The Influence of Glufosinate in Auxinic-Based Herbicide Systems By

Home , Acetochlor, Glufosinate

The influence of glufosinate in auxinic-based herbicide systems

Grace Flusche Ogden, 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

Peter A. Dotray, Ph.D. Chair of Committee

Katie L. Lewis, Ph.D. J. Wayne Keeling, Ph.D. Mark Sheridan Dean of the Graduate School

December, 2020

Texas Tech University, Grace Flusche Ogden, December 2020

ACKNOWLEDGMENTS I would like to give my sincere appreciation and gratitude to Dr. Peter Dotray for serving as a mentor and providing the opportunity to study weed science. I would like to thank Drs. Katie Lewis and Wayne Keeling for their guidance, feedback, and support of this project. I admire and respect all who served on my committee and am honored to have had the opportunity to work with them.

My fellow graduate students Ubaldo Torres, Delaney Foster, and Kyle Russell have been an invaluable help throughout this process. Bobby Rodrigues was always there for technical support and to lend a helping hand. Andrew Dotray provided a boost of morale on hot summer days. Thank you to each of them for their time and effort to help this project reach completion.

This project would not have come to fruition without the financial support of Legacy Monsanto Company and the field presence of John Everitt. Thank you to John for his field support and inquisitive nature.

Texas Tech University and Texas A&M Agricultural Experiment Station provided the facilities for this project and countless opportunities to share this research with the community. Dr. Jaroy Moore and Beau Henderson kindly let the Weed Science team grow our rouge weeds and understood the need for our unsightly research blocks. Kathy, Robyn, Gail, Joann, and Diann encouraged me and always met my questions or requests with kindness. Thank you to both institutions and their wonderful staff.

My family served as my biggest source of encouragement throughout this journey and I am immensely thankful for their support. My husband, Jace, spent many evenings and weekends helping with these projects and encouraging me to keep the faith. My parents gave the best pep talks and were always eager to hear about what I was learning. Thank you to all my family for fostering my weed science interests and playing the “What Weed is This?” game on Saturday afternoons.

Texas Tech University, Grace Flusche Ogden, December 2020

TABLE OF CONTENTS

DEDICATION...... i ACKNOWLEDGMENTS ...... ii ABSTRACT ...... iv LIST OF TABLES ...... vi CHAPTER I: LITERATURE REVIEW ...... 1 Literature Cited ...... 11 CHAPTER II: THE INFLUENCE OF GLUFOSINATE IN DICAMBA BASED HERBICIDE SYSTEMS ...... 17 Abstract ...... 17 Introduction ...... 19 Materials and Methods ...... 21 Sequential Applications of Glufosinate and Dicamba with and without Acetochlor ...... 21 Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals ...... 23 Results and discussion ...... 25 Sequential Applications of Glufosinate and Dicamba with and without Acetochlor ...... 25 Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals ...... 29 Literature Cited ...... 32 CHAPTER III: THE INFLUENCE OF GLUFOSINATE IN 2,4-D BASED HERBICIDE SYSTEMS ...... 41 Abstract ...... 41 Introduction ...... 43 Materials and Methods ...... 45 Sequential Applications of Glufosinate and 2,4-D Choline ...... 46 2,4-D Choline Tank-Mix Applications ...... 47 Results and Discussion ...... 49 Sequential Applications of Glufosinate and 2,4-D Choline ...... 49 2,4-D Choline Tank-Mix Applications ...... 50 Literature Cited ...... 53 SUMMARY AND CONCLUSIONS ...... 62

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Texas Tech University, Grace Flusche Ogden, December 2020

ABSTRACT

Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern United States and for decades has been one of the most common weeds in West Texas. In recent years, this weed has been considered one of the most troublesome across the southern United States. The management of Palmer amaranth has changed since the discovery of glyphosate resistant populations in 2005. Dicamba and 2,4-D tolerant cotton (Gossypium hirsutum L.) systems were introduced in 2017 and provide a new opportunity to manage glyphosate resistant populations of Palmer amaranth. The use of glufosinate (Liberty® 280 SL) in auxinic-based herbicide systems may not only improve the management of glyphosate-resistant Palmer amaranth, but also be effective against new developments of herbicide resistance to group 4 modes of action. Four field studies were conducted in a non-crop environment in Lubbock, Texas in 2018 and 2019 to determine the influence of sequential spray order and role of glufosinate when used in a system with dicamba or 2,4-D to control Palmer amaranth at different growth stages. Palmer amaranth control decreased as Palmer amaranth size at initial application increased for both dicamba and 2,4-D studies. A difference in efficacy based on herbicide order was observed for < 10 cm Palmer amaranth in the sequential applications of glufosinate and dicamba with and without acetochlor study. Glufosinate followed by dicamba was less effective than dicamba followed by glufosinate at multiple rating dates in both years. Dicamba + acetochlor followed by glufosinate provided greater Palmer amaranth control than dicamba followed by dicamba at one or more rating dates across all weed sizes. The addition of acetochlor reduced new weed flushes when applied to > 30 cm Palmer amaranth in 6 out of 8 treatments. No treatment controlled > 30 cm Palmer amaranth more than 55%. No consistent difference in efficacy based on herbicide order was observed for > 10 cm Palmer amaranth. Sequential applications of glufosinate or dicamba at 3, 7, and 10 day intervals evaluated the influence of sequential spray timing of dicamba and glufosinate. Sufficient evidence was not obtained in these studies to alter sequential application timing interval recommendation from current label restrictions. Sequential applications of glufosinate and 2,4-D choline evaluated the influence of sequential spray order when applying glufosinate and 2,4-D choline with or without pyrithiobac when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. When applied to 7 to 15 cm

Texas Tech University, Grace Flusche Ogden, December 2020

Palmer amaranth, 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 100% 10 days after sequential application in 2018 and 97% 21 days after sequential application in 2019. When applied to 25 to 30 cm, Palmer amaranth 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 98% 11 and 21 days after sequential application in 2018. 2,4-D choline provided greater control when used in the initial application and followed by glufosinate than glufosinate followed by 2,4-D choline in both weed sizes. 2,4-D choline tank-mix applications evaluated the efficacy of tank-mixing 2,4-D choline, 2,4-D choline + glyphosate, and glufosinate when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. Seven days after initial application, 2,4-D choline + glufosinate controlled 7 to 15 cm Palmer amaranth 92%, which was greater than 2,4-D choline + glyphosate (88%). Palmer amaranth control ranged from 96 to 99% 7 days after sequential application, but control declined within the following two weeks as a thick, uniform flush of Palmer amaranth emerged and grew quickly, demonstrating the need for residual herbicide in a Palmer amaranth system. Glufosinate served as a complimentary sequential application partner when paired with dicamba or 2,4-D. There was no penalty observed for adding glufosinate to an auxin- based system, and additional modes of action will be more effective to slow the development of weeds resistant to group 4 herbicides when compared to group 4 herbicides used alone.

