Crop Response, Weed Management Systems, and Tank Mix Partners with Isoxaflutole in HPPD Tolerant Cotton

Home , Trifluralin

Delaney Caitlin Foster, B.S.

A Thesis

Plant and Soil Science

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCE

Approved

Dr. Peter A. Dotray Chair of Committee

Dr. Frederick T. Moore

Dr. J. Wayne Keeling

Dr. Cade Coldren

Mark Sheridan Dean of the Graduate School

May, 2021

ACKNOWLEDGMENTS

First, I’d like to thank my committee chair and mentor Dr. Peter Dotray for taking a chance on this South Georgia girl from ABAC. Dr. Dotray truly provided me with every resource I could ever need to succeed and for that I am forever thankful.

His goal was for me to leave west Texas and share with others about my time here in the same manner I talked about my experiences in Georgia (excitedly)… I’d say he accomplished just that. Thank you for being a great mentor and providing me endless learning experiences.

Thanks are also in order to my committee members. Dr. Wayne Keeling always helped me see the big picture, I hope one day growers trust my professional opinion as much as they do his. Dr. Keeling challenged my weed ID skills more than a few times each summer with samples brought to him by growers and consultants; this seemingly small real-world experience was invaluable. Thank you to Dr. Frederick

Moore with BASF Corporation for bringing an industry perspective to the table and for advocating the importance of the IFT work, as well as helping me share my results at conferences and meetings. Dr. Cade Coldren taught the first graduate class I attended and gave me a great outside look on statistics while always being excited to learn about weed science.

A special thank you to Dr. Corey Thompson with BASF for the great advice, technical support, and answering my unending questions about this project… he could’ve easily defended my thesis better than I. Bobby, Grace, Kyle, Jon, Ubaldo, and Andrew, thank you for your companionship and support. Without all of you, I’d still be measuring plant heights in the field. To my best friend Taylor Randell, thank ii Texas Tech University, Delaney Caitlin Foster, May 2021 you for the advice and weekly phone calls from Georgia. Even moving halfway across the country, we pick up where we left off every time.

I appreciate the BASF Corporation employees who attended my field tours and conference presentations. Each of you met my project with excitement for this future technology and I’m thankful I was able to share this work with the folks who will be promoting it soon. I have no doubt these opportunities helped prepare me for professional life in the future. I would also like to thank Texas Tech University for the opportunity to research and learn in and outside of the classroom as well as Texas

A&M AgriLife Research and Extension for the use of resources and personnel.

Without either institution, my research would not have been possible.

Finally, thank you to my family – Mom, Dad, Brody, and Kelsi, for all of their love and support while I “make a career out of school”. Your encouragement means the world.

iii Texas Tech University, Delaney Caitlin Foster, May 2021

TABLE OF CONTENTS ACKNOWLEDGMENTS ...... ii

LIST OF TABLES ...... v

CHAPTER I: LITERATURE REVIEW ...... 1

Literature Cited ...... 12 CHAPTER II: COTTON RESPONSE TO HERBICIDE SYSTEMS USING ISOXAFLUTOLE ...... 18

Abstract ...... 18 Introduction ...... 20 Materials and Methods ...... 22 Results and Discussion ...... 25 Literature Cited ...... 30 CHAPTER III: WEED MANAGEMENT IN HERBICIDE SYSTEMS USING ISOXAFLUTOLE ...... 47

Abstract ...... 47 Introduction ...... 49 Materials and Methods ...... 51 Results and Discussion ...... 52 Literature Cited ...... 55 CHAPTER IV: TANK MIX PARTNERS WITH ISOXAFLUTOLE ACROSS THE COTTON BELT ...... 63

Abstract ...... 63 Introduction ...... 65 Materials and Methods ...... 67 Results and Discussion ...... 68 Literature Cited ...... 80

iv Texas Tech University, Delaney Caitlin Foster, May 2021

LIST OF TABLES Table 2.1. Treatments, application timing, herbicide, and rates used in crop tolerance experiments at New Deal and Lubbock in 2019 and 2020...... 33 Table 2.2. Cotton response 14 days after planting at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 34 Table 2.3. Cotton response 14 days after the early postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 35 Table 2.4. Cotton response 7 days after the mid-postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 36 Table 2.5. Cotton response 10 days after the postemergence-directed application at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 37 Table 2.6. Cotton density 21 days after planting at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 38 Table 2.7. Cotton heights 14 days after the early postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 39 Table 2.8. Cotton lint yield at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 40 Table 2.9. Cotton heights at harvest at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 41 Table 2.10. Cotton micronaire measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 42 Table 2.11. Cotton length measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 43 Table 2.12. Cotton uniformity measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 44 Table 2.13. Cotton strength measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 45 Table 2.14. Cotton elongation measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials...... 46 Table 3.1. Herbicide treatments, rates, and application timings used in non- crop weed control experiments at Halfway, TX in 2019 and 2020...... 57 Table 3.2 Palmer amaranth control 14 days after the preemergence application at Halfway, TX in 2019 and 2020 systems trials...... 58 Table 3.3. Palmer amaranth control 21 days after the preemergence application at Halfway, TX in 2019 and 2020 systems trials...... 59

v Texas Tech University, Delaney Caitlin Foster, May 2021

Table 3.4. Palmer amaranth control and counts 21 days after the early postemergence application at Halfway, TX in 2019 and 2020 systems trials...... 60 Table 3.5. Palmer amaranth control 21 days after the mid-postemergence application at Halfway, TX in 2019 and 2020 systems trials...... 61 Table 3.6. Palmer amaranth control 10 days after the postemergence-directed application at Halfway, TX in 2019 and 2020 systems trials...... 62 Table 4.1. Details for locations of field experiments in 2019 and 2020...... 83 Table 4.2. Details for applications of field experiments in 2019 and 2020...... 84 Table 4.3. Preemergence treatments and herbicide rates used in weed control experiments across the cotton belt in 2019 and 2020...... 85 Table 4.4. Palmer amaranth control and density in Halfway, TX in 2019 and 2020...... 86 Table 4.5. Palmer amaranth control and density in Marianna, AR in 2019 and 2020...... 87 Table 4.6. Palmer amaranth control in Bixby, OK in 2019 and 2020...... 88 Table 4.7. Palmer amaranth control and density in College Station, TX in 2019 and 2020...... 89 Table 4.8. Palmer amaranth control and density in Ideal, GA in 2019 and 2020...... 90 Table 4.9. Palmer amaranth control and density in Jackson, TN in 2019 and 2020...... 91 Table 4.10. Palmer amaranth control and density in Dundee, MS in 2019 and 2020...... 92 Table 4.11. Large crabgrass control and density in Marianna, AR in 2019...... 93 Table 4.12. Large crabgrass control and density in Ideal, GA in 2019 and 2020...... 94 Table 4.13. Large crabgrass control in Bixby, OK in 2019 and 2020...... 95 Table 4.14. Morningglory species control and density in Marianna, AR in 2020...... 96 Table 4.15. Morningglory species control in Bixby, OK in 2019 and 2020...... 97 Table 4.16. Morningglory species control and density in College Station, TX in 2019 and 2020...... 98 Table 4.17. Morningglory species control and density in Jackson, TN in 2020...... 99 Table 4.18. Weed control and density in Stillwater, OK in 2019 and 2020...... 100

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Table 4.19. Weed control and density in San Angelo, TX in 2019 and 2020...... 101

vii Texas Tech University, Delaney Caitlin Foster, May 2021

CHAPTER I

LITERATURE REVIEW

The history of cotton production dates back 10 to 20 million years where older varieties became domesticated in southern Asia and Africa. Over time the species evolved into the upland cotton, Gossypium hirsutum, that is grown today and has origins from Mesoamerica. Upland cotton is a woody perennial plant that humans have domesticated and grow as an annual crop. United States cotton production began circa 1605 in the southeastern colonies of Virginia, the Carolinas, and Georgia.

Between 1850 and 1950, United States cotton production increased five-fold over this

100 year period (Smith and Cothren 1999). Much of this increase can be attributed to the growth of the American textile industry as well as technological advances in both ginning and farm equipment.

More than five million hectares of cotton were planted in the United States in

2018, of which 56% were planted in Texas (USDA NASS 2018). The High Plains spanning from the northern and western borders of Texas to the Pecos River, is the largest contiguous cotton producing region in the nation and home to 66% of Texas cotton and cottonseed production (Plains Cotton Growers 2020). Cotton growers in the

United States and on the High Plains face a number of challenges such as drought, extreme weather events, diseases, nematodes, insects, weeds, and other agronomic pests.

Average annual precipitation in the High Plains is 45 centimeters, which is low in comparison to other cotton producing regions of the United States that can receive 1

Texas Tech University, Delaney Caitlin Foster, May 2021 up to 150 centimeters (National Weather Service 2008). In addition to low rainfall, the

High Plains is located above the Ogallala aquifer, a large underground water reservoir from which humans are drawing faster than it can be recharged (McGuire 2017).

