbon to nitrogen plants can use. As cover crop biomass (C:N) in the cover crop biomass is the most decomposes, these nutrients are released for common way to estimate how quickly use by cash crops. Farmers should make an biomass N will be mineralized and released effort to understand this complex process for use by cash crops. As a general rule, because it will help them to select the cover crop residues with C:N ratios lower proper legumes for their cropping plan, than 25:1 will release N quickly. In the calculate when to incorporate cover crops southeastern U. S., legume cover crops, and plant cash crops that follow, and plan such as hairy vetch and crimson clover, fertilizer rates and schedules for those cash killed immediately before corn planting crops. Above all, they need to inoculate generally have C:N ratios of 10:1 to 20:1 legume seed before planting with the (Ranells and Wagger, 1997). Residues with appropriate Rhizobium species. C:N ratios greater than 25:1, such as cereal rye and wheat, decompose more slowly and Cover Crop Tip their N is more slowly released. Nonleguminous cover crops, typically grasses A study conducted in 1989 reported that 75 or small grains, do not fix nitrogen. Nonethe- to 80 percent of the biomass N produced by less, they can be effective in recovering min- hairy vetch and crimson clover residues was eralized nitrogen from soil after crops are released eight weeks after the cover crops harvested. were incorporated into the soil (Wagger, 1989a). This amounted to 71 to 85 pounds The N associated with cover crop biomass of N per acre. However, not all of the undergoes many processes before it is ready released N was taken up by the subsequent to be taken up for use by cash crops. The corn crop. The corn utilized approximately process begins with biomass N, which is the 50 percent of the N released by both nitrogen contained in mature cover crops. residues. (This value may be con-sidered the From 75 to 90 percent of the nitrogen N uptake efficiency of corn from legume content in legume cover crops is contained residues. This value is similar to the N in the aboveground portions of the plant, uptake efficiency of corn from inorganic with the remaining N in its roots and fertilizer sources, such as ammonium nodules (Shipley et al., 1992). nitrate.) The N not taken up by the follow- ing crops may still contribute to soil health. When legume or grass cover crops are killed Living microbes in the soil may use the and incorporated into the soil, living nitrogen to support population growth and in the soil go to work to microbial activity in the soil.

Organic Production—Cover Crops for Organic Farms 7 As with just about everything else when it Grass species establish ground cover more comes to farming, the practice of growing quickly than legume , and cover crops to produce nitrogen in the soil their root growth remains active in the is complicated by many variables. Weather, cooler temperatures of autumn (Ranells and differences in growing seasons, the types of Wagger, 1997). Cover crop mixtures that cover crops involved, and the timing of include grasses can, therefore, prevent soil cover crop desiccation to produce the opti- erosion more effectively. Growing deep- mum benefit all come into play. Amassing rooted and shallow-rooted cover crops experience in cover crop production is together will also help a farmer to make simply the best way for a farmer to learn better use of water and other resources how to deal with the interplay of all these throughout the soil profile. variables.

Legumes Versus Grasses. As we’ve Legumes or Grasses? How To Choose a seen above, legume cover crops play a vital Cover Crop role in producing N for subsequent cash crops. What part do nonleguminous cover Generally, cover crop selection is based on each farmer’s situation and production goals. crops, which do not produce nitrogen, have For example, if the purpose of a cover is to in the cropping scheme? provide readily available, biologically fixed nitrogen for cash crops, then the farmer Organic farmers often plant nonlegumi- should choose a legume, such as hairy vetch nous winter cover crops to trap the soil N or cowpea. that is left over from summer cash crops and to prevent this N from out of If the cover crop will be managed as a surface the root zone or running off the field. mulch for weed suppression or incorporated Generally, grass cover crops are very to improve soil quality, then the farmer should effective—much more so than legumes—in choose a grass cover crop, such as cereal rye trapping and recovering N from the soil. or a sorghum-sudangrass mix. Both of these Grass and legume cover crop mixtures are grass cover crops can produce large amounts more efficient than legumes alone in of biomass with high C:N ratios at maturity, and both are reported to suppress some trapping leftover N in the soil, but don’t do weeds. as effective a job as straight grass cover crops. Farmers can also more effectively There are many advantages, however, to manipulate nitrogen cycling with mixed planting cover crops that are grass and cover crop species. Combining mature legume mixtures called bicultures. cereals, which have high carbon to nitro- Researchers (Clark et al., 1994) reported gen (C:N) ratios and break down slowly, that a cereal rye-hairy vetch biculture with legumes, which have low C:N ratios successfully scavenged potentially leach- and break down more quickly, can influ- able N from the soil, and also added fixed N ence decomposition of cover crop residues. for use by an ensuing corn crop. Addi- The decomposition of such cover crop tionally, the cover crop used excess water in mixtures will occur more quickly than that the soil, which also helped limit N leaching of cereal alone, releasing N more quickly for losses. uptake by cash crops.

Organic Production—Cover Crops for Organic Farms 8

Planting mixtures of cover crops can help a To estimate yield, take cuttings from several farmer to use the allelopathic potential of areas in the field. Dry and weigh the samples. the cover crops to suppress weeds. Allelo- Use a yardstick or metal frame of known pathic suppression of weeds depends on dimensions and clip the plants at ground level both the cover crop and the weed. There- within the known area. Dry the samples in an oven at about 140°F for 24 to 48 hours until fore, a broader spectrum of weed control they are crunchy dry. Use the following may be possible by growing a mixture of equation to determine per acre yield of dry cover crops, with each species contributing matter: allelopathic activity towards specific weed species (Creamer and Bennett, 1997). Yield (lb/acre) =

Total weight of dried samples (lb) X 43,560 sq ft Area (sq ft) sampled 1 acre Mixtures of cover crops can also be planted to influence insect populations. Species that For example, two 3 feet by 3 feet (9 sq ft or 1 sq may not produce much biomass or biomass yd) samples weigh 2.5 pounds. The dried N may be included in mixtures to attract biomass yield equals: beneficial insects into the cropping system. Yield (lb/acre) =