Texas Tech University, Grace Flusche Ogden, December 2020

LIST OF TABLES

2.1 Palmer amaranth control following initial and sequential applications of glufosinate and dicamba in 2018…………………………………...... 33 2.2 Palmer amaranth control following initial and sequential applications of glufosinate and dicamba in 2019……………………………...... ……34 2.3 Palmer amaranth dry weight 33 to 62 days after sequential applications of glufosinate and dicamba in 2019………………………………………………...35 2.4 Palmer amaranth density 33 to 62 days after sequential applications of glufosinate and dicamba in 2019……………………………….………………..36 2.5 Palmer amaranth control following 3, 7, and 10 day sequential applications of glufosinate and dicamba in Halfway, Texas in 2018 and 2019……...………..37 2.6 Palmer amaranth density and dry weight 36 days after the final sequential application of 3, 7, and 10 day sequential applications of glufosinate and dicamba in 2019……………………………………………………………...….38 2.7 Palmer amaranth control following 3, 7, and 10 day sequential applications of glufosinate and dicamba in Altus, Oklahoma in 2019……………………..….39 3.1 Palmer amaranth control following initial and sequential applications of glufosinate and 2,4-D choline in 2018…………………….……………………..54 3.2 Palmer amaranth control following initial and sequential applications of glufosinate and 2,4-D choline in 2019……….…………………………………..55 3.3 Palmer amaranth density 36 to 43 days after sequential applications of glufosinate and 2,4-D choline in 2019………………….…………………..……56 3.4 Palmer amaranth dry weight 36 to 43 days after sequential applications of glufosinate and 2,4-D choline in 2019………………………………………...…57 3.5 Palmer amaranth control following initial and sequential tank-mix applications of glufosinate and 2,4-D choline in 2019………………...………...58 3.6 Palmer amaranth dry weight 34 to 37 days after sequential tank-mix applications of glufosinate and 2,4-D choline in 2019…………………………..59 3.7 Palmer amaranth density 35 days after sequential tank-mix applications of glufosinate and 2,4-D choline in 2019………………………………………...60

Texas Tech University, Grace Flusche Ogden, December 2020

CHAPTER I

LITERATURE REVIEW Cotton (Gossypium hirsutum L.) is a perennial, tree-like plant that modern breeding techniques have transformed into a crop that is grown as an annual, bush-like plant that yields fiber and seed used for a variety of purposes. It is an important cash crop, with approximately 2.5% of the world’s arable land planted to cotton (Fangueiro and Sohel 2016). Over 23.5 million metric tons of lint were produced globally in 2013, making up 79% of the total natural fiber production of nearly 30 million metric tons (Fangueiro and Sohel 2016). Cotton not only produces a fiber suitable for clothing, but also a seed that is high in roughage for feeding livestock and oil that is high in fat for a variety of products. In 2018 in the United States, all cotton sales measured 6.7 billion dollars from 4 million upland cotton hectares harvested (NASS 2019). Over 40% of the nation’s cotton crop is produced in Texas, where 1.76 million hectares were harvested in 2018 (NASS 2019). The majority of Texas cotton is produced on the High Plains, within a 160-kilometer radius of Lubbock. Over 970,000 hectares of cotton were produced on the Northern and Southern High Plains in 2018 (USDA 2019). A multitude of potential pests can impact crop success, but pathogens, insects or animals, and weeds are the main groups of concern when considering crop losses. Among the three groups, weeds pose the greatest potential crop loss estimated at 34% annually (Oerke 2006). Weeds compete with crops for water, light, and nutrients (Stuart et al. 1984). Oerke (2006) estimated potential cotton yield loss attributed to weeds at 36% worldwide. Managing weeds is an essential aspect of any successful cropping system. Each cropping system faces weed management challenges dependent on characteristics of the crop and season of growth. Palmer amaranth (Amaranthus palmeri S. Watson) is an important weed native to the Southwest United States. Ten Palmer amaranth per 9 m of row reduced cotton yield up to 54% (Morgan et al. 2001). Palmer amaranth can use up to 1.2 g of water cm-2 d-1, which is twice that of cotton (Berger et al. 2015). This weed can produce more than half a million seeds under ideal conditions, and 100,000 seeds when competing with a crop

Texas Tech University, Grace Flusche Ogden, December 2020

(Legleiter and Johnson 2013; Webster and Grey 2015). Palmer amaranth is dioecious, allowing for cross-pollination between populations, which results in wide genetic diversity. It has a wide window of emergence from early spring to late fall, which suggests that it can adapt and spread quickly. These traits are particularly advantageous in the evolution of herbicide resistant populations (Guo and Al-Khatib 2003; Keeley et al. 1987; Steckel et al. 2004). Growing over 3 cm a day during optimal conditions, Palmer amaranth can reach heights of nearly 2 meters in a growing season (Keeley et al. 1987; Legleiter and Johnson 2013). These qualities make for a weed that is incredibly competitive and a challenge to manage. Little literature is available detailing the beginning of weed control. Wooden plows and hand-weeding were the weed control methods of choice for millennia, performed by abundant labor found in the number of women and children available for the task on homesteads and farms (Bell 2015). As the industrial age approached, women and children were moved from farm to factory and the labor available for weed management shifted. Quicker methods of control were needed. Other innovations aided in refining weed control, such as the implementation of the grain drill to the western hemisphere in 1700. Grain drills placed seed in a row as opposed to scattered by hand seeding. This opened the door for between row cultivation (Bell 2015). Many advancements have changed the way weeds are managed in the last 300 years, but the classic principles and techniques have remained the same. There are five areas of modern weed control: physical, mechanical, cultural, biological, and chemical. Physical control of weeds refers to the “physical” removal or suppression of weeds from the surrounding environment by hand pulling, chopping, burning, hoeing, spudding, mulching, microwaving, steam, or electricity. Mechanical means of weed control involve tillage practices. Primary tillage methods are used to “plow” or cut, turn over, and pulverize soil into rough aggregates. This method buries crop residue, moves weed seed below the soil surface, and redistributes soil nutrients. Secondary tillage is used to refine soil into smaller aggregates suitable for seed beds or to incorporate herbicides. Tillage may be used in row crops at preplant, preemergence, postemergence, or post-harvest.

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Cultural practices for weed control involve management decisions that promote advantages to the desirable crop and reduce the competitive edge of weeds. Cultural practices may involve planting rate and date, row spacing, fertility and water management, crop rotation, cover cropping, and variety selection. Transgenic cotton was first introduced to the market when bromoxynil and glyphosate tolerant cotton were commercialized 1995 and 1996, respectively. This added a new component to variety selection. Current transgenic cotton varieties include traits that allow cotton tolerance to herbicides and have added Bacillus thuringiensis (BT) genes for defense against boll worms. Variety selection is often the foundation for weed control programs and dictates what herbicide control options may be used. Another example of a cultural practice is planting cereal rye as a winter cover crop following cotton harvest, which Palhano et al. (2017) found to suppress Palmer amaranth emergence up to 83% when compared to areas without a cover crop. Biological control involves the use of a biological organism to control weeds, such as an insect, pathogen, plant, microorganism, fish, or animal. The biological organism used is generally a natural enemy of the weed and must not use the desirable crop as a host. Three great examples of biological control agents are moths, puncturevine weevils, and bindweed mites. Small moths from Argentina (Cactoblastis cactorum Berg), were introduced to Australia in 1925 to control over 24 million hectares of prickly pear (Opuntia polyacantha Haw.) cactus. These small moths made a huge impact, laying eggs in the prickly pear cacti and diminishing infected hectares to just 1% of what they were in 1925 (BWCC, 2000; Cruttwell McFadyen 1998). Similarly, seed weevils (Microlarinus lareynii Jacquelin du Val) and stem-boring weevils (Microlarinus lypriformis Wollaston) were introduced to California in 1961 to reduce the population of puncturevine (Tribulus terrestris L.) (Maddox and Andres 1979). One of the more recent biological control discoveries, the bindweed mite (Aceria malherbae Nuzzaci) is used to suppress field bindweed (Convolvulus arvensis L.). The mite creates gall-like symptoms on bindweed leaves and stunts growth. Populations of the bindweed gall mite have been established in several areas of the United States (Lauriault et al. 2004). Chemical control is using a phytotoxic chemical to kill or suppress plants, usually by way of herbicide. The Weed Science Society of America defines a herbicide as a