These challenging environmental conditions not only make it difficult to grow dryland cotton on the High Plains, but also make it difficult to provide full irrigation because of reduced well capacity. The driest year on record was 2011 and Texas cotton producers suffered an estimated economic loss of $2.2 billion caused by drought

(Anderson et al. 2012). The High Plains also differs from other cotton regions by having a shorter growing season due to cold temperatures late in the spring and early in the fall. Other natural disasters and extreme weather events are uncontrollable challenges that cotton producers face. Hailstorms can have a significant impact on cotton production, causing total loss in some cases. McGinty et al. (2019) reported anywhere between 18 to 84% stand loss from simulated hail damage to cotton.

Hurricanes, in other parts of the country, can be detrimental as well. Hurricane

Michael caused a multi-billion dollar loss to the row crop industry in the southeast in

2018 (Martin 2018).

While the forces of nature are out of producers’ control, the negative impact of agronomic pests such as pathogens, plant parasitic nematodes, insects, and weeds can be mitigated to a large degree. In west Texas, diseases such as verticillium wilt,

Verticillium dahliae, can cause cotton yield loss of 2,400 kilograms per hectare; however, many of these disease-induced losses can be overcome by planting resistant varieties (Chawla et al. 2012). Nematodes are the leading plant pathogen in terms of

Texas Tech University, Delaney Caitlin Foster, May 2021 yield loss in Texas, causing a 3.1% loss in 2018. The impact of nematodes can successfully be reduced by the use of crop rotation and in-furrow nematicides

(Lawrence et al. 2019; Texas A&M AgriLife Extension 2020). Perhaps the most important insect to devastate the cotton industry was the boll weevil, Anthonomus grandis. Prior to the success of the Boll Weevil Eradication Program in the 1970’s, cotton loss to the boll weevil were estimated around one-third of the yield potential

(Lange et al. 2009). More recent estimates of Texas High Plains cotton loss due to insects, commonly bollworms, thrips, and stinkbugs is 1% (Cook and Cutts 2018).

One of the most detrimental pests to cotton production are weeds. Weeds cause an average yield loss of 34% if not controlled properly (Oerke 2006). According to the

North American 2019 survey of most common and troublesome weeds in broadleaf crops, Palmer amaranth (Amaranthus palmeri S. Watson) and morningglory (Ipomoea spp) were the top two both most common and troublesome weeds in cotton production

(Van Wychen 2019). Other weeds on those lists include horseweed (Conyza canadensis L.), crabgrass (Digitaria spp), and barnyardgrass (Echinochloa crus-galli

L.). Common and troublesome weeds found in the Texas High Plains include Palmer amaranth, kochia (Bassia scoparia L.), and Russian thistle (Salsola tragus L.).

Weed management decisions have been a driving factor in agricultural production for almost 10,000 years, beginning with hand-weeding when humans began to domesticate crops (Bell 2015). In more recent centuries, mechanical weed control decreased the need for hand-pulling with the invention of mule-drawn plows, followed by self-propelled tractors and various implements to mechanically remove

Texas Tech University, Delaney Caitlin Foster, May 2021 weeds (Timmons 2005). Current methods of mechanical weed control still include inter-row cultivation as well as implements such as rod-weeders used prior to planting to remove weeds and prepare the soil for planting. In the 1990’s, using a moldboard plow or chisel plow were common practices; however, a moldboard plow was found to be more effective than the chisel plow in both soybean (Glycine max L. Merr.) and corn (Zea mays L.) (Cox et al. 1999). These two methods of mechanical weed control suppress weed emergence by deep burial of seed. When weed pressure is moderately low, in-row cultivation and rotary hoeing can be successful means of weed control without suffering stand loss in dry beans (Phaseolus vulgaris L.) and corn (Amador-

Ramirez et al. 2001; Vangessel et al. 1995; Vangessel et al. 1998). Current research in specialty crops looking at “futuristic” means of mechanical weed control, such as precision robotic weeders and drone-mounted sprayers, is being conducted; however, these methods are still years from being adopted commercially if they succeed.

Chemical control of weeds was discovered in the most recent century. In the early 1900’s, European scientists began noticing that mixtures of heavy metal salts such as iron sulfate could be used for selective weed control in cereal crops (Zimdahl

2010). This was followed by the discovery and use of petroleum fractions (diesel oil) called naphtha and smudge-pot oil that were first used to kill vegetation on railways and roadsides (Crafts and Reiber 1948; Robinson 1973). The first phenoxy herbicides were discovered in the 1940’s, which provided growers a selective broadleaf herbicide and began the chemical era of agriculture. This was followed by the discovery and use of the triazines in following decades (LeBaron and Muller 2008). Prior to the 1960’s,

Texas Tech University, Delaney Caitlin Foster, May 2021 flame cultivation was an attempted control method, but was replaced by more effective chemical herbicide options due to elevated gas prices and unacceptable crop injury (Seifert and Snipes 1998). In 1963, the first dinitroaniline herbicide, trifluralin, was available in cotton and was quickly and widely accepted. Other herbicides such as fluometuron and prometryn soon followed. These herbicides, which are still used today, introduced a new concept of soil applied herbicides and provided exceptional control of many annual grass and small-seeded broadleaf weeds (Buchanan 1992).

In 1960, herbicides accounted for 18% of United States pesticide use; however, that number grew to 76% by 2008 because of new herbicide chemistries and advances in herbicide tolerant crop genetics (Fernandez-Cornejo et al. 2014). Diclofop-methyl was introduced in 1975 as the first acetyl CoA carboxylase inhibitor. This group of herbicides controlled grass weeds in broadleaf crops exceptionally well (Appleby

2005). In 1993, pyrithiobac was available as the first selective postemergence over the top herbicide for control of broadleaf weeds in cotton (Keeling et al. 1993). The most recent class of chemistry to be developed were the HPPD, or p- hydroxyphenylpyruvate dioxygenase inhibiting herbicides, which were introduced in the 1980’s (Duke 2011; Mitchell et al. 2001).

Weed species present in agricultural production fields have shifted over time based on weed management decisions made each year and over years. Selective herbicides are more effective on certain weeds than others, causing a shift in weed populations with each new herbicide discovery and new herbicide tolerant crop technology (Weber et al. 1974). Bromoxynil tolerant cotton (BXN) was

Texas Tech University, Delaney Caitlin Foster, May 2021 commercialized in 1995 as the first transgenic herbicide tolerant cotton. The BXN gene, which results in bromoxynil metabolism, was cloned from bacteria and transferred to plants. The expression of the BXN gene allowed cotton to tolerate postemergence over the top applications of bromoxynil, a herbicide that is phytotoxic to wild type cotton (Reddy 2004; Stalker et al. 1988). Bromoxynil was successful at controlling many problematic weeds including morningglory, common cocklebur

(Xanthium strumarium L.), and spurred anoda (Anoda cristata L. Schltdl); however, the herbicide exhibited little to no activity on Palmer amaranth. This technology was quickly phased out following the introduction of first-generation glyphosate tolerant

(Roundup Ready) cotton in 1997 (Troxler et al. 2002).

In 1970, glyphosate was synthesized by Monsanto to be screened in greenhouse trials as an herbicide (Dill et al. 2010). Initially used for burndown and spot treatments, glyphosate was an effective nonselective herbicide in many cropping systems. Glyphosate was first available as an over-the-top option in transgenic cotton in 1997 and its use rapidly expanded while use of other herbicide groups such as photosystem II inhibitors and dinitroanilines declined. By 2002, glyphosate became the leading herbicide used in cotton in terms of treated area (Webster and Nichols

2012).

Glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, a key enzyme for aromatic amino acid synthesis. In order for cotton to achieve tolerance to glyphosate, a modified CP4 EPSP synthase enzyme was introduced (Kishore et al.

1992). The initial Roundup Ready® cotton was tolerant to applications of glyphosate

Texas Tech University, Delaney Caitlin Foster, May 2021 through the four-leaf stage, but later applications at reproductive stages caused yield reductions of 21 to 55% (Light et al. 2003). Second generation glyphosate tolerant

(Roundup Ready Flex®) cotton became available in 2006 and allowed for an extended window of application in-season (Huff et al. 2010; Miller et al. 2008). When first introduced, in-season applications of glyphosate over-the-top were effective at controlling many Amaranthus and annual grass species, but were inadequate at controlling most morningglory, dayflower (Commelina spp), and sedges (Cyperus spp). These weeds became more problematic because tillage practices decreased with the adoption of Roundup Ready cropping systems (Culpepper 2006).

In the first five years after adopting glyphosate resistant cotton, the average number of glyphosate applications nearly doubled because of the ability to make in- season over-the-top applications (Young 2006). Over-reliance on postemergence applications of glyphosate did not properly steward the Roundup Ready technology and in 2001, glyphosate resistant weeds were reported in Roundup Ready cotton systems in Tennessee when horseweed was confirmed as the first glyphosate resistant weed in cotton (Heap 2020; Steckel and Gwathmey 2009). Four years later, the first case of glyphosate resistant Palmer amaranth was confirmed in Georgia (Culpepper et al. 2006). Today, every cotton producing state in the nation has confirmed glyphosate resistant Palmer amaranth populations (Heap 2020). The spread of glyphosate resistant

Palmer amaranth across the southern United States initiated another weed species shift, placing Palmer amaranth at the top of the list of problematic weeds in the coming years.