2.5 lb X 43,560 sq ft = 6,050 lb/acre. Measuring Cover Crop Nitrogen. To 18 sq ft 1 acre ensure that cash crops receive enough nutrients, farmers must accomplish these Though not as accurate, yield can be estimated calculations in this sequence: from the height of the cover crop and the percentage of ground it covers. At 100 percent 1. Determine the biomass produced. ground coverage and a 6-inch height, most 2. Determine the nutrient levels in that nonwoody legumes contain roughly 2,000 biomass. pounds per acre of dry matter. For each additional 3. Predict how quickly the biomass will inch, add 150 pounds.

decompose, releasing nutrients for cash For example, a hairy vetch cover crop is 18 crops. inches tall and has 100 percent ground 4. Calculate whether additional nutrients coverage. The first 6 inches of dry biomass are required for the desired crop yields. weighs roughly 2,000 pounds. The 12 additional inches of growth weighs 150 pounds Cover Crop Tip per inch. The additional weight is:

Calculating the nutrient levels released by 12 X 150 = 1,800 lb, green manures for a subsequent cash crop normally requires three measurements: and the total weight of the cover crop dry matter is: • The amount of biomass (dry weight). 2,000 + 1,800 = 3,800 lb

• The nutrient composition of the cover If the stand has less than 100 percent ground crop. coverage, multiply the total weight by the • The decomposition rate of the cover crop percentage of ground covered, represented as a during the cash cropping season. decimal number (the percentage divided by 100). If the percentage of ground covered in the example above is 60 percent, then the weight of the dry matter is:

3,800 X 0.60 (60/100) = 2,280 pounds of dry biomass (Adapted from Sarrantonio, 1998)

Organic Production—Cover Crops for Organic Farms 9 These calculations normally require three ground biomass prior to flowering, and 3.0 measurements: the amount of biomass (dry to 3.5 percent at flowering. After flowering, weight), the nutrient composition of the nitrogen in the leaves decreases quickly as it cover crop, and the decomposition rate of accumulates in the growing seeds. Most the cover crop during the cash cropping cover crop grasses contain 2.0 to 3.0 per- season. cent nitrogen before flowering and 1.5 to 2.5 percent after flowering. Other cover Farmers can estimate the amount of crops, such as Brassica species and buck- nitrogen in a cover crop by estimating the wheat, will generally be similar to, or total biomass yield of the cover crop and slightly below, grasses in their N concen- the percentage of nitrogen in the plants tration. To precisely determine the per- when they’re killed. A simple process for centage of nitrogen in the cover crop, send the assessment is explained in Managing a plant sample to a laboratory for a chem- Cover Crops Profitably (Sarrantonio, 1998: ical analysis. The N.C. Department of Agri- culture Plant Analysis Lab provides that For cereal rye, the height relationship is a service for $4 per sample. bit different. Cereal rye produces approxi- mately 2,000 pounds per acre of dry matter As discussed previously, not all of the at an 8-inch height and 100 percent ground nitrogen contained in the cover crop coverage. For each additional inch, add 150 residue will be available to the cash crop. pounds, as before, and multiply by the To conservatively estimate the amount that percentage of ground covered, represented will be available to the following crop, as a decimal (the percentage divided multiply legume biomass nitrogen, as by100). For most small grains and other calculated above, by 0.50 if the cover crop annual grasses, start with 2,000 pounds per residue will be incorporated and by 0.40 if acre at a 6-inch height and 100 percent of the residue will be left on the soil surface. ground covered. Add 300 pounds for each additional inch, and multiply by the per- centage of ground covered, represented as a From the example above, if the hairy vetch is decimal (the percentage divided by 100). incorporated in the soil in early May in a normal spring, then the nitrogen available from the hairy vetch to the cash crop will be: To calculate the amount of nitrogen in the dried cover crop biomass, multiply the dry biomass Available N (lb/acre) = yield times the percentage of nitrogen expressed 152 lb N per acre X .50 = 76 lb N per acre. as a decimal (percentage of N divided by 100). For the hairy vetch cover crop example above If the hairy vetch is left on the soil surface in with 100 percent cover and an estimated 4 early May in a normal spring, then the nitrogen percent nitrogen at flowering: available from the hairy vetch to the next crop will be: Total N (lb/acre) = 3,800 lb/acre X .04 (4 ÷ 100) = 152 lb N per acre Available N (lb/acre) = 152 lb N per acre X .40 = 61 lb N per acre

Annual legumes typically have between 3.5 These availability coefficients will change, and 4.0 percent nitrogen in the above- depending on the weather. In dry or cold

Organic Production—Cover Crops for Organic Farms 10 and wet springs, the soil microorganisms another publication within this series: responsible for mineralization of the “Conservation Tillage on Organic Farms.” organic nitrogen in the cover crop residue will be less active. The mineralization rate Should Farmers Leave Cover Crops on the will be reduced, and a lesser fraction of the Surface Or Incorporate Them? nitrogen will be available to the following cash crop. In almost all cases, the availa- In a wet growing season, incorporating bility coefficient of a small grain cover crop, legumes into the soil may produce the highest such as cereal rye, will be very low. Very yields in cash crops that follow. However, little N in the residue will be released for under relatively dry growing conditions, cover use by the cash crop. crop residue left on the surface will help to conserve soil moisture. ESTABLISHING COVER CROPS In no-till organic production systems, cover Using a drill to sow cover crops into a crops are usually killed mechanically and conventionally prepared seedbed is the left on the surface as a mulch. Each kill most reliable way to obtain a uniform method has its benefits and drawbacks: stand. However, in a no-till situation, a no- till grain drill can also be used successfully, Undercutting utilizes a steel bar that is provided that the residue from the previous drawn several inches underneath the soil crop is not excessive and the soil is moist surface (usually beneath a plant bed), enough to allow the drill to penetrate to severing the top growth and crown of the the desired planting depth. Seeds may be plant from the roots and leaving the sur- broadcast if the soil has been disked and face and aboveground biomass undisturb- partially smoothed, but seeding rates ed. The main advantage of undercutting is should be increased by 20 percent. that it leaves the cover crop intact and the large pieces break down more slowly, It is best to cultipack the field after broad- enhancing weed suppression. casting to firm the soil around the seeds. Crimson clover, in particular, can be Mowing with a flail mower leaves established quite easily with this method. the finely chopped residue evenly distri- In a limited number of trials, aerial seeding buted over the bed. The residue tends to of crimson clover into a standing crop, such decompose quickly, so high biomass is as , has proven successful. An desirable. innovative system that has shown promise in North Carolina and other southeastern Rolling the cover crop often includes states is to allow crimson clover to reseed crimping the cover crop stems, which itself naturally. damages each plant’s vascular system and causes it to die. Rolling keeps the above- MANAGING COVER CROP RESIDUE ground part of the plant attached to the root system. As with undercutting, rolled In organic systems, cover crops may be plants decompose more slowly than those killed and incorporated into the soil by killed by mowing and, consequently, tillage, mowing, undercutting, or rolling. control weeds for a longer period of time Details about these methods are included in (Lu et al., 2000). To facilitate seeding into