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“chemical substance or cultured biological organism used to kill or suppress the growth of plants” (WSSA 2020). The first chemicals used to control weeds were inorganic salts and acids, used as fumigants and desiccants in the mid-19th Century (Bell 2015). The style of herbicide use that best represents modern practices started with the introduction of synthetic auxin herbicides in the 1940s (Hamner and Tukey 1944), which started the “Chemical Era of Agriculture”. New herbicides were steadily developed, with the last unique herbicide mode of action released in the 1980s. Chemical control may be selective, in which weeds are killed but the desired crop is not, or nonselective where all plants treated are affected alike. The selectivity of chemical control may be based on application timing. The four general herbicide application timings include preplant incorporated (PPI), preemergence (PRE), postemergence (POST), and postemergence- directed (PDIR). The first widely used herbicide in cotton production was soil applied. Trifluralin, a dinitroaniline (WSSA group 3 herbicide), was introduced in 1963 and quickly accepted by cotton producers as an effective weed management tool to control grasses and small- seeded broadleaf weeds. Trifluralin is applied PPI because it is susceptible to photodecomposition and volatilization. Monosodium methanearsonate (MSMA) was applied POST for control of annual grasses and nutsedge (Cyperus spp.). This herbicide was effective, but its mammalian toxicity concern has reduced its value to a small fraction of cotton hectares. Selective postemergence herbicides that control broadleaf weeds and cause little to no crop injury were not available for use in cotton until the introduction of pyrithiobac- sodium (Staple®) in 1996. Pyrithiobac effectively controlled broadleaf weeds such as Palmer amaranth and devil’s-claw (Proboscidea louisianica (Mill.) Thellung) (Dotray et al. 1996). Prior to pyrithiobac, chlorpropham, diuron, and dalapon were available for selective postemergence use in cotton but caused moderate to severe crop injury (Reed 2012). Bromoxynil-resistant cotton was the first transgenic herbicide tolerant cotton. In 1995, this technology allowed cotton producers over-the-top postemergence options to control many annual broadleaf weeds (Culpepper and York 1997; Jordan et al. 1993; Paulsgrove and Wilcut 1999). Unfortunately, bromoxynil was not effective at controlling

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Palmer amaranth (Culpepper and York 1997). Nine crops have been developed with herbicide tolerance and approved for field release in the United States, although not all are currently on the market. These crops include alfalfa (Medicago sativa L.), canola (Brassica napus L.), cotton, flax (Linum usitatissimum L.), corn (Zea mays L.), rice (Oryza sativa L.), soybean (Glycine max (L.) Merrill), sugar beet (Beata vulgaris L.), and winter wheat (Triticum aestivum L.) (National Academies of Sciences, Engineering, and Medicine 2016). Of these nine crops, herbicide tolerant cotton is the most economically important to the Texas High Plains (USDA 2019; USDA-NASS 2019). Postemergence herbicides currently labeled for use in their respective tolerant cotton varieties are glyphosate, glufosinate, dicamba, and 2,4-D. Glyphosate is a nonselective, postemergence herbicide that inhibits 5- enolpyruvylshikimate-3-phoshate synthase (EPSPS) in plants. This enzyme is part of an essential pathway for aromatic amino acid production in plants. Susceptible plants treated with glyphosate die slowly. First discovered in 1970, glyphosate use increased dramatically when glyphosate-tolerant crops were commercially released in 1997, beginning a new era of cotton weed control. Prior to this point, glyphosate was applied as a preplant burndown, PDIR, or with “wicking” applicators that relied on crop-weed height differences to apply glyphosate to weeds that grew above the cotton canopy. Glyphosate-tolerant crops, also known as Roundup Ready crops, quickly were adopted due to their cost efficiency, effective weed control, and simplicity of use (Shaner 2000). Before 1998, trifluralin and fluometuron dominated most cotton hectares as a primary means of chemical weed control (Young 2006). From 1990 to 2002, glyphosate use in cotton increased from 0.7 to 3.8 million kg ai per year (Young 2006). The first generation of glyphosate-tolerant cotton was limited because applications could only be made over- the-top up to the 5-leaf stage, with injury and yield loss from glyphosate application possible even under recommended conditions (Light et al. 2003; Pline et al. 2001; Viator et al. 2004). However, improvements to glyphosate tolerant cotton allowed higher rates with a wider treatment window and often contained tolerance to other postemergence herbicides and other pest management genes (Green 2009). Glyphosate provided excellent control of Palmer amaranth, given that the population wasn’t glyphosate resistant (Bond et al. 2006; Whitaker et al 2010).

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Widespread overuse of glyphosate after successful commercialization of glyphosate tolerant crops created a favorable environment for the evolution of resistance, with the first case of glyphosate resistant Palmer amaranth confirmed in 2005 (Culpepper et al. 2006; Garetson et al. 2019). Today, glyphosate-resistant Palmer amaranth has been found in 28 states in the United States (Heap 2020). This wide-spread resistance has been problematic for producers and industry professionals alike. Weed managers have had to resort to using more creative weed management plans than one or two pass glyphosate- only systems. Cotton varieties tolerant to glufosinate (Liberty® 280 SL), known as Liberty Link® cotton, were commercialized in 2004. Glufosinate, a nonselective herbicide, is applied POST over-the-top to this transgenic cotton. Additional lines of glufosinate tolerant cotton have been released with stacked genes to confer resistance to other herbicides and pest management genes. Blair et al. (2001) reported glufosinate tolerance up to 8X in genetically transformed cotton. Weed control with glufosinate varies by weed species and size and is influenced by time of day during application, humidity, and air temperature (Coetzer et al 2001; Kumaratilake and Preston 2005; Steckel et al. 1997). In some regions, glufosinate may control weeds better than glyphosate (Culpepper et al. 2009). Glufosinate is a contact herbicide, and symptoms of herbicide injury typically are noticeable 2 to 7 days after treatment. This rapid injury is opposite of glyphosate, where plants may not exhibit injury until 10 to 14 days after treatment. The true mechanism for plant death from glufosinate wasn’t clear until recently. It was hypothesized that ammonia accumulation and carbon assimilation inhibition may be the cause of death; however, a study conducted by Takano et al. (2019) determined that reactive oxygen species cause lipid peroxidation of cell membranes and thus cell death. Glufosinate is a suitable tank-mix partner for a variety of herbicides; however, studies have shown antagonism when mixing glufosinate and glyphosate (Besancon et al. 2018; Bethke et al. 2013; Reed 2012). Weed control with glufosinate may be improved by adding a nitrogen source, ammonium sulfate (AMS), to the tank (Maschhoff et al. 2000). Optimal weed control has been observed when glufosinate is used in a system with other postemergence herbicides or in sequential applications (Butts et al. 2016; Manuchehri et al. 2017; Reed 2012). Small weeds (< 10 cm) are the ideal target size for

Texas Tech University, Grace Flusche Ogden, December 2020 glufosinate. Glufosinate is less effective on weeds > 20 cm (Coetzer et al. 2002; Merchant et al. 2013). Sequential applications of glufosinate alone do not control Palmer amaranth consistently (Coetzer et al 2002; Reed 2012). Sequential applications of glufosinate followed by glyphosate followed by glyphosate controlled Palmer amaranth at 99%. Coetzer et al. (2002) found that single applications of glufosinate did not reduce Amaranthus populations compared to a non-treated control. Sequential applications can offer more weed control than single applications. Steckel et al. (2002) reported reduced rates of sequentially applied soil applied herbicides and glyphosate were more effective at controlling common waterhemp (Amaranthus rudis J. Sauer) in corn than single applications of a PRE herbicide 56 days after planting. Gonzini et al. (1999) found that sequential applications of glyphosate, tank-mixing glyphosate, or using a PRE followed by glyphosate offered greater weed control than a single application of glyphosate. Sequential applications of tank-mixed glyphosate and dicamba controlled glyphosate resistant common ragweed (Ambrosia artemisiifolia L.) 99%, compared to 70% control provided by a single application of the tank-mixture (Byker et al. 2018). The “Chemical Era of Agriculture”, the era of weed control that most weed managers are familiar with, started after World War II with the introduction of 2,4-D and MCPA. These herbicides are synthetic auxinic herbicides that mimic naturally occurring plant hormones at low doses, and at high doses cause a cascading effect of rapid plant growth that result in injury and eventual death. The success of these two herbicides spurred the discovery of more auxin-type herbicides such as dicamba, picloram, clopyralid, and quinclorac. Although 2,4-D was one of the first commercially available, widely used herbicide, novel applications for this herbicide and dicamba have recently progressed with the commercialization of auxin tolerant corn, soybean, and cotton. EnlistTM cotton, released by Phytogen in 2016, is tolerant to the choline salt of 2,4-dichlorophenoxyacetic acid (2,4-D choline), glufosinate, and glyphosate. Dow AgroSciences has released two 2,4-D formulations for use with these crops: Enlist One® with Colex-D® Technology (2,4-D choline) and Enlist Duo® with Colex-D® Technology (2,4-D choline + glyphosate).