Texas Tech University, Delaney Caitlin Foster, May 2021

In 2004, glufosinate tolerant (LibertyLink®) cotton was commercialized.

Transgenic cotton tolerant to both glufosinate and glyphosate (GlyTol LibertyLink®) was commercialized in 2011 (Reed et al. 2014). Crops tolerant to glufosinate have the bialaphos resistance (BAR) gene inserted that results in glufosinate detoxification

(Blair-Kerth et al. 2001). One weakness of glufosinate is its inconsistent weed control in dry, hot, and low humidity environments (Steckel et al. 1997). Glufosinate also should be applied so that adequate coverage is obtained because it is a contact herbicide.

In 2016, the next transgenic events became commercially available in cotton when 2,4-D (Enlist™) and dicamba (XtendFlex®) tolerant cotton varieties were available for use. These technologies were developed through the insertion of the

AAD-1 (aryloxyalkanoate dioxygenase-1) transgene and dicamba monooxygenase gene, respectively, which results in herbicide detoxification (Behrens et al. 2007;

Braxton et al. 2017; Inman et al. 2016). The following year, low volatility herbicide formulations received Federal 3 label status for use in these new cotton technologies.

These two auxinic herbicides selectively control broadleaf weeds such as Palmer amaranth, and when applied in a timely manner are effective at controlling weeds postemergence (Cahoon et al. 2015; Manuchehri et al. 2017). Use of these herbicides comes with the risk of off-target movement and tank contamination, which can be detrimental to sensitive crops such as peanut, non-tolerant cotton and soybean, and specialty crops at low concentrations (Culpepper et al. 2018).

Texas Tech University, Delaney Caitlin Foster, May 2021

Palmer amaranth is a weed native to the dry southwestern United States and

Mexico (Sauer 1950). In a 2019 survey, Palmer amaranth was ranked as the most common and troublesome weed species among all broadleaf crops, fruits, and vegetables (Van Wychen 2019). Among the four most common Amaranthus species,

Palmer amaranth has the greatest leaf number, dry matter, and fastest growth rate per growing degree days (Horak and Loughin 2000). Steckel et al. (2004) determined that

Palmer amaranth has the greatest germination rate of the Amaranthus species. Over reliance on herbicides has led to populations of two herbicide resistant Amaranthus species in Texas (Heap 2020). In a recent state-wide survey of Palmer amaranth populations, samples collected from the High Plains Region had the greatest number of populations that were resistant or less sensitive to glyphosate, pyrithiobac, and atrazine (Garetson et al. 2019).

Palmer amaranth has the potential to make mechanical cotton harvest more difficult when weeds become entangled in harvest equipment. In dryland stripper cotton, it has been reported that machine stoppages required to dislodge large weed stems added an additional 74 to 183 minutes per hectare of harvest time in weedy cotton (Smith et al. 2000). Harvesting cotton with a picker was considered impractical when Palmer amaranth densities exceeded six plants per 9.1 m of row (Morgan et al.

2001).

Soil residual herbicides are an important tool for cotton growers because of their efficacy and ability to control potentially resistant Palmer amaranth before they emerge. Critical weed-free periods vary by crop. In cotton, an eight week weed-free

Texas Tech University, Delaney Caitlin Foster, May 2021 period after emergence is needed to ensure minimal yield loss due to weed competition (Buchanan and Burns 1970). In Georgia, Palmer amaranth that emerged in cotton between 12- and 17-leaves had no effect on yield while earlier emerging

Palmer amaranth decreased cotton lint yield (MacRae et al. 2013). Fortunately, there are several options for preemergence herbicides in cotton; however, the majority of these herbicides are photosystem II or acetolactate synthase (ALS) inhibitors, which are two groups where a large number of resistant weed species have developed.

Isoxaflutole is a Herbicide Resistance Action Committee (HRAC) Group F2 herbicide that inhibits the plant essential enzyme p-hydroxyphenylpyruvate dioxygenase (Herbicide Resistance Action Committee 2020). P- hydroxyphenylpyruvate inhibitors are part of a larger group of carotenoid biosynthesis inhibitors. Herbicides in this group deplete plastoquinones, an essential electron acceptor in the carotenoid biosynthetic pathway, which are essential for plant life as they protect chlorophyll molecules from photooxidation caused by light. This generates singlet oxygen in the absence of carotenoids (Beaudegnies et al. 2009).

Once the carotenoid biosynthesis pathway is blocked and the formation of new carotenoids is stopped, all new plant growth show symptomology that resembles

“bleaching” or white colored meristematic tissue (Lee et al. 1997). Lipid peroxidation causes eventual plant death.

Isoxaflutole received Federal 3 label status in 1998 and has been an effective herbicide at controlling a number of annual grass and broadleaf weed species in field corn (Environmental Protection Agency 1998). When used as part of a preemergence

Texas Tech University, Delaney Caitlin Foster, May 2021 herbicide program, isoxaflutole provided up to 95% Palmer amaranth control three weeks after application (Meyer et al. 2016). Johnson et al. (2012) found that isoxaflutole applied alone provided from 87 to 99% Palmer amaranth control eight weeks after application.

While current cotton varieties do not tolerate HPPD inhibitors, BASF

Corporation has developed HPPD-tolerant cotton that will allow growers to use isoxaflutole in future weed management programs. Adding a new mode of action to cotton weed management such as an HPPD inhibitor like isoxaflutole may increase the diversity of weed management programs, delaying the development of herbicide resistant weeds (Anderson and Hartzler 2016). According to the International Survey of Herbicide Resistant Weeds database (Heap 2020), Palmer amaranth and tall waterhemp (Amaranthus rudis Sauer) have confirmed resistance to HPPD inhibitors in the midwestern United States where these herbicides are commonly used in corn. It would be beneficial to determine the best way to integrate isoxaflutole into current local weed management programs. In addition, there would be value determining effective tank mix partners labelled for use in cotton to extend soil residual weed control and delay the development of resistance.

Texas Tech University, Delaney Caitlin Foster, May 2021

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LeBaron HM, Muller G (2008) Biology and ecology of weeds and the impact of triazine herbicides in LeBaron H, McFarland J, Burnside O, eds. The Triazine Herbicides: 50 Years Revolutionizing Agriculture 63-72

Lee DL, Prisbylla MP, Cromartie TH, Dagarin DP, W.Howard S, Provan WM, Ellis MK, Fraser T, Mutter LC (1997) The discovery and structural requirements of inhibitors of p-hydroxyphelynpyruvate dioxygenase. Weed Technol 45:601- 609

Texas Tech University, Delaney Caitlin Foster, May 2021

Light GG, Baughman TA, Dotray PA, Keeling JW, Wester DB (2003) Yield of glyphosate-tolerant cotton as affected by topical glyphosate applications on the Texas High Plains and Rolling Plains. J of Cotton Sci 7:231-235

MacRae AW, Webster TM, Sosnoskie LM, Culpepper AS, Kichler JM (2013) Cotton yield loss potential in response to length of Palmer amaranth (Amaranthus palmeri) interference. J of Cotton Sci 17:227-232

Manuchehri MR, Dotray PA, Keeling JW (2017) Enlist weed control systems for Palmer amaranth (Amaranthus palmeri) management in Texas High Plains cotton. Weed Technol 31:793-798

Martin J (2018) Georgia farmers face long recovery from hurricane michael crop losses. Insurance Journal. 1 p

McGinty J, Morgan G, Mott D (2019) Cotton response to simulated hail damage and stand loss in central texas. J of Cotton Sci 23:1-6

McGuire VL (2017) Water-level and recoverable water in storage changes, High Plains aquifer, predevelopment to 2015 and 2013-2015. U.S. Geological Survey Scientific Investigations Report 2017. 14 p https://doi.org/10.3133/sir20175040

Meyer CJ, Norsworthy JK, Young BG, Steckel LE, Bradley KW, Johnson WG, Loux MM, Davis VM, Kruger GR, Bararpour MT, Ikley JT, Spaunhorst DJ, Butts TR (2016) Early-season Palmer amaranth and waterhemp control from preemergence programs utilizing 4-hydroxyphenylpyruvate dioxygenase– inhibiting and auxinic herbicides in soybean. Weed Technol 30:67-75

Miller DK, Zumba JX, Blouin DC, Bagwell R, Burris E, Clawson EL, Leonard BR, Scroggs DM, Stewart AM, Vidrine PR (2008) Second-generation glyphosate- resistant cotton tolerance to combinations of glyphosate with insecticides and mepiquat chloride. Weed Technol 22:81-85

Mitchell G, Bartlett DW, Fraser TE, Hawkes TR, Holt DC, Townson JK, Wichert RA (2001) Mesotrione: a new selective herbicide for use in maize. Pest Manage Sci 57:120-128