Organic Production—Cover Crops for Organic Farms 11 rolled or undercut cover crops, farmers nitrogen in a corn cropping system. If should plant in the same direction as they nitrogen prices, which increase with energy rolled the cover crop. prices, continue to rise, legume cover crops may become cost-effective N sources. THE ECONOMICS OF PLANTING COVER CROPS In studies where cover crop systems are reported to be less profitable than conven- Researchers are just beginning to investi- tional systems, the lower profitability is gate the long-term profitability of using attributed to the establishment cost of the cover crops in horticultural systems. cover crop. In these studies, the benefits of Farmers using cover crops have additional using the cover crop (increased yields and expenses due to seed, seeding, and man- reduced amounts of applied N) do not agement. However, cover crops allow a outweigh the establishment cost of the farmer to reduce costs for , pest cover crop (Bollero and Bullock, 1994; and disease control, and extensive tillage. Hanson et al., 1993). They also represent a long-term investment in soil resources. Relative to these benefits Each farmer must determine how to and to the potential returns from high- account for the less apparent, long-term value horticultural crops, the cost of cover benefits—such as reduced soil erosion, cropping is comparatively minor. increased organic matter content, improved soil physical properties, reduced nitrate Legume cover crops are generally reported leaching, and enhanced nutrient cycling. to have more profitability potential than grass cover crops because they contribute nitrogen to the subsequent cash crop, COVER CROPS FOR THE SOUTHEAST reducing input costs (Roberts et al., 1998). Grass cover crops may, in fact, consume soil Which winter and summer legume and N that could be used by the cash crop. grass cover crops perform well in the Hairy vetch may be among the more southeastern United States? Our promising cover crops. It contributes N to descriptions of recommended cover crops the soil, and it also improves the soil’s are drawn primarily from three sources: structure and water-holding capacity. In Duke (1981), Sarrantonio (1994), and doing so, it also increases the effectiveness Bowman et al. (1998). An electronic source of fertilizer-applied N (Lichtenberg et al., we found very useful was the University of 1994; Hanson et al., 1993). California at Davis (UCD) Sustainable Cover Crop Resource Page at We would like to emphasize that all of http://www.sarep.ucdavis.edu/ccrop/ (UCD, these benefits may not always lead to 2001). For more information on the cover increased profits for farmers. Allison and crops listed, or to find information about Ott (1987) reviewed studies investigating other potential cover crops, refer to these the economics of using legume cover crops references, which are listed in the in conservation tillage systems. They “Recommended Reading” section of this concluded that legume cover crops increase publication. profitability if they enhance the yield of the succeeding crop. But they decrease profitability when used as the sole source of

Organic Production—Cover Crops for Organic Farms 12

Hairy vetch is probably the most Recommended Winter Species commonly used cover crop in the United States, in part because it is so widely Examples of winter legume cover crops adapted. Hairy vetch is seeded at 20 to 30 include crimson clover, hairy vetch, pounds per acre, with the lower rate used if Austrian winter pea (Pisum sativum arvense), the vetch is drilled or planted in mixtures. and subterranean clover (Trifolium subter- At mid-bloom, hairy vetch can be easily raneum). Cereal rye, wheat, and oats are also killed by undercutting or mowing. Be aware commonly used as small grain cover crops that hairy vetch can harbor root-knot and in mixtures with the legumes nematodes (Meloidogyne spp.), cyst mentioned above. Generally, winter cover (Heterodera glycines), and various cutworms. crops are planted in early fall and allowed Susceptible vegetable crops should be to grow until mid-spring, at which time the temporarily separated in a rotation. If crop is incorporated by tillage, or killed and allowed to produce mature seed, vetch can left as a surface mulch into which another also be viewed as a weed in subsequent crop is planted. small grain crops.

Winter Legumes Crimson clover (Trifolium incarnatum). Crimson clover stands upright and blooms about three to four weeks earlier than hairy Clover, Vetch & Winter Peas—What Are vetch. It grows vigorously in fall and winter Their Limitations? and has good reseeding ability. It is not widely adapted, however, and is more Hairy vetch tends to be more winter hardy appropriate in warmer climates. Crimson than the other winter legumes and can clover has good shade tolerance and can be generally be planted later in the season. Hairy overseeded into fall vegetable crops in vetch also adapts better to sandy soils than September. Seeding rates vary from 15 to 25 crimson clover, although crimson clover will pounds per acre, with the lower rate being provide adequate dry matter production on used when the seed is drilled. most well-drained, sandy loams. Subterranean clover (Trifolium In contrast, crimson clover grows faster in the subterraneum). Subterranean clover is a spring, thereby maturing and obtaining peak relatively low growing winter annual with dry matter production approximately three to prostrate stems. In late spring, subterra- four weeks before hairy vetch. Adequate dry nean clover develops seeds below ground matter and nitrogen production will be (much like peanuts), which gives it an obtained with a soil pH from 5.8 to 6.0. Soil excellent reseeding ability. Subterranean testing will help determine P and K fertilizer clover forms a thick mat when left on the requirements. It is important to inoculate surface as a mulch and has been shown to legumes with the proper strain of N-fixing suppress weeds in vegetable crops planted bacteria. into the mulch. Subterranean clover does

not produce as much biomass as other cool- Hairy vetch ( villosa). Hairy vetch season legumes grown in the South, but forms a very dense cover and, if planted biomass yield can reach 5,500 pounds per with a tall growing species like rye, will acre with a nitrogen concentration of climb and produce a great deal of biomass. between 2 and 3 percent. Subterranean