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Bollguard II® XtendFlex® cotton, released by Bayer CropScience (formerly Monsanto/DeltaPine) in 2016, is tolerant to dicamba, glufosinate, and glyphosate. Formulations of dicamba designed for use in XtendFlex cotton include diglycolamine salt (DGA) of dicamba (XtendiMax® with VaporGrip Technology® or FeXapanTM) or dicamba: N,N-Bis-(3-aminopropyl)methylamine salt of 3,6-dichloro-o-anisic acid (BAPMA). Another formulation of the DGA salt is Tavium® Plus VaporGrip® Technology, which is dicamba + S-metolachlor premixed. Both auxin-tolerant varieties allow postemergence over-the-top applications of their respective synthetic auxin herbicide. This is an important formulation improvement since even low rates of 2,4-D and dicamba can cause symptomology and yield loss in susceptible cotton (Byrd et al. 2016; Everitt and Keeling 2009; Johnson et al. 2012; Marple et al. 2007; Russell et al. 2020). One valuable use of these auxin technologies is controlling herbicide resistant weeds (Butts et al. 2018; Ou et al. 2017; Shergill et al. 2017). Although 2,4-D and dicamba provide fair to good control of Palmer amaranth, adding glufosinate to the tank can increase control up to 98% (Merchant et al. 2013). Historically, off-target movement and volatilization have complicated the use of these auxin herbicides. Since reintroducing these products as in-season weed control options off-target movement, volatilization, and weed resistance are increased concerns. Current literature suggests that sequential applications of glufosinate, glyphosate, dicamba, or 2,4-D are more effective for Palmer amaranth control in cotton than single applications (Butts et al. 2016; Cahoon et al. 2015a; Manuchehri et al. 2017; Merchant et al. 2014; Reed et al. 2014). Merchant et al. (2014) found that two sequential applications of glufosinate tank-mixed with 2,4-D followed by a layby application of diuron (Direx®) controlled Palmer amaranth 95 to 97%. Manuchehri et al. (2017) reported that systems that included applications of trifluralin PPI, 2,4-D choline + glyphosate or glufosinate early POST followed by 2,4-D choline + glyphosate mid POST controlled Palmer amaranth at 94% or greater. Sequential applications of glufosinate provided greater control compared to glyphosate (Cahoon et al. 2015b). Inman et al. (2020) reported 98% control or greater 8 weeks after cotton planting when 3 or more herbicides were used, regardless of herbicide order in sequential applications. Growers who are successful at controlling weeds including Palmer amaranth are using a systems approach that involves

Texas Tech University, Grace Flusche Ogden, December 2020 multiple application timings, multiple herbicide modes of action, and the addition of soil residual herbicides, as well as the incorporation of mechanical, cultural, and biological methods where appropriate (Manuchehri et al. 2017). Volatilization and off-target movement of new formulations of 2,4-D and dicamba are under investigation. Sosnoskie et al. (2015) found that 2,4-D choline formulations were less volatile than 2,4-D ester under field conditions. Kalsing et al. (2018) found that 2,4-D dimethylamine (DMA) drifted farther from the location of application than an experimental formulation of 2,4-D with Colex-D® Technology regardless of nozzle tip used, plant species used as a bio-indicator, or application date. Egan and Mortensen (2012) noted that DGA dicamba was less volatile compared to DMA dicamba or free acid dicamba, and DGA dicamba formulations reduced vapor drift by 94%. Jones et al. (2019) found few differences in off-target movement of DGA and BAPMA forms of dicamba. Herbicide resistance in weeds has become an important topic within the scientific community and agricultural sector. As of May 2020, there are 512 unique cases of herbicide resistance in 262 weed species (Heap 2020). Of the 26 currently known herbicide sites of action, 23 have at least one weed species that possess resistance (Heap 2020). Palmer amaranth populations have developed resistance to 8 modes of action (Heap 2020), making it one of the most troublesome weeds in modern agriculture production. The first case of glyphosate resistance in Palmer amaranth was discovered in Macon County, Georgia in 2005 and since that time producers have reverted to age-old weed control methods such as hand-weeding. Cost of weed control per hectare has increased up to 475% compared to years prior to resistance (Sosnoskie and Culpepper 2014). In Texas, Palmer amaranth has confirmed resistance to photosynthesis II inhibitors and EPSP synthase inhibitors (Heap 2020). Several herbicide tolerant crops have been brought to the market and changed herbicide use patterns, raising concern that the use of these cropping systems and their respective herbicide systems are increasing the rate of herbicide resistant weeds. However, recent data has indicated the contrary. The overall rate of new discoveries of herbicide resistant weed species has decreased since 2005 (Kniss 2018). Low levels of 2,4-D and dicamba resistance are present in current weed populations. There are no

Texas Tech University, Grace Flusche Ogden, December 2020 confirmed cases of Palmer amaranth resistant to dicamba (Heap 2020). A survey of Palmer amaranth in Texas conducted from 2014 to 2016 further confirmed that populations on the High Plains did not contain dicamba resistant biotypes; however, 22% of the population were less sensitive to the labeled field rate (Garetson et al. 2019). Kansas is the only state with a confirmed population of 2,4-D resistant Palmer amaranth as of May 2020 (Heap 2020). The overall objective of this research was to determine effective postemergence sequential applications to control glyphosate resistant Palmer amaranth using dicamba, 2,4-D choline, and glufosinate. Projects included a.)10 day sequential applications of glufosinate and DGA dicamba with and without acetochlor, b.) 3, 7, and 10 day sequential applications of glufosinate or BAPMA dicamba, c.) 10 day sequential applications of glufosinate, 2,4-D choline, or 2,4-D choline + glyphosate, and d.) tank- mixing glufosinate with 2,4-D choline or 2,4-D choline + glyphosate. Each of these projects aimed to determine the role of glufosinate in auxin systems and best management approaches to Palmer amaranth control when using glufosinate and auxinic herbicides. Low levels of 2,4-D and dicamba resistance, paired with no current confirmed cases of glufosinate resistant Palmer amaranth (Heap 2020) make the use of glufosinate in auxin herbicide systems a potentially effective and sustainable portion of a larger, complex integrated weed management system.