Morgan GD, Baumann PA, Chandler JM (2001) Competitive impact of Palmer amaranth (Amaranthus palmeri) on cotton (Gossypium hirsutum) development and yield. Weed Technol 15:408-412

National Weather Service Lubbock TX (2008) Official Precipitation and its Collection History Accessed March 28, 2020

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Oerke EC (2006) Crop losses to pests. J of Agricultural Sci 144:31-43

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Timmons FL (2005) A history of weed control in the United States and Canada. Weed Sci 18:294-307

Troxler SC, Askew SD, Wilcut JW, Smith WD, Paulsgrove MD (2002) Clomazone, fomesafen, and bromoxynil systems for bromoxynil-resistant cotton (Gossypium hirsutum). Weed Technol 16:838-844

USDA NASS (2018) Annual Cotton Review https://www.nass.usda.gov/Statistics_by_State/Texas/Publications/Current_Ne ws_Release/2018_Rls/tx-cotton-review-2018.pdf. Accessed March 4, 2020

Van Wychen L (2019) 2019 Survey of the most common and troublesome weeds in broadleaf crops, fruits, & vegetables in the United States and Canada. Accessed March 19, 2020

Vangessel MJ, Schweizer EE, Lybecker DW, Westra P (1995) Compatibility and efficiency of in-row cultivation for weed management in corn (Zea mays). Weed Technol 9:754-760

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Zimdahl RL (2010) A History of Weed Science in the United States. Burlington, MA: Elsevier. Pp. 79-81

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CHAPTER II COTTON RESPONSE TO HERBICIDE SYSTEMS USING ISOXAFLUTOLE D. C. Foster, P. A. Dotray, C. N. Thompson, G. Baldwin, F. T. Moore

Abstract

Over half of the nation’s cotton is planted in Texas with 1.6 million hectare residing in the High Plains region. Since 2011, glyphosate resistant Palmer amaranth has increased weed control costs in cotton production and alternatives to glyphosate- based systems are needed. Integrating soil residual herbicides into a weed management system is an effective strategy to control glyphosate resistant weeds before they emerge. BASF Corporation is developing p-hydroxyphenylpyruvate dioxygenase (HPPD) tolerant cotton, which will allow growers to use isoxaflutole, an

HPPD inhibiting HRAC Group F2 herbicide, in future weed management programs.

Field experiments were conducted at New Deal and Lubbock, Texas in 2019 and 2020 to determine HPPD-tolerant cotton response to isoxaflutole applied preemergence or early postemergence to 2- to 4-leaf cotton. Visual cotton response was evaluated 14 days after the preemergence and early postemergence applications, 7 days after the mid-postemergence application, and 10 days after the postemergence-directed application. Cotton stand, height, and lint yield and quality also were evaluated. At

New Deal, cotton response was greatest following the early postemergence application, but never exceeded 10%. Cotton response was greatest following the preemergence application at Lubbock in 2019, but never exceeded 14%. In 2020 at

Lubbock, cotton was replanted due to severe weather. There was <1% cotton response following the preemergence application and maximum cotton response observed was 18

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9% following the early postemergence and mid-postemergence applications. Cotton response was undetectable by late season. There were no differences in cotton stand at either location. Cotton lint yields ranged from 1,214-1,425 kg ha-1 at New Deal and

675-758 kg ha-1 and 1,544-1729 kg ha-1 in Lubbock in 2019 and 2020, respectively.

Lint yields and fiber quality parameters following isoxaflutole treatments were not different from the nontreated weed-free control.

Nomenclature: Cotton, Gossypium hirsutum L.; isoxaflutole; p- hydroxyphenylpyruvate dioxygenase.

Key words: Crop response; preemergence

Texas Tech University, Delaney Caitlin Foster, May 2021

Introduction

In 2018, 56% of the more than five million hectares of cotton (Gossypium hirsutum L.) in the United States were planted in Texas (USDA NASS 2018). The

High Plains region is the largest contiguous cotton producing region in the nation where 66% of Texas cotton and cottonseed production is located (Plains Cotton

Growers 2020). One of the most detrimental pests to cotton production are weeds.

Weeds cause an average yield loss of 34% if not controlled properly (Oerke 2006).

According to the 2019 survey of most common and troublesome weeds in broadleaf crops, Palmer amaranth (Amaranthus Palmeri S. Watson) and morningglory (Ipomoea spp) are the top two most common and most troublesome weeds in cotton production

(Van Wychen 2019). Other weeds listed include horseweed (Conyza canadensis L.), crabgrass (Digitaria spp), and barnyardgrass (Echinochloa crus-galli L.).

Troublesome weeds found in the Texas High Plains include Palmer amaranth, kochia

(Bassia scoparia L.), and Russian thistle (Salsola tragus L.).

Palmer amaranth is native to the semi-arid southwestern United States and

Mexico (Sauer 1950). Palmer amaranth was ranked as the most common and troublesome weed among all broadleaf crops, as well as in fruit and vegetable production (Van Wychen 2019). Among the four most common Amaranthus species,

Palmer amaranth has the greatest leaf number and dry matter and has the fastest growth rate per growing degree days (Horak and Loughin 2000). Steckel et al. (2004) determined that Palmer amaranth has the greatest germination rate of the Amaranthus species. Intensive use of herbicides has led to populations of seven different herbicide

Texas Tech University, Delaney Caitlin Foster, May 2021 resistant weeds in Texas (Heap 2020). In a recent state-wide survey of Texas Palmer amaranth, samples collected from the High Plains region of west Texas had the greatest number of populations that were resistant or less sensitive to glyphosate, pyrithiobac, and atrazine (Garetson et al. 2019).

Controlling herbicide resistant Palmer amaranth before emergence can be achieved using soil residual herbicides (Young 2006), making them an important tool for cotton growers. In cotton, an eight week weed-free period after emergence is needed to ensure minimal yield loss due to weed competition (Buchanan and Burns

1970). In Georgia, Palmer amaranth that emerged between the 12- and 17- leaf stage of cotton had no effect on yield while earlier emerging weeds decreased lint yield

(MacRae et al. 2013). Fortunately, there are several options for preemergence herbicides in cotton; however, the majority of these herbicides are photosystem II and acetolactate synthase (ALS) inhibitors, two groups where large numbers of resistant weed species have developed (Heap 2020).

Isoxaflutole is an Herbicide Resistance Action Committee (HRAC) Group F2 herbicide that inhibits the plant essential enzyme p-hydroxyphenylpyruvate dioxygenase, also known as HPPD (Herbicide Resistance Action Herbicide 2020). P- hydroxyphenylpyruvate inhibitors are part of a larger group of carotenoid biosynthesis inhibitors. Herbicides in this group deplete plastoquinones, an essential electron acceptor in the carotenoid biosynthetic pathway that are essential for plant life as they protect chlorophyll molecules from photooxidation. This generates singlet oxygen in the absence of carotenoids (Beaudegnies et al. 2009). Once the carotenoid biosynthesis

Texas Tech University, Delaney Caitlin Foster, May 2021 pathway is blocked and the formation of new carotenoids is stopped, all new plant growth show symptomology that resembles “bleaching” or white colored meristematic tissue (Lee et al. 1997). Lipid peroxidation causes eventual plant death.

Isoxaflutole received Federal 3 label status in 1998 and has been an effective herbicide at controlling a number of annual grass and broadleaf weeds in field corn

(Zea mays L.) (Environmental Protection Agency 1998). When used as part of a preemergence herbicide program, isoxaflutole provided up to 95% Palmer amaranth control three weeks after application (Meyer et al. 2016). Johnson et al. (2012) found that isoxaflutole controlled Palmer amaranth 87 to 99% eight weeks after application.

While current cotton varieties do not tolerate HPPD inhibitors, BASF Corporation has developed HPPD-tolerant cotton that will allow growers to use isoxaflutole in future weed management programs pending germplasm approval. The objective of this study was to determine HPPD-tolerant cotton response to isoxaflutole used in a season-long herbicide system.

Materials and Methods

Field experiments were conducted in 2019 and 2020 at the Texas Tech

University New Deal Research Farm (33.730881°N, -101.734796°W) near New Deal,

TX and at the BASF Corporation Breeding and Trait Development Research Farm

(33.581785°N, -101.779416°W) in Lubbock, TX. The soil type at New Deal was a

Pullman sandy clay loam (19% sand, 42% silt, and 39% clay) with a pH of 8.2 and 1% organic matter (USDA-NRCS). The New Deal location was equipped with subsurface

Texas Tech University, Delaney Caitlin Foster, May 2021 drip irrigation. Cotton was planted using a John Deere 1700 MaxEmerge cone planter at New Deal on May 15, 2019 and May 16, 2020. The target planting density at both locations was 145,000 plants per hectare. At New Deal, total in-season irrigation was

364 mm and 371 mm and rainfall was 469 mm and 218 mm in 2019 and 2020, respectively. At New Deal, trifluralin at 1.12 kg ai ha-1 was applied and incorporated twice to a depth of 5 cm using a rolling cultivator on April 9, 2019 and March 25,

2020. Plots were maintained weed-free throughout the season by hand-weeding, cultivation, and use of clethodim at 0.25 kg ai ha-1 plus 1% v/v crop oil concentrate. At

New Deal, harvest aids ethephon at 1.26 kg ai ha-1 and thidiazuron+diuron at

0.11+0.05 kg ai ha-1 were applied on September 30, 2020. On October 7, 2020, paraquat at 0.28 kg ai ha-1 plus 0.25% v/v nonionic surfactant was applied to finish preparing cotton for harvest. Harvest aids were not applied in 2019.