Organic Production—Cover Crops for Organic Farms 13 clover is seeded at 8 to 15 pounds per acre Normally seeded in the fall, seeding rates between mid-September and mid-October. are between 20 and 30 pounds per acre.

Austrian winter pea (Pisum sativum Other cereal grasses. All of the cereal arvense). Austrian winter pea is succulent grasses will produce biomass ranging from and viney and can climb when planted 2,000 to 6,000 pounds per acre with with small grain crops. It grows vigorously nitrogen concentrations between 1 and 2 and will suppress weeds while growing, but percent. Biomass accumulation depends, in it decomposes rapidly and is not a good part, on how early in the spring the cover choice for a surface mulch and weed crop is killed. The high end of the range control. Austrian winter pea does well in a represents a kill date in mid- to late May. At mixture with oats, barley, rye, or wheat. this late date, however, the biomass carbon Seed is drilled at 60 to 90 pounds per acre to nitrogen (C:N) ratio will normally be and can be sown through October. greater than 50:1, a ratio at which soil microbes would immobilize any Winter Nonlegumes mineralized nitrogen. Small grains are normally drilled at 100 pounds of seed per Cereal rye (Secale cereale). Rye is one of acre. the most commonly used winter cover • Wheat (Triticum aestivum) provides a crops. It grows 3 to 6 feet tall and has an good overwintering ground cover and extensive, fibrous root system. It performs also provides the option of harvesting well when mixed with hairy vetch, which the grain. will use it for climbing support. Rye can • Barley (Hordeum vulgare) biomass tolerate a wide variety of soil types and production peaks about two weeks climatic conditions and is considered to be earlier than wheat, and about the same weed suppressive when managed as a time as crimson clover. Barley, grown as mulch. Of all the small grains, rye is the a , has been shown to best scavenger of excess soil nitrogen in the suppress winter annual weeds in fall. The seeding rate is 100 pounds per cropping systems, but must be planted acre. in September or early October to reduce winter kill. Annual ryegrass (Lolium multiflorum). • Oats (Avena sativa) grow well in cool Annual ryegrass is a noncreeping weather and provide rapid ground cover bunchgrass. In the spring it can grow 2 to 4 in the fall. Some growers plant spring feet tall. Annual ryegrass can be difficult to oats in the fall to produce a winter- control and can become a serious weed if it killed mulch for early spring no-till produces seed. Ryegrass requires vegetable plantings. However, spring considerable nitrogen and water. If these oats may not always winter-kill in mild are limited, it may not be a good choice. winters. Ryegrass has a very fibrous, dense root system that protects against soil erosion Recommended Summer Species while improving water infiltration and soil tilth. Dry matter yield can average between There is growing interest in the use of 1,300 and 2,000 pounds per acre, with an short-season summer annual legumes or average nitrogen content of 1.5 percent. grasses as cover crops and green manures in cropping systems. Summer annual legumes

Organic Production—Cover Crops for Organic Farms 14 and grasses can provide benefits between maturing cultivars usually give the highest the of spring vegetable crops and biomass yield and fix the most nitrogen. If the planting of fall vegetables or small well established, soybean will withstand grains. While additional legumes and short periods of drought. The viney, forage grasses are being evaluated for use, the types (for example, the cultivars Quail- following species are currently the best haven and Laredo) have the potential to options. produce more biomass than traditional soybean cultivars. Summer Legumes Velvetbean (Mucuna deeringiana). Cowpea (Vigna unguiculata). Other Velvetbean is a vigorously growing, warm- common names for this species are season annual legume native to the tropics blackeyed, crowder, and southern pea. and well adapted to southern U. S. condi- Cowpea is a fast growing, summer cover tions. It performs well in sandy and infer- crop that adapts to a wide range of soil tile soils. Most cultivars are viney, and conditions. Cowpeas have a deep taproot, stems can grow as much as 10 meters. tolerate drought, and compete well against Velvetbean is an excellent green manure weeds. Cowpeas produce 3,000 to 4,000 crop, producing high amounts of biomass pounds of dry biomass per acre, which that decompose readily to provide nitrogen contains 3 to 4 percent nitrogen. Maxi- for a cash crop. Velvetbean does best when mum biomass is achieved in 60 to 90 days. direct-seeded into warm soils in 38-inch Residues are succulent and decompose rows. Velvetbean seed should not be readily when incorporated into the soil. drilled, because the very large seed can be Cowpeas can be planted in the spring (after damaged in conventional drills. all danger of frost) through late summer. Cowpea seeds can be drilled in rows 6 to 8 Sunnhemp (Crotalaria juncea). Sunnhemp inches apart at 40 pounds per acre or broad- is a tall, herbaceous, warm-season annual cast at approximately 75 pounds per acre. legume with erect fibrous stems. It has been Higher seeding rates are necessary in late used extensively for soil improvement and summer when soil moisture is likely to be green manuring in the tropics. It competes limited. Recommended cultivars include with weeds, grows rapidly, and can reach a Iron Clay and Red Ripper. Plants normally height of 8 feet in 60 days. It can tolerate grow up to 24 inches tall, but some culti- poor, sandy soils and drought, but requires vars can climb when planted in mixtures good drainage. Sunnhemp tolerates with other species. Good mixture options moderate acidity, but a soil pH below 5 can are sorghum-sudangrass and German limit growth. Sunnhemp should be drilled foxtail millet. When mowed or undercut, or seeded in rows 38 inches apart at 30 cowpeas have the potential for consider- pounds per acre. The growing season in the able regrowth in some years. continental U.S. is not long enough to produce viable seed. Sunnhemp becomes Soybean (Glycine max). Soybean is one of fibrous with age, but the plants are the most economical choices for a summer succulent for about eight weeks after legume cover crop. It is an erect, bushy seeding. Sunnhemp is often planted in plant that grows 2 to 4 feet tall, establishes midsummer after cool-season vegetables or quickly, and competes well with weeds. sweet corn crops are harvested. It will When grown as a green manure crop, late produce high biomass and biomass