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Literature Cited Bell C (2015) A historical view of weed control technology. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17593. Accessed October 28, 2019. Berger ST, Ferrell JA, Rowland DL, Webster TM (2015) Palmer amaranth (Amaranthus palmeri) competition for water in cotton. Weed Sci 64:928-935. Besancon TE, Penner D, Everman WJ (2018) Reduced translocation is associated with antagonism of glyphosate by glufosinate in giant foxtail (Setaria feberi) and velvetleaf (Abutilon theophrasti). Weed Sci 66:159-167. Bethke RK, Molin WT, Sprague C, Penner D (2013) Evaluation of the interaction between glyphosate and glufosinate. Weed Sci 61:41-47. Blair LK, Dotray PA, Keeling JW, Gannaway JR, Oliver MJ, Quisenberry JE (2001) Tolerance of transformed cotton to glufosinate. Weed Sci 49:375-380. Bond JA, Oliver LR, Stephenson DO (2006) Response of Palmer amaranth (Amaranthus palmeri) accessions to glyphosate, fomesafen, and pyrithiobac. Weed Technol 20:885-892. Butts TR, Norsworthy JK, Kruger GR, Sandell LD, Young BG, Steckel LE, Loux MM, Bradley KW, Conley SP, Stoltenberg DE, Arriaga FJ, Davis VM (2016) Management of pigweed (Amaranthus spp.) in glufosinate-resistant soybean in the Midwest and Mid-South. Weed Technol 30:355-365. Butts TR, Samples CA, Franca LX, Dodds DM, Reynolds DB, Adams JW, Zolinger RK, Howatt KA, Frits BK, Hoffmann WC, Luck JD, Kruger GR (2018) Droplet size impact on the efficacy of dicamba-plus-glyphosate mixture. Weed Technol 33:66- 74. [BWCC] Biological Weed Control Committee of the Weed Science Society of America (2000) Biological control of weeds – it’s a natural! http://wssa.net/wp- content/uploads/BCBrochure.pdf. Accessed November 15, 2019. Byker HP, Van Wely AC, Soltani N, Lawton MB, Robinson DE, Sikkema PH (2018) Single and sequential applications of dicamba for the control of glyphosate- resistant common ragweed in glyphosate- and dicamba-resistant soybean. Can. J. Plant Sci. 98:552-556. Byrd SA, Collins GD, Culpepper AS, Dodds DM, Edmisten KL, Wright DL, Morgan GD, Baumann PA, Dotray PA, Manuchehri MR, Jones A, Grey TL, Webster TM, Davis JW, Whitaker JR, Roberts PM, Snider JL, Porter WM (2016) Cotton stage of growth determines sensitivity to 2,4-D. Weed Technol 30:601-610. Cahoon CW, York AC, Jordan DL, Everman WJ, Seagroves RW, Culpepper AS, Eure PM (2015a) Palmer amaranth (Amaranthus palmeri) management in dicamba- resistant cotton. Weed Technol 29:758-770.

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Cahoon CW, York AC, Jordan, DL, Seagroves RW, Everman WJ, Jennings KM (2015b) Sequential and co-application of glyphosate and glufosinate in cotton. J Cotton Sci 19:337-350. Coetzer E, Al-Khatib K, Loughin TM (2001) Glufosinate efficacy, adsorption, and translocation in amaranth as affected by relative humidity and temperature. Weed Sci 49:8-13. Coetzer E, Al-Khatib K, Peterson DE (2002) Glufosinate efficacy on Amaranthus species in glufosinate-resistant soybean (Glycine max). Weed Technol 16:326-331. Cruttwell McFadyen RE (1998) Biological control of weeds. Annu Rev Entomol 43:369- 93. Culpepper AS, York AC (1997) Weed management in no-tillage bromoxynil-tolerant cotton (Gossypium hirsutum). Weed Technol 11:335-345. Culpepper AS, Grey TL, Vencill WK, Kichler JM (2006) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia. Weed Sci 54:620-626. Culpepper AS, York AC, Roberts P, Whitaker JR (2009) Weed control and crop response to glufosinate applied to ‘PHY 485 WRF’ cotton. Weed Technol 23:356-362. Dotray PA, Keeling JW, Henniger CG, Abernathy JR (1996) Palmer amaranth (Amaranthus palmeri) and devil’s-claw (Proboscidea louisianica) control in cotton (Gossypium hirsutum) with pyrithiobac. Weed Technol 10:7-12. Egan JF, Mortensen DA (2012) Quantifying vapor drift of dicamba herbicides applied to soybean. Environ Toxicol Chem 31:1023-1031. Everitt JD, Keeling JW (2009) Cotton growth and yield response to simulated 2,4-D and dicamba drift. Weed Technol 23:503-506. Fangueiro R, Sohel R, ed. (2016) Natural Fibres: Advances in Science and Technology Towards Industrial Applications. Vol 12. SpringerNature. Pp 381-388. Garetson R, Singh V, Singh S, Dotray P, Bagavathiannan M (2019) Distribution of herbicide-resistant Palmer amaranth (Amaranthus palmeri) in row crop production systems in Texas. Weed Technol 33:355-365. Gonzini LC, Hart SE, Wax LM (1999) Herbicide combinations for weed management in glyphosate-resistant soybean (Glycine max). Weed Technol 13:354-360. Green JM (2009) Evolution of glyphosate-resistant crop technology. Weed Sci 57:108- 117. Guo P, Al-Khatib K (2003) Temperature effects on germination and growth of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis). Weed Sci 51:869-875. Hamner CL, Tukey HB (1944) The herbicidal action of 2,4 dichloprophenoxyacetic and 2,4,5 trichlorophenoxyacetic acid on bindweed. Science 100:154-155.

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Heap I (2020) The international survey of herbicide resistant weeds. http://www.weedscience.org. Accessed March 12, 2020. Inman MD, Jordan DL, Vann MC, Hare AT, York AC, Cahoon CW (2020) Influence of timing of Palmer amaranth control in dicamba-resistant cotton on yield and economic return. Weed Technol 34:1-7 Johnson VA, Fisher LR, Jordan DL, Edmisten KE, Stewart AM, York AC (2012) Cotton, peanut, and soybean response to sublethal rates of dicamba, glufosinate, and 2,4- D. Weed Technol 26:195-206. Jones GT, Norsworthy JK, Scott RC, Barber LT (2019) Off-target movement of DGA and BAMPA dicamba to sensitive soybean. Weed Technol 33:51-65. Jordan DL, Smith CM, McClelland MR, Frans RE (1993) Weed control with bromoxynil applied alone and with graminicides. Weed Technol 7:835-839. Kalsing A, Rossi CVS, Lucio FR, Zobiole LHS, da Cunha LCV, Minozzi GB (2018) Effect of formulations and spray nozzles on 2,4-D spray drift under field conditions. Weed Technol 32:379-384. Keeley PE, Carter CH, Thullen RJ (1987) Influence of planting date on growth of Palmer amaranth (Amaranthus palmeri). Weed Sci 35:199-204. Kniss AR (2018) Genetically engineered herbicide-resistant crops and herbicide-resistant weed evolution in the United States. Weed Sci 66:260-273. Kumaratilake AR, Preston C (2005) Low temperature reduce glufosinate activity and translocation in wild radish (Raphanus raphanistrum). Weed Sci 53:10-16. Lauriault LM, Thompson DC, Pierce JB, Michels GJ, Hamilton WV (2004) Managing Aceria malherbae gall mites for control of field bindweed. New Mexico State Cooperative Extension Service. https://web.archive.org/web/20090115214047/http://www.cahe.nmsu.edu/pubs/_c irculars/CR%20600.pdf. Accessed October 28, 2019. Legleiter T, Johnson B (2013) Palmer amaranth biology, identification, and management. Purdue Extension Local Faces. https://www.extension.purdue.edu/extmedia/WS/WS-51-W.pdf. Accessed October 28, 2019. 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 Cotton Sci. 7:231-235. Maddox DM, Andres LA (1979) Status of puncturevine weevils and their host plant in California. California Agriculture 33:7-9. Manuchehri MR, Dotray PA, Keeling JW (2017) EnlistTM weed control systems for Palmer amaranth (Amaranthus palmeri) management in Texas High Plains cotton. Weed Technol 31:793-798.