The Lubbock location soil was an Amarillo fine sandy loam (66% sand, 15% silt, and 19% clay) with a pH of 8.4 and less than 1% organic matter. The Lubbock location was equipped with overhead center pivot sprinkler irrigation. Cotton was planted using a John Deere 1700 MaxEmerge cone planter at Lubbock on May 29,

2019 and May 26, 2020. At Lubbock in 2020, high force winds destroyed emerged cotton on June 9, so cotton was replanted on existing beds with minimal soil disturbance on June 11. At Lubbock, total in-season irrigation was 102 mm and 377 mm and rainfall was 497 mm and 167 mm in 2019 and 2020, respectively. At

Lubbock, a blanket treatment of trifluralin at 0.84 kg ai ha-1 was applied and incorporated to a depth of 5 cm using a tandem plow with double disc lister on March

Texas Tech University, Delaney Caitlin Foster, May 2021

18, 2019 and April 1, 2020. To aid in weed control at the Lubbock location, S- metolachlor at 1.07 kg ai ha-1 was applied over the entire trial area on May 28 and July

11 in 2019 and May 28 in 2020 and diuron at 0.9 kg ai ha-1 was applied under a hooded sprayer on July 28, 2020. At Lubbock, harvest aids ethephon at 1.26 kg ai ha-1 and thidiazuron+diuron at 0.11+0.05 kg ai ha-1 were applied on October 2, 2019 and

October 20, 2020. On October 14, 2019, pyraflufen ethyl at 0.006 kg ai ha-1 was applied to finish preparing cotton for harvest.

Treatments were arranged in a randomized complete block design with four replications and listed in Table 2.1. Plot size was four 101.6 cm rows by 7.6 m in length with the center two rows receiving herbicide treatments. All herbicides were applied using a CO2-pressurized backpack sprayer equipped with AIXR 11002 nozzles

(TeeJet® Technologies, Glendale Heights, IL) calibrated to deliver 140 L ha-1 at 4.85 kph using 220 kPa. Ammonium sulfate at 2.82 kg ha-1 was added to all treatments containing glufosinate.

Visual cotton response was evaluated on a 0-100% scale (0 being no visual response and 100 being all plants dead) (Frans et al. 1986) at both locations 14 days after the preemergence (PRE) and early postemergence (EPOST) applications, 7 days after the mid-postemergence (MPOST) application, and 10 days after the postemergence directed (PDIR) application. Cotton stand was determined in 2 m from the center two rows 21 days after the PRE application except in Lubbock in 2020 where stand was recorded 21 days after the replanting, which was 37 days after the initial

PRE application. The height of six plants chosen randomly per plot (three each from

Texas Tech University, Delaney Caitlin Foster, May 2021 the center two rows) was recorded by measuring plants to the tallest part of the growing point 14 days after the EPOST application and just prior to harvest.

At New Deal, yield data was collected per plot using a 2 row John Deere 7445 cotton stripper and at Lubbock with a John Deere 7460 cotton stripper, both equipped with calibrated load cells to determine plot yield. A 25-boll sample was collected at random from the center two rows of each plot just prior to mechanical harvest.

Samples were ginned on a 20-saw table top gin to calculate lint percentage. Fiber samples were sent to Texas Tech’s Fiber and Biopolymer Institute in Lubbock, TX in

2019 and BASF’s internal lab in Leland, MS in 2020 for fiber quality analysis using

High Volume Instrument testing. Lint yield was calculated on a per plot basis by multiplying plot yield by the lint percentage from the 25-boll sample.

Data were analyzed using the GLIMMIX procedure (2014 Version 9.4, SAS

Institute Inc., Cary, NC) for analysis of variance and Tukey’s HSD at alpha = 0.05.

Locations were analyzed separately and at Lubbock, years also were analyzed separately because of the need to replant cotton in 2020. Year was considered a random effect at New Deal to broaden the inference space and account for environmental variability when making a recommendation (Blouin et al. 2011; Carmer et al. 1989; Moore and Dixon 2014).

Results and Discussion

At 14 days after the PRE treatment, cotton response (stunting and discoloration) was ≤6% for all treatments at New Deal (Table 2.2). The only response

Texas Tech University, Delaney Caitlin Foster, May 2021

>5% was from the isoxaflutole plus prometryn plus pendimethalin treatment. At

Lubbock in 2019, cotton response ranged from 1-14%. The lowest response occurred in treatments with isoxaflutole plus fluometuron or pendimethalin (1%) and the greatest response occurred in plots treated with isoxaflutole plus prometryn (14%). In

2020 at Lubbock, cotton response was <3% when evaluated 14 days after replanting

(31 days after the PRE application) and was similar for all treatments.

At New Deal, cotton response 14 days after the EPOST treatment ranged from

8-10% (Table 2.3). Cotton response at Lubbock 14 days after the EPOST treatment ranged from 6-9% in both 2019 and 2020. Cotton response across treatments at all locations were similar 14 days after the EPOST applications. Seven days after the

MPOST treatment, no cotton response was observed at New Deal (Table 2.4). At

Lubbock, cotton response ranged from 6-9% 7 days after the MPOST treatment in

2019 and 2020 and were not different among treatments.

At 10 days after the PDIR treatment, cotton response was ≤1% at New Deal and no response was observed either year at Lubbock (Table 2.5). Similarly in HPPD- tolerant soybean (Glycine max L. Merr.), Schultz et al. (2015) observed ≤2% injury following isoxaflutole PRE at rates up to 0.14 kg ai ha-1. Smith et al. (2019) observed less than 10% bleaching from isoxaflutole applied PRE at 9 of 10 field sites tested.

Cotton density 21 days after the PRE application ranged from 89,000-110,000 plants ha-1 at New Deal (Table 2.6). In 2019 at the Lubbock location, cotton density ranged from 131,000-168,000 plants ha-1 21 days after the PRE application. Twenty- one days after replanting and 37 days after the PRE application, cotton density ranged

Texas Tech University, Delaney Caitlin Foster, May 2021 from 141,000-157,000 plants ha-1 in 2020. Cotton densities were similar among treatments and the nontreated control at all locations and years.

At New Deal, cotton height from the nontreated control was 25 cm while cotton height in herbicide treated plots ranged from 23-24 cm 14 days after the

EPOST treatments (Table 2.7). In 2019 at Lubbock, cotton height in the nontreated control was 22 cm while herbicide treatments reduced cotton height 1-3 cm 14 days after the EPOST treatments. Cotton height was always highest in the nontreated control.

At New Deal, cotton lint yield ranged from 1,214-1,425 kg ha-1 and yield following all herbicide treatments were similar to the nontreated control (1,416 kg ha-1) (Table 2.8). All treatments yielded >1,400 kg ha-1 except the systems containing isoxaflutole plus pendimethalin and isoxaflutole plus prometryn plus pendimethalin, both followed by dimethenamid plus glufosinate (1,214-1,235 kg ha-1). At Lubbock in

2019, cotton lint yield ranged from 675-758 kg ha-1 and yield following all herbicide treatments did not differ from the nontreated control (730 kg ha-1). In 2020, lint yield ranged from 1,544-1729 kg ha-1 and all herbicide treatments were similar to the nontreated control (1,729 kg ha-1).

In 2020 at Lubbock, herbicide treatments reduced cotton height 1-4 cm when compared to the nontreated control (28 cm) 14 days after the EPOST treatment. At harvest, cotton height at New Deal ranged from 70-83 cm and all herbicide treatments were similar to the nontreated control (74 cm) (Table 2.9). At Lubbock in 2019, cotton height from the nontreated control was 64 cm while cotton height in herbicide treated

Texas Tech University, Delaney Caitlin Foster, May 2021 plots ranged from 59-64 cm at harvest. In 2020 at Lubbock, the nontreated control cotton height was 71 cm. Isoxaflutole plus prometryn (½ rate) and isoxaflutole plus prometryn both followed by S-metolachlor plus glufosinate and prometryn followed by isoxaflutole plus glufosinate with and without glyphosate decreased cotton height at harvest (64-65 cm).