Organic Production—Cover Crops for Organic Farms 15 nitrogen in 45 to 60 days. Seed is not effective at suppressing weeds and has been readily available at this writing in early shown to have allelopathic properties. The 2006, but availability may increase if roots of sorghum sudangrass are good demand increases. While some Crotalaria foragers for nutrients and help control species are toxic to animals, sunnhemp erosion. Sorghum sudangrass does well forage is not. Sunnhemp should not to be when planted in mixtures, providing confused with showy crotalaria, a noxious effective support for viney legumes like weed species. velvetbean. If frost-killed, the residue can provide a no-till mulch for early planted Summer Nonlegumes spring crops like broccoli. When stressed by drought or by frost, this cover crop can (Fagopyrum esculentum). produce prussic acid, which is toxic to Buckwheat is a very rapidly growing . broadleaf summer annual, which can flower in four to six weeks. It reaches 30 German (foxtail) millet (Setaria inches in height and is single-stemmed italica). German or foxtail millet is an with many lateral branches. It has both a annual warm-season grass that matures deep taproot and fibrous roots. It can be quickly in the hot summer months. It is grown to maturity between spring and fall one of the oldest of cultivated crops. vegetable crops, suppressing weed growth German millet has a fairly low water and recycling nutrients during that period. requirement. Because of its shallow root It is succulent, easy to incorporate, and system, however, it doesn’t recover easily decomposes rapidly. Buckwheat flowers are after a drought. Grain formation requires very attractive to insects, and some farmers 75 to 90 days. German millet forms slen- use this cover to attract beneficial insects der, erect, and leafy stems that can vary in into cropping systems. Buckwheat is an height from 2 to 4 feet. The seed can be effective phosphorous scavenger. The main drilled from mid-May through August at a disadvantage to buckwheat is that it sets rate of 10 to 15 pounds per acre. A small seed quickly and, if allowed to go to seed, seeded crop, German millet requires a may become a weed in cash crops. Thus, relatively fine, firm seedbed for adequate the optimal time to incorporate buckwheat germination. To avoid early competition is one week after flowering, before seed is from germinating weed seed, it should be set. Buckwheat can be planted anytime in closely drilled in the row or sown in a stale the spring, summer, or fall, but is frost- seedbed—a seedbed that has been prepared, sensitive. with early emerged weeds killed just before planting the cover crop. Coarse, sandy soils Sorghum sudangrass (Sorghum bicolor × should be avoided. Sorghum sudanense). Sorghum sudangrass is a hybrid of grain sorghum and sudangrass. Pearl millet (Pennisetum glaucum). Pearl It is a warm-season annual grass, most often millet is a tall annual bunchgrass that planted from late spring through mid- grows 4 to 10 feet tall. It is also often summer. It grows well in hot, dry referred to as cattail millet because its long, conditions and produces a large amount of dense, spike-like inflorescences resemble biomass. Often reaching 6 feet in height, it cattails. Though it performs best in sandy can be mowed to enhance biomass loam soils, pearl millet is well adapted to production. Sorghum sudangrass is very soils that are sandy, infertile, or both. Pearl

Organic Production—Cover Crops for Organic Farms 16 millet can be planted from late April decomposition of Brassica juncea tissues in through July at a rate of 15 to 20 pounds soil. Soil Biology and Biochemistry. per acre. Pearl millet matures in 60 to 70 31:695-703. days. In North Carolina studies, pearl millet Bollero, G.A., and D.G. Bullock. 1994. Cover was not as readily killed by mechanical cropping systems for the Central Corn Belt. Journal of Production Agriculture. methods (mowing and undercutting) as 7:55-58. German or Japanese millet. Bugg, R.L., S.C. Phatak, and J.D. Dutcher. 1990. Insects associated with cool-season Japanese millet (Enchinochloa frumen- cover crops in southern Georgia: tacea). Japanese millet is an annual grass Implications for biological control in that grows 2 to 4 feet tall. It resembles and truck-farm and pecan agroecosystems. may have originated from barnyard grass. Biological Agriculture and Horticulture. Japanese millet is commonly grown as a 7:17-45. late-season green forage. If weather Clark, A.J., A.M. Decker, and J.J. Meisinger. conditions are favorable, it grows rapidly 1994. Seeding rate and kill date effects on and will mature from seed in as little as 45 hairy vetch-cereal rye cover crop mixtures days. Japanese millet can be planted from for corn production. Journal. April to July at a rate of 10 to 15 pounds per 86:1065-1070. acre. It performs poorly on sandy soils. Clement, A.J., J.K. Ladha, and F.P. Chalifour. 1995. Crop residue effects on nitrogen mineralization, microbial biomass, and RECOMMENDED READING rice yield in submerged soils. Soil Science Society of America Journal. 59:1595-1603. Creamer, N.G., and M.A. Bennett. 1997. Sources Cited Evaluation of cover crop mixtures for use Altieri, M.A., and D.K. Letourneau. 1982. in vegetable production systems. Vegetation management and biological HortScience. 32:866-870. control in agroecosystems. Crop Protection. Creamer, N.G., M.A. Bennett, B.R. Stinner, J. 1:405-430. Cardina, and E.E. Regnier. 1996a. Andow, D.A. 1988. Management of weeds for Mechanisms of weed suppression in cover insect manipulation in agroecosystems. In crop-based production systems. M.A. Altieri and M. Liebman (Eds.). Weed HortScience. 31:410-413. Management in Agroecosystems: Ecological Duke, J.A. 1981. Handbook of Legumes of World Approaches. pp.265-294. CRC Press. Boca Economic Importance. Plenum Press. New Raton, FL. York, NY. Allison, J.R., and S.L. Ott. 1987. Economics of Fawcett, R.S. 1987. Overview of pest using legumes as a nitrogen source in management for conservation tillage conservation tillage systems. In (J.F. systems. In R.J. Logan (Ed.). Effects of Power, Ed.) The Role of Legumes in Conservation Tillage on Groundwater Conservation Tillage Systems. pp.145-150. Quality: Nitrates and Pesticides. pp. 17-37. Society of America. Lewis Publishers. Chelsea, MI. Barnes, J.P., and A.R. Putnam. 1986. Evidence Forney, D.R., and C.L. Foy. 1985. for allelopathy by residues and aqueous Phytotoxicity of products from extracts of rye (Secale cereale). Weed rhizospheres of a sorghum-sudangrass Science. 86:384-390. hybrid (Sorghum bicolor X Sorghum Bending G.D., and S.D. Lincoln. 1999. sudanese). Weed Science. 33:597-604. Characterization of volatile sulphur- Fujii, Y., T. Shibuya, and T. Yasuda. 1992. containing compounds produced during Allelopathy of velvetbean: Its