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Marple ME, Al-Khatib K, Shoup D, Peterson DE (2007) Cotton response to simulated drift of seven hormonal-type herbicides. Weed Technol 21:987-992. Maschhoff JR, Hart SE, Baldwin JL (2000) Effect of ammonium sulfate on efficacy, absorption, and translocation of glufosinate. Weed Sci 48:2-6. Merchant RM, Sosnoskie LM, Culpepper AS, Steckel LE, York AC, Braxton LB, Ford JC (2013) Weed response to 2,4-D, 2,4-DB, and dicamba applied alone or with glufosinate. J Cotton Sci 17:212-218. Merchant RM, Culpepper AS, Eure PM, Richburg JS, Braxton LB (2014) Controlling glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in cotton with resistance to glyphosate, 2,4-D, and glufosinate. Weed Technol 28:291-297. 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. [NASS] National Agricultural Statistics Service (2019) http://nass.usda.gov. Accessed August 27, 2019. National Academies of Sciences, Engineering, and Medicine (2016) Genetically Engineered Crops: Experiences and Prospects. Washington, DC: The National Academies Press. Pp 77-79. Oerke EC (2006) Crop losses to pests. J Agric Sci 144:31-43. Ou J, Thompson CR, Stahlman PW, Jugulam M (2017) Preemergence application of dicamba to manage dicamba-resistant kochia (Kochia scoparia). Weed Technol 32:309-313. Palhano MG, Norsworthy JK, Barber T (2017) Cover crop suppression of Palmer amaranth (Amaranthus palmeri) in cotton. Weed Technol 30:60-65. Paulsgrove MD, Wilcut JW (1999) Weed management in bromoxynil-resistant Gossypium hirsutum. Weed Sci 47:596-601. Pline WA, Price AJ, Wilcut JW, Edmisten KL, Wells R (2001) Absorption and translocation of glyphosate in glyphosate-resistant cotton as influenced by application method and growth state. Weed Sci 49:460-467. Reed J (2012) Palmer amaranth and ivyleaf morningglory management In GlyTol® + LibertyLink® cotton. Ph.D. dissertation. Lubbock, TX: Texas Tech University. 95 p. Reed JD, Keeling JW, Dotray PA (2014) Palmer amaranth (Amaranthus palmeri) management in GlyTol® LibertyLink® cotton. Weed Technol 28:592-600. Russell KR, Dotray PA, Pabuayon ILB, Ritchie GL (2020) Dicamba effects on fruiting sensitivity in cotton. Weed Technol 34:1-6. Shaner D (2000) The impact of glyphosate-tolerant crops on the use of other herbicides and on resistant management. Pest Manag Sci 56:320-326.

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Shergill LS, Bish MD, Biggs ME, Bradley KW (2017) Monitoring the changes in weed populations in a continuous glyphosate- and dicamba-resistant soybean system: a five-year field-scale investigation. Weed Technol 32:166-173. Sosnoskie LM, Culpepper AS (2014) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) increases herbicide use, tillage, and hand-weeding in Georgia cotton. Weed Sci 62:393-402. Sosnoskie LM, Culpepper AS, Braxton LB, Richburg JS (2015) Evaluating the volatility of three formulations of 2,4-D when applied in the field. Weed Technol 23:177- 184. Steckel GJ, Wax LM, Simmons FW, Phillips WH (1997) Glufosinate efficacy on annual weeds is influenced by rate and growth stage. Weed Technol 11:484-488. Steckel LE, Sprague CL, Hager AG (2002) Common waterhemp (Amaranthus rudis) control in corn (Zea mays) with single preemergence and sequential applications of residual herbicides. Weed Technol 16:755-761. Steckel LE, Sprague CL, Stoller EW, Wax LM (2004) Temperature effects on germination of nine Amaranthus species. Weed Sci 52:217-221. Stuart BL, Harrison SK, Abernathy JR, Kreig DR, Wendt CW (1984) The response of cotton (Gossypium hirsutum) water relations to smooth pigweed (Amaranthus hybridus) competition. Weed Sci 32:126-132. Takano HK, Beffa R, Preston C, Westra P, Dayan FE (2019) Reactive oxygen species trigger the fast action of glufosinate. Planta 249:1837-1849. [USDA] United States Department of Agriculture, Agricultural Marketing Service- Cotton and Tobacco Program (2019) Cotton varieties planted 2019 crop. https://www.ams.usda.gov/mnreports/cnavar.pdf. Accessed September 23, 2019. [USDA-NASS] United State Department of Agriculture- National Agricultural Statistics Service (2019) Annual cotton review. Southern Plains Regional Field Office. 4 p. Viator RP, Jost PH, Senseman SA, Cothren JT (2004) Effect of glyphosate application timings and methods on glyphosate-resistant cotton. Weed Sci 52:147-151. Webster TM, Grey TL (2015) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology, growth, and seed production in Georgia. Weed Sci 63:264- 272. [WSSA] Weed Science Society of America (2020) Glossary. http://wssa.net/wssa/wssaglossary/#:~:text=Herbicide%20%E2%80%93%20A%2 0chemical%20substance%20or,suppress%20the%20growth%20of%20plants. Accessed July 10, 2020. Whitaker JR, York AC, Jordan DL, Culpepper AS (2010) Palmer amaranth (Amaranthus palmeri) control in soybean with glyphosate and conventional herbicide systems. Weed Technol 24:403-410.

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

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CHAPTER II

THE INFLUENCE OF GLUFOSINATE IN DICAMBA BASED HERBICIDE SYSTEMS Abstract

Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern region of the United States and for decades has been one of the most common weeds in west Texas. In recent years, this weed has been considered one of the most troublesome weeds across the southern United States. The management of Palmer amaranth has changed since the discovery of glyphosate resistant populations in 2005. Dicamba tolerant cotton systems that utilize XtendiMax® with VaporGrip Technology® were introduced in 2016 and provide a new opportunity to manage glyphosate resistant populations. The use of glufosinate (Liberty® 280 SL) in a dicamba-based system may not only improve the management of glyphosate resistant Palmer amaranth, but also be effective against the development of herbicide resistance to new modes of action. Field studies were conducted in Lubbock and Halfway, Texas in 2018 and 2019 and Altus, Oklahoma in 2019 to determine the influence of sequential order and timing when applying dicamba and glufosinate to < 10 cm, 10 to 20 cm, and > 30 cm Palmer amaranth. The study in Lubbock, Texas also evaluated adding acetochlor in tank mixture in one of the sequential postemergence applications. Dicamba + acetochlor followed by glufosinate controlled Palmer amaranth greater than dicamba followed by dicamba at one or more rating events across all weed sizes. A difference in efficacy based on herbicide order was observed for < 10 cm Palmer amaranth, where glufosinate followed by dicamba was less effective than dicamba followed by glufosinate 7 and 14 days after sequential application in 2018 and 7 and 21 days after sequential application in 2019. No difference in efficacy based on herbicide order was observed for > 10 cm Palmer amaranth. Palmer amaranth control decreased as size increased, where > 30 cm Palmer amaranth was controlled < 57% 30 days after sequential application in 2018. Acetochlor reduced subsequent weed flushes when applied to > 30 cm Palmer amaranth in 6 out of 8 treatments and were the only treatments that reduced density. The study in Halfway, Texas and Altus, Oklahoma evaluated Palmer amaranth control when varying sequential application timing at a 3, 7,

Texas Tech University, Grace Flusche Ogden, December 2020 or 10 day interval after the initial application. Palmer amaranth control ranged from 88 to 98% 17 days after initial application. Sufficient evidence was not obtained in this study to alter sequential application timing interval recommendations from current label restrictions. Glufosinate serves as a compliment to a dicamba-based system, with acetochlor offering soil residual activity to help manage Palmer amaranth.