At New Deal, cotton micronaire following all herbicide systems ranged from

4.0-4.3 and all systems were similar to the nontreated control (4.2) (Table 2.10). At

Lubbock in 2019, micronaire ranged from 4.3-4.6 and in 2020 ranged from 3.3-3.8 and micronaire following all systems were similar to the nontreated control (4.4 and

3.5, respectively). Length measurements at New Deal were 1.1 cm for all herbicide systems (Table 2.11). At Lubbock, length was 1.0 cm for all herbicide systems in 2019 and ranged from 1.1-1.2 cm in 2020 and length following all herbicide systems were not different from the nontreated control (1.2 cm). Cotton uniformity at New Deal ranged from 81.3-82.2% and all treatments were similar to the nontreated control

(81.6%) (Table 2.12). At Lubbock in 2019, uniformity ranged from 80.0-81.6% and in

2020 uniformity ranged from 82.4-83.6%. All treatments had similar cotton uniformity when compared to the nontreated control in both years (80.6 and 82.8%, respectively).

At New Deal, cotton strength ranged from 29.4-30.8 g tex-1 and all treatments were similar to the nontreated control (29.4 g tex-1) (Table 2.13). Strength at Lubbock ranged from 26.8-28.5 g tex-1 and 30.1-31.8 g tex-1 in 2019 and 2020, respectively. All treatments had similar cotton strength when compared to the nontreated control in both years (26.8 and 31.7 g tex-1, respectively). Cotton elongation at New Deal ranged

Texas Tech University, Delaney Caitlin Foster, May 2021 from 6.0 to 6.2% and no treatment differed from the nontreated control (6.2%) (Table

2.14). At Lubbock in 2019, elongation ranged from 5.6-5.8% and in 2020 ranged from

5.8-7.2% and all treatments had similar cotton elongation when compared to the nontreated control (5.6 and 7.1%, respectively).

This study suggests that the opportunity to use isoxaflutole in cotton weed management systems will present no adverse effects on cotton yield and fiber quality.

Cotton height was decreased at all locations following the EPOST treatment and at harvest at Lubbock in 2020. Cotton density was not affected by treatments of isoxaflutole when compared to the nontreated control. Isoxaflutole will provide cotton growers a novel site of action for weed control without risk of negative crop response.

Texas Tech University, Delaney Caitlin Foster, May 2021

Literature Cited

Blouin DC, Webster EP, Bond JA (2011) On the analysis of combined experiments. Weed Technol 25:165-169

Buchanan GA, Burns ER (1970) Influence of weed competition on cotton. Weed Sci 18:149-154

Carmer SG, Nyquist WE, Walker WM (1989) Least significant differences for combined analyses of experiments with two- or three- factor treatment designs. Agron J 81:665-672

Environmental Protection Agency (1998) Pesticide Fact Sheet: Isoaxflutole: Environmental Protection Agency. 15 p

Frans RE, Talbert R, Marx D, Crowley H (1986) Experimental design and techniques for measuring and analyzing plant response to weed control practices. Pages 29-46 in Camper ND, ed. Research Methods in Weed Science. Champaign: Southern Weed Science Society

Heap I (2020) The International Survey of Herbicide Resistant Weeds. Accessed December 30, 2020.

Herbicide Resistance Action Committee (2020) HRAC Mode of Action Classification 2020. Accessed December 30, 2020.

Horak MJ, Loughin TM (2000) Growth analysis of four Amaranthus species. Weed Sci 48:347-355

Johnson WG, Chahal GS, Regehr DL (2012) Efficacy of various corn herbicides applied preplant incorporated and preemergence. Weed Technol 26:220-229

Texas Tech University, Delaney Caitlin Foster, May 2021

Lee DL, Prisbylla MP, Cromartie TH, Dagarin DP, SWHoward, Provan W, Ellis MK, Fraser T, Mutter LC (1997) The discovery and structural requirements of inhibitors of p-hydroxyphelynpyruvate dioxygenase. Weed Technol 45:601- 609

MacRae AW, Webster TM, Sosnoskie LM, Culpepper AS, Kichler JM (2013) Cotton yield loss potential in response to length of Palmer amaranth (Amaranthus palmeri) interference. J of Cotton Sci 17:227-232

Moore KJ, Dixon PM (2014) Analysis of combined experiments revisited. Agron J 107:763-771

Oerke EC (2006) Crop losses to pests. J Agric Sci 144:31-43

Plains Cotton Growers (2020) Cotton 101. https://plainscotton.org/cotton-101/. Accessed February 22, 2020

Sauer JD (1950) The Grain Amaranths: A survey of Their History and Classification. Annals of the Missouri Botanical Garden 1990 37:561-632

Schultz JL, Weber M, Allen J, Bradley KW (2015) Evaluation of weed management programs and response of FG72 soybean to HPPD-inhibiting herbicides. Weed Technol 29:653-664

Smith A, Soltani N, Kaastra AJ, Hooker DC, Robinson DE, Sikkema PH (2019) Annual weed management in isoxaflutole-resistant soybean using a two-pass weed control strategy. Weed Technol 33:411-425

Steckel LE, Sprague CL, Stoller EW, Wax LM (2004) Temperature effects on germination of nine Amaranthus species. Weed Sci 52:217-221

USDA NASS (2018) Annual Cotton Review https://www.nass.usda.gov/Statistics_by_State/Texas/Publications/Current_Ne ws_Release/2018_Rls/tx-cotton-review-2018.pdf. Accessed March 4, 2020.

USDA NRCS: Official soil series. https.//soilseries.sc.egov.usda.gov/ Accessed January 11, 2021

Texas Tech University, Delaney Caitlin Foster, May 2021

Van Wychen L (2019) 2019 Survey of the most common and troublesome weeds in broadleaf crops, fruits, and vegetables in the United States and Canada. Accessed March 19, 2020.

Young BG (2006) Changes in herbicide use patterns and production practices resulting from glyphosate-resistant crops. Weed Technol 20:301-307

Texas Tech University, Delaney Caitlin Foster, May 2021

Table 2.1. Treatments, application timing, herbicide, and rates used in crop tolerance experiments at New Deal and Lubbock in 2019 and 2020.

Herbicide Rate Application timinga kg ai or ae ha-1 Nontreated Control --

Prometryn 1.35 PRE S-metolachlor + glufosinate 1.4 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR

Isoxaflutole + prometryn 0.11 + 1.35 PRE Dimethenamid + glufosinate 0.84 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST

Isoxaflutole + pendimethalin 0.11 + 1.12 PRE Dimethenamid + glufosinate 0.84 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST

Isoxaflutole + prometryn + pendimethalin 0.11 + 1.35 + 1.12 PRE Dimethenamid + glufosinate 0.84 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST

Isoxaflutole + prometryn 0.11 + 0.67 PRE S-metolachlor + glufosinate 1.4 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR

Isoxaflutole + prometryn 0.11 + 1.35 PRE S-metolachlor + glufosinate 1.4 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR

Isoxaflutole + fluometuron 0.11 + 1.12 PRE S-metolachlor + glufosinate 1.4 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR

Prometryn 1.35 PRE Isoxaflutole + glufosinate 0.11 + 0.88 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR

Prometryn 1.35 PRE Isoxaflutole + glufosinate + glyphosate 0.11 + 0.88 + 2.1 EPOST Glyphosate + glufosinate 2.1 + 0.88 MPOST Diuron 1.12 PDIR a Abbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence.

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Table 2.2. Cotton response 14 days after planting at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton responsebc New Deal Lubbock PRE Herbicidesd 2019/2020 2019 2020 ------%------Nontreated NA NA NA Prometryn 3 bc 6 bc 0 Isoxaflutole + fluometuron 1 c 1 c 1 Isoxaflutole + prometryn 5 ab 14 a 0 Isoxaflutole + prometryn + pendimethalin 6 a 9 ab 1 Isoxaflutole + ½ prometryn 2 c 4 bc 2 Isoxaflutole + pendimethalin 1 c 1 c 1 aAbbreviations: PRE, preemergence. bTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. c Cotton response data were combined across years at New Deal but separated at Lubbock. d All herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, and pendimethalin at 1.12.

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Table 2.3. Cotton response 14 days after the early postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton responsebc Herbicide systemd New Deal Lubbock PRE EPOSTe 2019/2020 2019 2020 ------%------Nontreated NA NA NA Prometryn S-metolachlor + glufosinate 8 7 8 Isoxaflutole + prometryn Dimethenamid + glufosinate 10 8 7 Isoxaflutole + pendimethalin Dimethenamid + glufosinate 10 7 6 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate 9 8 8 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate 9 6 9 Isoxaflutole + prometryn S-metolachlor + glufosinate 9 9 8 Isoxaflutole + fluometuron S-metolachlor + glufosinate 9 7 9 Prometryn Isoxaflutole + glufosinate 9 6 6 Prometryn Isoxaflutole + glufosinate + glyphosate 10 7 8 aAbbreviations: EPOST, early postemergence; PRE, preemergence. bCotton response data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. d All herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, and glyphosate at 2.1. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

Texas Tech University, Delaney Caitlin Foster, May 2021

Table 2.4. Cotton response 7 days after the mid-postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton responsebc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST 2019/2020 2019 2020 ------%------Nontreated NA NA NA Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate 0 7 8 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate 0 8 7 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate 0 7 6 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate 0 9 8 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate 0 8 9 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate 0 8 8 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate 0 7 9 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate 0 7 6 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate 0 7 8 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PRE, preemergence. b Cotton response data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, and glyphosate at 2.1. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.5. Cotton response 10 days after the postemergence-directed application at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton responsebc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------%------Nontreated NA NA NA Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1 0 0 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 0 0 0 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 0 0 0 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 0 0 0 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1 0 0 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1 0 0 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 0 0 0 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 1 0 0 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 1 0 0 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton response data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.6. Cotton density 21 days after planting at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton densitybc New Deal Lubbock PRE Herbicidesd 2019/2020 2019 2020 ------1,000 plants ha-1------Nontreated 110 168 148 Prometryn 109 142 142 Isoxaflutole + fluometuron 98 145 141 Isoxaflutole + prometryn 109 131 141 Isoxaflutole + prometryn + pendimethalin 89 131 157 Isoxaflutole + ½ prometryn 95 135 151 Isoxaflutole + pendimethalin 110 124 143 aAbbreviations: PRE, preemergence. bTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. cCotton density data were combined across years at New Deal but separated at Lubbock. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, and pendimethalin at 1.12.