Organic Production—Cover Crops for Organic Farms 17 discrimination and identification of L- McSorley, R., and R.N. Gallaher. 1991. DOPA as a candidate of allelopathic Nematode population changes and forage substances. Japan Agricultural Research yields of six corn and sorghum cultivars. Quarterly. 25:238-247. Supplement to the Journal of Hanson, J.C., E. Lichtenberg, A.M. Decker, Nematology. 23:673-677. and A.J. Clark. 1993. Profitability of no- Masiunas, J.B., L.A. Weston, and S.C. Weller. tillage corn following a hairy vetch cover 1995. The impact of rye cover crops on crop. Journal of Production Agriculture. weed populations in a tomato cropping 6:432-437. system. Weed Science. 43:318-323. Hepperly, P., E.H. Aguilar, R. Perez, M. Diaz, Phatak, S.C. 1998. Managing pests with cover and C. Reyes. 1992. Pigeon pea and velvet crops. In Managing Cover Crops Profitably, bean allelopathy. In S.J.H. Rizvi and V. 2nd Edition. pp.25-29. Sustainable Rizvi (Eds.). Allelopathy: Basic and Applied Agriculture Network, Handbook Series 3. Aspects. pp.357-370. Chapman and Hall. Beltsville. London. Phillips, D.J., A.G. Watson, A.R. Weinhold, Langdale, G.W., R.L. Blevins, D.L. Karlen, K.K. and W.C. Snyder. 1971. Damage of McCool, M.A. Nearing, E.L. Skidmore, lettuce seedlings related to crop residue A.W. Thomas, D.D. Tyler, and J.R. decomposition. Plant Disease Report. Williams. 1991. Cover crop effects on soil 55:837-841. erosion by wind and water. In W.L. Putnam, A.R., 1988. Allelopathy: Problems Hargrove (Ed.). Cover Crops for Clean and opportunities in weed management. Water. pp. 15-22. Soil and Water In M.A. Altieri and M. Liebman (Eds.). Conservation Society. Ankeny, IA. Weed Management in Agroecosystems: Lewis, J.A., and G.C. Papavizas. 1971. Effect of Ecologic Approaches. pp.77-78. CRC Press. sulfur-containing volatile compounds and Boca Raton, FL. vapors from cabbage decomposition on Radke, J.K., R.W. Andrews, R.R. Janke, and Apanomyces euteiches. Phytopathology. S.E. Peters. 1988. Low input cropping 61:208-214. systems and efficiency of water and Lichtenberg, E., J.C. Hanson, A.M. Decker, nitrogen use. In W.L. Hargrove (Ed.). and A.J. Clark. 1994. Profitability of Cropping Strategies for Efficient Use of Water legume cover crops in the mid Atlantic and Nitrogen. ASA Special Publication 51. region. Journal of Soil and Water pp.193-218. ASA, CSSA, and SSSA. Conservation. 49:582-585. Madison, WI. Liebl, R.A., and A.D. Worsham. 1983. Ranells, N.N., and M.G. Wagger. 1997. Inhibition of pitted morning glory Winter grass-legume bicultures for (Ipomoea lacunose L.) and certain other efficient nitrogen management in no-till weed species by phytotoxic components corn. Agriculture, Ecosystems and of wheat (Triticum aestivum L.) straw. Environment. 65:23-32. Journal of Chemical . 9:1027- Reddy, K.C., A.R. Soffes, G.M. Prine, and R.A. 1043. Dunn. 1986 Tropical legumes for green Lu, Y.C., K.B. Watkins, J.R. Teasdale, and A.A. manure. II. Nematode populations and Abdul-Baki. 2000. Cover crops in their effects on succeeding crop yields. sustainable food production. Food Agronomy Journal. 78:5-10. Reviews International. 16:121-157. Rhoades, H.L., and R.B. Forbes. 1986. Effects McSorley, R., and D.W. Dickson. 1995. Effect of fallow, cover crops, organic , of tropical rotation crops on Meloidogyne and fenamiphos on nematode incognita and other plant-parasitic populations, soil nutrients, and cash crop nematodes. Supplement to the Journal of growth. Nematropica. 16:141-151. Nematology. 27:535-544.