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Introduction

Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern United States and is a significant weed of concern in west Texas. Palmer amaranth compete for important resources and can cause crop loss. Using up to twice as much water as cotton (Gossypium hirsutum L.) (1.2 g of water cm-2 d-1) and producing more than half a million seeds per female plant, this weed is a challenge to manage (Berger et al. 2015; Legleiter and Johnson 2013; Webster and Grey 2015). Morgan et al. (2001) reported cotton yields were reduced 54% with 10 Palmer amaranth per 9 m of row. When glyphosate resistant crops where introduced in 1997, glyphosate use increased significantly (Young 2006), simplifying into systems of multiple applications of glyphosate alone. Over-reliance on glyphosate created a favorable environment for herbicide resistance (Garetson 2019). In 2005 the first glyphosate resistant Palmer amaranth was discovered in Georgia and glyphosate resistant populations have since been discovered across the United States, including West Texas (Culpepper et al. 2006; Heap 2020). Glyphosate resistance complicates the management of Palmer amaranth. With nearly 40% of the United States cotton grown in the High Plains region that encompasses West Texas and the surrounding areas (USDA-NASS 2019), finding sustainable and effective weed management strategies to control Palmer amaranth is essential to successful cotton production. Cotton varieties tolerant to dicamba (XtendiMax® with VaporGrip Technology®) were introduced in 2017 and may provide a new strategy to manage glyphosate resistant Palmer amaranth. This new technology involves a triple stack of genes that conveys herbicide tolerance to dicamba, glufosinate, and glyphosate. Prior to the development of this technology, dicamba was not labelled for use in cotton. Dicamba is a volatile herbicide that can move off-target and cause crop injury to non-tolerant plants. Non- dicamba tolerant cotton that is exposed to dicamba can sustain serious crop injury or die completely. Dicamba resistance is currently low in the Great Plains, which is one benefit to using this system. With tolerance to three postemergence broadleaf or broad spectrum herbicides, options are available to applicators that allow for herbicide rotation to minimize the development of herbicide resistance and crop injury. Weed managers who use this herbicide system need to have accurate, relevant information to decide when and 19

Texas Tech University, Grace Flusche Ogden, December 2020 how to use these products in order to successfully manage weeds and mitigate risk to surrounding crops. Research regarding the influence of glufosinate in dicamba-based systems is required to evaluate what use options are available and what factors allow for success of this new technology. Two studies were performed to evaluate variables such as sequential application timing, sequential spray order, weed size, the benefit of the addition of a soil residual herbicide, and whether glufosinate is an acceptable sequential application partner in this system. The data gathered from these studies could serve to improve the management of Palmer amaranth in West Texas and prolong the efficacy of dicamba- based cotton systems on the High Plains.

Texas Tech University, Grace Flusche Ogden, December 2020

Materials and Methods

Sequential Applications of Glufosinate and Dicamba with and without Acetochlor

A field experiment was conducted in 2018 and 2019 at the Texas A&M AgriLife Research and Extension Center (33°69’N, -101°82’W) near Lubbock, Texas to evaluate Palmer amaranth control following sequential applications of glufosinate (glufosinate- ammonium) and dicamba (diglycolamine salt; 3,6-dichloro-o-anisic acid) with and without acetochlor (2-chloro-N-ethoxymethyl-N-(2-ethyl-6-mehtylphenyl) acetamide). The trial location was a fallow field with a dense population of Palmer amaranth (approximately 70 per m2). Plot size was 4.05 m by 9.14 m. The soil was an Acuff Loam with 1% organic matter and a pH of 7.5 (USDA-NRCS). In-furrow irrigation was used to promote weed emergence, but no supplemental irrigation was used during the duration of the trial. Rainfall during the duration of the trial amounted to 100 mm from June 1 to August 31 in 2018 and 302 mm during this time ® -1 period in 2019. Pendimethalin (Prowl H2O ) at 0.86 kg active ingredient (ai) ha was applied preplant and incorporated twice to a depth of 5 to 8 cm using a rolling cultivator immediately after application on May 3, 2018 and April 25, 2019 to lessen Palmer amaranth density. Pendimethalin was applied using a tractor mounted three-point sprayer equipped with Turbo TeeJet 11002 nozzles calibrated to deliver 140 L ha-1 at 207 kPa. Treatments consisted of a non-treated weedy check, glufosinate followed by (fb) glufosinate, glufosinate fb dicamba, glufosinate + acetochlor fb glufosinate, glufosinate fb glufosinate + acetochlor, glufosinate + acetochlor fb dicamba, glufosinate fb dicamba + acetochlor, dicamba fb dicamba, dicamba fb glufosinate, dicamba + acetochlor fb glufosinate, dicamba + acetochlor fb dicamba, dicamba fb dicamba + acetochlor, and dicamba fb glufosinate + acetochlor. Treatments were applied to < 10 cm, 10 to 20 cm, and > 30 cm Palmer amaranth. Weed size was based on plant height at the time of the initial application. Strategic irrigation in 2018 allowed for all Palmer amaranth to be at these heights simultaneously. Thus, treatments to all sizes of Palmer amaranth were applied on June 15 and sequential applications were made 10 days later (June 25). Strategic watering in 2019 was attempted but did not yield all plant heights at a given date; therefore, treatments were applied as Palmer amaranth reached the desired height.

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Palmer amaranth < 10 cm were treated on July 1 fb the sequential application on July 12; Palmer amaranth 10 to 20 cm were treated on July 9 fb the sequential application on July 19; Palmer amaranth > 30 cm were treated on June 13 fb the sequential application on June 24. Weeds were hand-pulled or hoed in each size block to allow consistent population heights as necessary in both years. Glufosinate rate was dependent on the time of sequential treatment order. If glufosinate was applied in sequence, the second application rate was 0.59 kg ai ha-1 due to restrictions per the 2018 label (Anonymous 2018). Initial applications of glufosinate and those applied sequentially after dicamba or dicamba + acetochlor were applied at 0.88 kg ai ha-1. Dicamba was applied at 0.56 kg acid equivalent (ae) ha-1 and acetochlor -1 at 1.261 kg ai ha . A handheld 1.93m CO2-pressurized backpack calibrated to deliver 140 L ha-1 at 207 kPa-1 was used to apply all treatments. Application speed was 4.8 km hour-1. Turbo TeeJet 11002 nozzles were used for all glufosinate treatments whereas 11002 Turbo TeeJet Induction nozzles were used for all dicamba treatments. All glufosinate treatments included ammonium sulfate at 2.86 kg ha-1. Treatments were arranged in a randomized complete block design within weed size and replicated three times. Palmer amaranth control was evaluated 7 days after initial application (DAIA) and 7, 14, 21, and 30 days after sequential application (DASA). Control was estimated from 0 to 100, with 0 indicating no control and 100 indicating complete necrosis (Frans et al. 1986). The number of Palmer amaranth were recorded and harvested from one m2 within each plot and placed in mesh bags once weed control was less than 50% for the majority of plots, and fresh weights recorded in 2019. Palmer amaranth were harvested at 33 days after sequential application for Palmer amaranth < 10 cm at initial application, 62 days for 10 to 20 cm Palmer amaranth at initial application, and 37 days for Palmer amaranth > 30 cm at initial application. Mesh bags were dried in a plant dryer at 35°C for one to two weeks and dry weights were recorded. Data were subjected to ANOVA using the PROC GLIMMIX procedure of SAS 9.1 (SAS Institute Inc., SAS Campus Drive, Cary, NC 27513). Means of treatment effects were separated by year, weed size, and at an alpha level of 0.05. A year by treatment and weed size by treatment interaction was observed; therefore, data were analyzed separately by year and by weed size.

Texas Tech University, Grace Flusche Ogden, December 2020

Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals

This study was conducted in 2018 and 2019 at the Texas A&M AgriLife Research and Extension Center near Halfway, Texas (34°18’N ,101°92'W) and in 2019 in Altus, Oklahoma (34°36’N, 99°19’W) to evaluate Palmer amaranth control with 3, 7, and 10 day sequential applications of glufosinate-ammonium or dicamba (N,N-Bis-93- aminopropyl) methylamine salt of 3,6-dichloro-o-anisic acid). Treatments consisted of two sequential applications of glufosinate (Liberty® 280 SL) at 0.66 kg ai ha-1 glufosinate or dicamba (Engenia®) at 0.56 kg ae ha-1 at 3, 7, or 10 day intervals. Random variables were year and replication. A significant location by treatment effect was determined in visual rating data; therefore, means of treatment effects were separated by location at an alpha level of P=0.05.