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Table 2.7. Cotton heights 14 days after the early postemergence application at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton heightbc Herbicide systemd New Deal Lubbock PRE EPOSTe 2019/2020 2019 2020 ------cm------Nontreated 25 a 22 a 28 a Prometryn S-metolachlor + glufosinate 24 ab 21 a 26 ab Isoxaflutole + prometryn Dimethenamid + glufosinate 23 b 20 ab 25 ab Isoxaflutole + pendimethalin Dimethenamid + glufosinate 23 b 20 ab 24 b Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate 23 b 19 b 26 ab Isoxaflutole + ½ prometryn S-metolachlor + glufosinate 24 ab 20 ab 27 ab Isoxaflutole + prometryn S-metolachlor + glufosinate 24 ab 20 ab 24 b Isoxaflutole + fluometuron S-metolachlor + glufosinate 24 ab 21 a 25 ab Prometryn Isoxaflutole + glufosinate 23 b 20 ab 24 b Prometryn Isoxaflutole + glufosinate + glyphosate 24 ab 21 a 26 ab aAbbreviations: EPOST, early postemergence; PRE, preemergence. bCotton height data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, and glyphosate at 2.1. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.8. Cotton lint yield at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton lint yieldbc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------kg ha-1------Nontreated 1416 730 1729 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1425 718 1685 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 1329 707 1632 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 1235 688 1686 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 1214 675 1628 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1332 706 1714 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1332 723 1613 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1367 758 1544 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 1353 680 1654 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 1367 688 1650 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton lint yield data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

Texas Tech University, Delaney Caitlin Foster, May 2021

Table 2.9. Cotton heights at harvest at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Cotton heightbc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------cm------Nontreated 74 64 71 a Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 75 63 67 ab Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 74 64 68 ab Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 74 62 67 ab Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 74 61 70 ab Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 72 59 65 b Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 70 62 65 b Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 83 60 66 ab Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 75 62 65 b Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 73 60 64 b aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton height data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.10. Cotton micronaire measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Micronairebc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 Nontreated 4.2 4.4 3.5 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 4.2 4.5 3.7 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 4.1 4.6 3.3 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 4.0 4.4 3.7 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 4.1 4.3 3.5 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 4.3 4.3 3.8 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 4.1 4.4 3.7 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 4.0 4.3 3.6 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 4.1 4.5 3.8 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 4.1 4.5 3.7 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton fiber quality data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.11. Cotton length measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Lengthbc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------cm------Nontreated 1.1 1.0 1.2 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1.1 1.0 1.2 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 1.1 1.0 1.2 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 1.1 1.0 1.2 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 1.1 1.0 1.2 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1.1 1.0 1.2 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1.1 1.0 1.1 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 1.1 1.0 1.1 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 1.1 1.0 1.2 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 1.1 1.0 1.2 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton fiber quality data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.12. Cotton uniformity measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Uniformitybc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------%------Nontreated 81.6 80.6 82.8 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 81.9 80.4 83.6 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 81.8 81.6 82.9 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 82.0 80.8 82.4 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 82.2 80.5 82.7 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 81.7 80.7 82.9 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 81.6 80.6 83.3 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 81.3 80.1 82.7 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 81.7 81.1 83.1 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 81.5 80.0 83.3 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton fiber quality data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.13. Cotton strength measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Strengthbc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------g tex-1------Nontreated 29.4 26.8 31.7 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 30.8 26.8 31.3 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 29.4 27.3 31.3 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 30.7 28.2 30.7 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 30.2 27.7 31.3 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 30.0 27.4 30.1 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 30.0 27.7 30.7 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 30.4 28.5 31.8 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 29.5 27.8 30.8 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 30.3 27.8 31.1 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton fiber quality data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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Table 2.14. Cotton elongation measurements at New Deal and Lubbock, TX in 2019 and 2020 systems trials.a Elongationbc Herbicide systemd New Deal Lubbock PRE EPOSTe MPOST PDIR 2019/2020 2019 2020 ------%------Nontreated 6.2 5.6 7.1 Prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 6.1 5.7 7.1 Isoxaflutole + prometryn Dimethenamid + glufosinate Glufosinate + glyphosate -- 6.0 5.6 7.0 Isoxaflutole + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 6.1 5.7 7.2 Isoxaflutole + prometryn + pendimethalin Dimethenamid + glufosinate Glufosinate + glyphosate -- 6.1 5.6 7.2 Isoxaflutole + ½ prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 6.0 5.7 7.2 Isoxaflutole + prometryn S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 6.1 5.8 7.2 Isoxaflutole + fluometuron S-metolachlor + glufosinate Glufosinate + glyphosate Diuron 6.1 5.6 6.8 Prometryn Isoxaflutole + glufosinate Glufosinate + glyphosate Diuron 6.1 5.7 7.2 Prometryn Isoxaflutole + glufosinate + glyphosate Glufosinate + glyphosate Diuron 6.1 5.6 7.2 aAbbreviations: EPOST, early postemergence; MPOST, mid-postemergence; PDIR, postemergence-directed; PRE, preemergence. bCotton fiber quality data were combined across years at New Deal but separated at Lubbock. cTreatment means within a column followed by the same or no letter do not statistically differ according to Tukey’s HSD test at an alpha level of 0.05. dAll herbicides used according to labelled rates in kg ai ha-1: prometryn at 1.35, isoxaflutole at 0.11, fluometuron at 1.12, pendimethalin at 1.12, S-metolachlor at 1.4, dimethenamid at 0.84, glufosinate at 0.88, glyphosate at 2.1, and diuron at 1.12. e2.52 kg/ha ammonium sulfate was included in all treatments containing glufosinate.

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CHAPTER III WEED MANAGEMENT IN HERBICIDE SYSTEMS USING ISOXAFLUTOLE D. C. Foster, P. A. Dotray, C. N. Thompson, G. Baldwin, F. T. Moore

Abstract

The southern United States produces 90% of the nation’s cotton and the Texas

High Plains is the largest contiguous production region. Since 2011, cotton production has been threatened by the rapid increase of glyphosate-resistant Palmer amaranth populations and alternatives to heavy reliance on glyphosate in glyphosate-based systems are needed. Integrating soil residual herbicides into a weed management system is an effective strategy to control glyphosate resistant weeds before they emerge. BASF Corporation is developing p-hydroxyphenylpyruvate dioxygenase tolerant cotton, which will allow growers to use isoxaflutole in future cotton weed management systems. In 2019 and 2020, non-crop field experiments were conducted at the Texas A&M AgriLife Research Center in Halfway, TX to determine the efficacy of isoxaflutole applied preemergence or early postemergence as part of a cotton weed management system. At 14 and 21 days after the preemergence treatment, all treatments containing isoxaflutole controlled Palmer amaranth ≥94%. All treatments controlled Palmer amaranth ≥94% 21 days after the early postemergence application. Palmer amaranth density 21 days after the early postemergence treatment was reduced 88-100% in herbicide treated plots and 40,365 plants ha-1 were present in the nontreated control. All systems that included isoxaflutole preemergence decreased

Palmer amaranth density 96-99% while treatments with isoxaflutole applied early postemergence decreased Palmer amaranth density 88-94% when compared to the 47

Texas Tech University, Delaney Caitlin Foster, May 2021 nontreated control. Twenty-one days after the mid-postemergence treatment, systems with isoxaflutole in the early postemergence application controlled Palmer amaranth

88-93% while systems with isoxaflutole preemergence controlled Palmer amaranth

94-98%. End of season weed control was lowest in the system without isoxaflutole

(88%) and when isoxaflutole was used in the early postemergence application (88-

91%).

Nomenclature: Isoxaflutole; Palmer amaranth, Amaranthus palmeri S. Wats.; p- hydroxyphenylpyruvate dioxygenase.