Organic Production—Cover Crops for Organic Farms 18 Roberts, R.K., J.A. Larson, D.D. Tyler, B.N. Sullivan, P.G., D.J. Parrish, and J.M. Luna. Duck, and K.D. Dillivan. 1998. Economic 1991. Cover crop contributions to N analysis of the effects of winter cover supply and water conservation in corn crops on no-tillage corn yield response to production. American Journal of applied nitrogen. Journal of Soil and Alternative Agriculture. 6:106-113. Water Conservation. 53:280-284. Teasdale, J.R. 1993. Interaction of light, soil Roberts, B.W., and B. Cartwright. 1991. Cover moisture, and temperature with weed crop, nitrogen, and insect interactions. In suppression by hairy vetch residues. Weed W.L. Hargrove (Ed.). Cover Crops for Clean Science. 41:46-51. Water. pp.164-167. Soil and Water Teasdale, J.R., and C.S.T. Daughtry. 1993. Conservation Society. Ankeny, IA. Weed suppression by live and desiccated Rodriguez-Kabana, R., J. Pinochet, D.G. hairy vetch. Weed Science. 41:207-212. Robertson, C.F. Weaver, and P.S. King. Tyler, D.D.,M.G. Wagger, D.V. McCracken, 1992. Horsebean (Canavalia ensiformis) W.L. Hargrove, and M.R. Carter. 1994. and crotalaria (Crotalaria spectabilis) for Role of conservation tillage in sustainable the management of Meloidogyne spp. agriculture in the southern United States. Nematropica. 22:29-35. Conservation Tillage in Temperate Sarrantonio, M. 1998. Building Agroecosystems. pp. 209-229. Lewis and tilth with cover crops. In A. Clark Publishers Inc. Boca Raton, FL. (Ed.). Managing Cover Crops Profitably, 2nd University of California at Davis (UCD). 2001. Ed. pp. 22-23. University of California Sarep Cover Crop Network, Handbook Series 3. Beltsville, Resource Page. Online: MD. http://www.sarep.ucdavis.edu/ccrop/ Sarrantonio, M. 1994. Northeast Cover Crop Wagger, M.G. 1989a. Time of dessication Handbook. Rodale Institute. Emmaus, PA. effects on plant composition and Schilling, D.G., R. A. Liebl, and A.D. subsequent nitrogen release from several Worsham. 1985. Rye (Secale cereat L.) and winter annual cover crops. Agronomy wheat (Triticum aestivum L.) mulch: The Journal. 81:236-241. suppression of certain broadleaved weeds Wagger, M.G. 1989b. Cover crop and the isolation and identification of management and nitrogen rate in relation phytotoxins. In A.C. Thompson (Ed.). The to growth and yield of no-till corn.. Chemistry of Allelopathy: Biochemical Agronomy Journal. 81:533-538. Interactions Among Plants. pp. 243-271. White, R.H., A.D. Worsham, and U. Blum. Symp. Ser. 268. American Chemical 1989. Allelopathic potential of legume Society. Washington, D.C. debris. Weed Science. 37:674-679. Shipley, P.R., J.J. Meisinger, and A.M. Decker. 1992. Conserving residual corn fertilizer Additional Reading nitrogen with winter cover crops. Araya, M., and E.P. Caswell-Chen. 1994. Agronomy Journal. 84:869-876. Penetration of Crotalaria juncea, Dolichos Smeda, R.J., and S.C. Weller. 1996. Potential lablab and Sesamum indicum roots by of rye (Secale cereale) for weed Meloidogyne javanica. Journal of management in transplant tomatoes Nematology. 26:238-240. (Lycopersicum esculentum). Weed Science. Bergersen, F.J., J. Brockwell, R.R. Gault, L. 44:596-602. Morthorpe, M.B. Peoples, and G.L. Subbarao, K.V., and J.C. Hubbard. 1996. Turner. 1989. Effects of available soil Interactive effects of broccoli residue and nitrogen and rates of inoculation on temperature on Verticillium dahliae micro- nitrogen fixation by irrigated soybeans sclerotia in soil and on wilt in cauliflower. and evaluation of delta 15N methods for Phytopathology. 86:1303-1310.

Organic Production—Cover Crops for Organic Farms 19 measurement. Australian Journal of Einhellig, F.A. 1996. Interactions involving Agricultural Research. 40:763-780. allelopathy in cropping systems. Boosalis, M.G., and G.E. Cook. 1973. Plant Agronomy Journal. 88:886-893. diseases. In Conservation Tillage, Conference Ess, D.R., D.H. Vaughan, J.M. Luna, and P.G. Proceedings. pp. 114-125. Soil Sullivan. 1994. Energy and economic Conservation Society of America. Ankeny, savings from the use of legume cover IA. crops in Virginia corn production. Brunson, K.E., C.R. Stark, Jr., M.E. Wetzstein, American Journal of Alternative and S.C. Phatak. 1995. Economic Agriculture. 9:178-185. comparisons of alternative and Hargrove, W.L. 1991. Cover Crops for Clean conventional production technologies for Water. Proceedings of an International eggplant in souther Georgia. Journal of Conference. West Tennessee Experiment . 13:159-173. Station. April 9-11, 1991. Jackson, TN. Bugg, R.L., F.L. Wackers, K.E. Brunson, J.D. Soil and Water Conservation Society. Dutcher, and S.C. Phatak. 1991. Cool- Ankeny. 198 pp. season cover crops relay intercropped Haynes, R.J. 1980. Competitive aspects of the with cantaloupe: Influence on a grass-legume association. Advances in generalistic predator, Geocoris punctipes Agronomy. 33:227-261. (Hemiptera: Lygaeidae). Journal of Holderbaum, J.F., A.M. Decker, J.J. Meisinger, Economic Entomology. 84:408-415. F.R. Mulford, and L.R. Vough. 1990. Fall Burgos, N.R., and R.E. Talbert. 1996. Weed seeded legume cover crops for no-tillage control by spring cover crops and corn in the humid East. Agronomy imazethapyr in no-till southern pea Journal. 82:117-124. (Vigna unguiculata). Weed Technology. Kelly, T.C., Y.C. Lu, A.A. Abdul-Baki, and J.R. 10:893-899. Teasdale. 1995. Economics of a hairy Chapman, A.L., and R.J.K. Myers. 1987. vetch mulch system for producing fresh- Nitrogen contributed by grain legumes to market tomatoes in the mid-Atlantic rice grown in rotation on the Cununurra region. Journal of the American Society of soils of the Ord irrigation area, Western Horticultural Science. 120:854-860. Australia. Australian Journal of Kloepper, J.W., R. Rodriguez-Kabana, J.A. Experimental Agriculture. 27:155-163. McInroy, and R.W. Young. 1992. Creamer, N.G., M.A. Bennett, B.R. Stinner, Rhizosphere bacteria antagonistic to and J. Cardina. 1996b. A comparison of soybean cyst (Heterodera glycines) and four processing tomato production root-knot (Meloidogyne incognita) systems differing in cover crop and nematodes: Identification by fatty acid chemical inputs. Journal of the American analysis and frequency of biological Society of Horticultural Science. 121:559- control activity. Plant and Soil. 139:75-84. 568. Kuo, S., E.J. Jellum, and U.M. Sainju. 1995. Dillard, H.R., and R.G. Grogan. 1985. The effect of winter cover cropping on Influence of green manure crops and soil and water quality. Proceedings of the lettuce on sclerotial populations of Western Nutrient Management Conference. Sclerotinia minor. Plant Disease. 69:579- pp.56-64. Salt Lake City, UT. 582. McCracken, D.V., M.S. Smith, J.H. Grove, C.T. Ditsch, D.C., M.M. Alley, K.R. Kelley, and Y.Z. MacKown, and R.L. Blevins. 1994. Nitrate Lei. 1993. Effectiveness of winter rye for leaching as influenced by cover cropping accumulating residual fertilizer N and nitrogen source. Soil Science Society following corn. Journal of Soil and Water of America Journal. 58:1476-1483. Conservation. 48:125-132. Mojtahedi, H., G.S. Santo, and R.E. Ingham. 1993. Suppression of Meloidogyne