Key words: Weed management; preemergence

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Introduction

One of the most detrimental impediments to efficient cotton production is the presence of weeds. According to the 2019 survey of most common and troublesome weeds in broadleaf crops, Palmer amaranth (Amaranthus palmeri S. Watson) was the most common and troublesome weed in cotton (Gossypium hirsutum L.) production

(Van Wychen 2019). Cotton growers rely on a number of strategies to manage weeds including cultivation, cultivar selection, cover crops, and herbicides. Many herbicide- resistant crop traits have come to market including cotton tolerance to bromoxynil, glyphosate, glufosinate, dicamba, and 2,4-D. Over-reliance on glyphosate led to weed populations rapidly evolving resistance to the herbicide that was once an effective stand-alone product. To be successful at managing weeds, cotton growers need to adopt a season-long systems approach that includes the use of several application timings and herbicide modes of action (Burke et al. 2005; Scott et al. 2002; Toler et al.

2002).

Soil residual herbicides are an important tool for cotton growers because of their efficacy and ability to control herbicide resistant Palmer amaranth before they emerge. In cotton, an eight week weed-free period after emergence was needed to ensure minimal yield loss due to weed competition (Buchanan and Burns 1970). In

Georgia, Palmer amaranth that emerged between 12- and 17-leaf cotton had no effect on yield while earlier emerging weeds decreased lint yield, which provides strong evidence on the importance of early-season weed control (MacRae et al. 2013).

Fortunately, there are several preemergence herbicide options in cotton production;

Texas Tech University, Delaney Caitlin Foster, May 2021 however, the majority of these herbicides are photosystem II and acetolactate synthase inhibitors, which are the two mode of action groups where the largest number of resistant weed species have developed (Heap 2020).

Some herbicides act at the plant essential enzyme p-hydroxyphenylpyruvate dioxygenase and are known as HPPD inhibitors (Herbicide Resistance Action

Herbicide 2020). P-hydroxyphenylpyruvate inhibitors are Weed Science Society of

America Group 27 herbicides and part of a larger group of carotenoid biosynthesis inhibitors. Herbicides in this group deplete plastoquinones, an essential electron acceptor in the carotenoid biosynthetic pathway, which are essential for plant life as they protect chlorophyll molecules from photooxidation. This generates singlet oxygen in the absence of carotenoids (Beaudegnies et al. 2009). Once the carotenoid biosynthesis pathway is blocked and the formation of new carotenoids stopped, all new plant growth show symptoms that resemble “bleaching” or white colored meristematic tissue (Lee et al. 1997).

BASF Corporation has recently developed HPPD-tolerant cotton. If approved, this technology will allow growers to use isoxaflutole in future weed management programs. Adding a new mode of action to cotton weed management such as an HPPD inhibitor will increase the diversity of weed management programs and delay the development of herbicide resistant weeds to currently available herbicides in cotton

(Anderson and Hartzler 2016). Determining the best way to integrate isoxaflutole into current cotton weed management systems would be beneficial. The objective of this

Texas Tech University, Delaney Caitlin Foster, May 2021 study was to evaluate season-long weed management programs in cotton by incorporating isoxaflutole applied preemergence or early postemergence.

Materials and Methods

In 2019 and 2020, non-crop field experiments were conducted at the Texas

A&M AgriLife Research Center (34.1861009°N, -101.9460551°W) in Halfway, TX.

The soil was a Pullman clay loam (22.5% sand, 44.5% silt, and 33% clay) with a pH of 8.4 and less than 1% organic matter (USDA-NRCS). This research site is equipped with overhead center pivot sprinkler irrigation. A blanket treatment of trifluralin at

1.12 kg ai ha-1 was applied and incorporated to a depth of 5 cm using a field cultivator on March 5, 2019 and March 6, 2020. Each year treatments were arranged in a randomized complete block design with four replications as listed in Table 3.1. All herbicides were applied using a CO2-pressurized backpack sprayer equipped with

AIXR 11002 nozzles (TeeJet® Technologies, Glendale Heights, IL) calibrated to deliver 140 L ha-1 at 4.82 kph using 220 kPa. Ammonium sulfate at 2.82 kg ha-1 was added to all treatments containing glufosinate. In 2019, herbicide applications were made on May 15, June 11, July 10, and July 31 for preemergence (PRE), early postemergence (EPOST), mid-postemergence (MPOST), and postemergence-directed

(PDIR) treatments, respectively. In 2020, herbicide applications were made on May

18, June 12, July 15, and August 6. Preemergence herbicides were sprinkler-activated within 48 hours of application using 1.9 cm water.

Palmer amaranth control was evaluated on a 0-100% scale (0 being no control and 100 being no Palmer amaranth present) (Frans et al. 1986) 14 and 21 days after

Texas Tech University, Delaney Caitlin Foster, May 2021 the PRE application, 21 days after the EPOST and MPOST applications, and 10 days after the PDIR application. Palmer amaranth density was recorded by counting the total number of plants present between the center two rows of four 101.6-cm rows by

102 cm 21 days after the EPOST applications. Total in-season irrigation was 95 mm and 398 mm in 2019 and 2020, respectively. Total in-season rainfall was 233 mm in

2019 and 123 mm in 2020. Data were analyzed using the GLIMMIX procedure (2014

Version 9.4, SAS Institute Inc., Cary, NC) for analysis of variance and Tukey’s HSD at alpha = 0.05. Year was considered a random effect to broaden the inference space and account for environmental variability when making a recommendation; therefore, data were pooled across years (Blouin et al. 2011; Carmer et al. 1989; Moore and

Dixon 2014).

Results and Discussion

At 14 and 21 days after the PRE application, all treatments containing isoxaflutole controlled Palmer amaranth ≥94%, while prometryn controlled Palmer amaranth ≤90% (Tables 3.2 and 3.3). At 21 days after the PRE application, isoxaflutole plus fluometuron and isoxaflutole plus prometryn plus pendimethalin completely controlled Palmer amaranth (Table 3.3). All treatments controlled Palmer amaranth ≥94% 21 days after the EPOST application (Table 3.4). Palmer amaranth density 21 days after the EPOST treatment ranged from 269-4,777 plants ha-1 in herbicide-treated plots and 40,365 plants ha-1 in the nontreated control. When compared to the nontreated control, all systems that included isoxaflutole PRE

Texas Tech University, Delaney Caitlin Foster, May 2021 decreased Palmer amaranth density 96-99% while treatments with isoxaflutole applied

EPOST decreased Palmer amaranth density 88-94%. Prometryn followed by (fb) S- metolachlor plus glufosinate decreased Palmer amaranth density by 90% when compared to the nontreated control.

Twenty-one days after the MPOST treatment, systems with isoxaflutole applied EPOST controlled Palmer amaranth 88-93% while systems with isoxaflutole

PRE controlled Palmer amaranth 94-98% (Table 3.5). The system without isoxaflutole controlled Palmer amaranth 90% 21 days after the MPOST treatment. Ten days after the PDIR treatment, systems containing isoxaflutole PRE and diuron PDIR controlled

Palmer amaranth 97-98% at the end of the season while systems containing isoxaflutole PRE without a PDIR application controlled Palmer amaranth 87-96%

(Table 3.6). End of season weed control was lowest in the system without isoxaflutole

(88%) and when isoxaflutole was used in the EPOST application (88-91%).

In corn, isoxaflutole applied PRE alone fb glufosinate postemergence (POST) controlled Palmer amaranth 91% at the end-of-season (Stephenson and Bond 2012).

Similar to this new technology in cotton, soybean (Glycine max L. Merr.) tolerance to isoxaflutole also has been developed. In soybean, isoxaflutole at 105 g plus metribuzin, a common PRE treatment in soybean, provided full-season residual control of Amaranthus species (Smith et al. 2019). In cotton, fluometuron and prometryn are common PRE herbicides, both of which increased season-long broadleaf weed control compared to a POST-only system (Porterfield et al. 2002;

Scroggs et al. 2007). Similarly, Grichar et al. (2004) found that tank mixing PRE

Texas Tech University, Delaney Caitlin Foster, May 2021 herbicides in cotton increased season-long control of Amaranthus species while at the same time diversifying weed control programs. Isoxaflutole was most effective on

Palmer amaranth when applied before weed emergence but had little effect on emerged weeds. The opportunity to use isoxaflutole in cotton will improve season- long control of Palmer amaranth and add a novel site of action in weed management systems when integrated as part of an overall weed management program.

Texas Tech University, Delaney Caitlin Foster, May 2021

Literature Cited

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Texas Tech University, Delaney Caitlin Foster, May 2021

MacRae AW, Webster TM, Sosnoskie LM, Culpepper AS, Kichler JM (2013) Cotton yield loss potential in response to length of Palmer amaranth (Amaranthus palmeri) interference. J of Cotton Sci 17:227-232

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Texas Tech University, Delaney Caitlin Foster, May 2021

Table 3.1. Herbicide treatments, rates, and application timings used in non-crop weed control experiments at Halfway, TX in 2019 and 2020. Herbicide Rate Application Timinga kg ai or ae ha-1 Nontreated Control --