Organic Production—Cover Crops for Organic Farms 20 chitwoodi with sudangrass cultivars as Soil Science Society of America Journal. green manure. Journal of Nematology. 59:1587-1594. 25:303-311. Roberson, E.B., S. Sarig, and M.K. Firestone. Muller, M.M., V. Sundman, O. Soininvaara, 1991. Cover crop management of and A. Merilainen. 1988. Effect of polysaccharide-mediated aggregation in chemical composition on the release of an soil. Soil Science Society of nitrogen from agricultural plant materials America Journal. 55:734-739. decomposing in soil under field Roberts, J.L., and F.R. Olson. 1942. conditions. Biology and Fertility of Soils. Interrelationships of legumes and grasses 6:78-83. grown in association. Agronomy Journal. Nwonwu, F.O.C., and P.C. Obiaga. 1988. 32:695-701. Economic criteria in the choice of weed- Sainju, U.M., and B.P. Singh. 1997. Winter control methods for young pine (Pinus cover crops for sustainable agricultural caribaea var. hundurensis Barr and Golf) systems: Influence on soil properties, plantations. Weed Research. 28:181-184. water quality, and crop yields. Ofori, C.F., and W.R. Stern. 1987. Cereal- HortScience. 32:21-28. legume intercropping systems. Advances Shennan, C. 1992. Cover crops, nitrogen in Agronomy. 26:177-204. cycling, and soil properties in semi- Patrick, Z.A., T.A. Toussoun, and W.C. irrigated vegetable production systems. Snyder. 1963. Phytotoxic substances in HortScience. 27:749-754. arable soils associated with Smith, M.S., W.W. Frye, and J.J. Varco. 1987. decomposition of plant residues. Legume winter cover crops. Advances in Phytopathology. A53:152-161. Soil Science. 7:95-139. Peoples, M.B, and E.T. Craswell. 1992. Wyland, L.J., L.E. Jackson, W.E. Chaney, K. Biological nitrogen fixation: Investments, Klonsky, S.T. Koike, and B. Kimple. 1996. expectations, and actual contributions to Winter cover crops in a vegetable agriculture. Plant and Soil. 141:13-39. cropping system: Impacts on nitrate Phillips, S.H. 1984. Other pests in no-tillage leaching, soil water, crop yield, pests and and their control. In R.E. Phillips and S.H. management costs. Agriculture, Phillips (Eds.). No Tillage Agriculture. pp. Ecosystems and Environment. 59:1-17. 171-189. Van Nostrand Reinhold Co. New York, NY. Power, J.F., and J.W. Doran. 1988. Role of Crop Residue Management in Nitrogen Cycling and Use. ASA Special Publication No. 51. pp. 101-113. American Society of Agronomy. Madison, WI. Ranells, N.N., and M.G. Wagger. 1992. Nitrogen release from crimson clover in relation to plant growth stage and composition. Agronomy Journal. 84:424- 430. Rice, E.L. 1974. Allelopathy. Academic. New York, NY. Roberson, E.B., S. Sarig, C. Shennan, and M.K. Firestone. 1995. Nutritional management of microbial polysaccharide production and aggregation in an agricultural soil.

Organic Production—Cover Crops for Organic Farms 21

The Organic Production publication series was developed by the Center for Environmental Farming Systems,

a cooperative effort between North Carolina State University, North Carolina A&T State University, and the North Carolina Department of Agriculture and Consumer Services.

The USDA Southern Region Sustainable Agriculture Research and Education Program and the USDA Initiative for Future Agriculture and Food Systems Program provided funding in support of the Organic Production publication series. David Zodrow and Karen Van Epen of ATTRA contributed to the technical writing, editing, and formatting of these publications.

Prepared by

Keith R. Baldwin, Program Leader, ANR/CRD Extension Specialist—Horticulture North Carolina A&T State University

Nancy G. Creamer, Director, Center for Environmental Farming Systems North Carolina State University College of Agriculture and Life Sciences

Published by NORTH CAROLINA COOPERATIVE EXTENSION SERVICE

AG-659W-03 07/2006—BS E06-45788

Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, disability, or veteran status. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.