Halfway

The Halfway trial location was a cotton field planted with ‘DP 1822 XF’ on May 16, 2018 and May 14, 2019 at 10 seeds m-1 in rows spaced 101 cm apart. Plot size was 4 m by 9 m. The soil type was a Pullman clay loam with less than 1% organic matter and a pH of 8.1 (USDA- NRCS). Palmer amaranth population was naturally occurring. The treatment area was conducted using an overhead center pivot irrigation system with 81 mm of water via low energy precision application (LEPA) irrigation during the duration of the trial, with an additional 37 mm of rainfall in 2018 from June 1 to August 31, 2018. In 2019 the trial area received 74 mm of water via sock irrigation during the duration of the trial and 122 mm of rainfall from June 1 to August 31, 2019. Treatments were applied on June 15, June 18, June 21, and June 25, 2018 and June 28, July 1, July 3, and July 5, 2019. Palmer amaranth were 5 to 10 cm in height at initial application in 2018 and 7 to 15 cm in 2019. A handheld 1.93 m CO2-pressurized backpack calibrated to deliver 140 L ha-1 at 207 kPa was used to apply all treatments. Turbo TeeJet 11002 nozzles were used for all glufosinate treatments and Turbo TeeJet Induction 11002 nozzles were used for all dicamba treatments. All glufosinate treatments included ammonium sulfate at 2.86 kg ha-1. The experimental design was a randomized complete block design with three replications. Palmer amaranth control was evaluated 3, 7, 10, 17 and 31 days after initial application (DAIA). Palmer amaranth control was

Texas Tech University, Grace Flusche Ogden, December 2020 estimated from 0 to 100, with 0 indicating no control and 100 indicating complete kill (Frans et al. 1986). The number of Palmer amaranth were recorded and harvested from one m2 within each plot and placed in mesh bags once weed control was less than 50% for the majority of plots and fresh weights recorded in 2019. Harvest occurred 36 days after the last sequential application. Mesh bags were dried in a plant dryer at 35°C for one to two weeks and dry weights recorded. Data were subjected to ANOVA using the PROC GLIMMIX procedure of SAS 9.1 (SAS Institute Inc., SAS Campus Drive, Cary, NC 27513). Random variables were year and replication. Means of treatment effects were separated an alpha level of 0.05. A year by treatment interaction was observed for the 30 DAIA Palmer amaranth control ratings; therefore, data were analyzed separately for that rating date.

Altus

The Altus trial location was planted to ‘PHY480 W3FE’ on May 30, 2019 at 13 seeds m-1 in rows spaced 76 cm apart. Plot size was 3 m by 8 m. The soil type was loam with 1.4% organic matter and a pH of 7. The trial area received 154 mm of rainfall from June 1 to August 31, 2019. Treatments were applied on June 18, June 21, June 25, and June 28, 2019. Initial treatments were applied when Palmer amaranth was 10 to 25 cm. A handheld 1.93m

CO2-pressurized backpack with Turbo TeeJet Induction 110015 nozzles calibrated to deliver 112 L ha-1 at 240 kPA was used to apply all treatments. The experimental design was a randomized complete block design with four replications. Palmer amaranth control was evaluated 10, 23 36, and 48 DAIA. Control was estimated from 0 to 100, with 0 indicating no control and 100 indicating complete weed kill (Frans et al. 1986). Data were subjected to ANOVA using the PROC GLIMMIX procedure of SAS 9.1 (SAS Institute Inc., SAS Campus Drive, Cary, NC 27513). Random variable was replication. Means of treatment effects were separated at an alpha level of P=0.05.

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Results and discussion

Sequential Applications of Glufosinate and Dicamba with and without Acetochlor

< 10 cm Palmer amaranth

Seven DAIA, glufosinate controlled < 10 cm Palmer amaranth 92 to 96%, which was greater than Palmer amaranth control following dicamba (69 to 73%) in 2018 (Table 2.1). When glufosinate was applied in the sequential application, at least 97% control was observed 7 DASA except for glufosinate fb glufosinate + acetochlor (92%) in 2018. Palmer amaranth was controlled 95% following both dicamba fb glufosinate + acetochlor and dicamba + acetochlor fb glufosinate 21 DASA. Palmer amaranth control ranged from 77 to 93% 30 DASA. In 2019, glufosinate controlled Palmer amaranth 89 to 90% 7 DAIA, which was greater than Palmer amaranth control following dicamba (78 to 79%) (Table 2.2). All treatments provided at least 89% Palmer amaranth control 7 DASA. Treatments with glufosinate in the sequential application controlled Palmer amaranth better than treatments with dicamba in the sequential application 7 DASA. Twenty-one DASA, dicamba + acetochlor fb glufosinate or dicamba controlled Palmer amaranth 91%, which was greater than treatments with glufosinate in the initial application. Dicamba + acetochlor fb glufosinate or dicamba controlled Palmer amaranth 75% and 73%, respectively, 30 DASA. Dicamba fb glufosinate provided as good as or better control of < 10 cm Palmer amaranth as two applications of dicamba or glufosinate across all rating dates in both years. Glufosinate fb dicamba was not as effective as dicamba fb glufosinate 7 and 14 DASA in 2018 and 7 and 21 DASA in 2019. Reed (2012) reported minimal control of Palmer amaranth of similar size with two or three sequential applications of glufosinate. In contrast, Corbett et al. (2004) reported 100% control of 2 to 5 cm and 8 to 10 cm Palmer amaranth following sequential applications of glufosinate made 10 days apart at lower rates of glufosinate per hectare. Less than 10 cm Palmer amaranth treated with dicamba + acetochlor fb glufosinate had 87% reduction in dry weight 33 DASA compared to the non-treated control (Table 2.3). Treatments of two applications of dicamba reduced dry weight by

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70% when compared to the non-treated control. No difference in dry weight reduction was observed between treatments of dicamba fb glufosinate (57%) or glufosinate fb dicamba (55%). All treatments reduced dry weight compared to the non-treated control. Less than 10 cm Palmer amaranth treated with dicamba + acetochlor fb glufosinate had 3 Palmer amaranth per m2 33 DASA (Table 2.4). The reverse of this treatment, glufosinate fb dicamba + acetochlor, had 42 Palmer amaranth per m2, where the greatest Palmer amaranth density out of all herbicide treatments was observed. Acetochlor applied in the initial application was able to control newly emerging Palmer amaranth seedlings, while the sequential application 10 days later likely occurred when Palmer amaranth were greater than 3 cm in height. This delay in application timing resulted in less effective Palmer amaranth control. Thus, including acetochlor in the initial application would create a more effective, sustainable herbicide system than including it in the sequential application or not at all. Treatments of two applications of dicamba had 28 Palmer amaranth per m2. All treatments reduced the number of Palmer amaranth per m2 compared to the non-treated control.

10 to 20 cm Palmer amaranth

In 2018, glufosinate controlled Palmer amaranth 81 to 83% 7 DAIA, which was greater than Palmer amaranth control following dicamba (64 to 66%). All treatments controlled Palmer amaranth greater than 90% 7 DASA except for treatments with two applications of dicamba alone (80%) (Table 2.1). Dicamba + acetochlor fb glufosinate controlled Palmer amaranth 91% 14 DASA, which was greater than Palmer amaranth control following dicamba fb dicamba (85%). Glufosinate + acetochlor fb dicamba controlled Palmer amaranth 75% 30 DASA, which was similar to dicamba fb dicamba (74%). In 2019, glufosinate controlled Palmer amaranth 90 to 92%, which was greater than Palmer amaranth control following dicamba (82 to 83%) 7 DAIA (Table 2.2). Similar to 2018, dicamba + acetochlor fb glufosinate controlled Palmer amaranth 85%, which was greater than Palmer amaranth control following dicamba fb dicamba (76%) 14 DASA. Palmer amaranth control ranged from 70 to 83% 21 DASA. Vann et al. (2017) found 16 to 23 cm Palmer amaranth were controlled 79% by two applications of

Texas Tech University, Grace Flusche Ogden, December 2020 glufosinate and 81% by two applications of dicamba 14 DASA. Sequential order of dicamba and glufosinate affected control of 10 to 20 cm Palmer amaranth at only one rating event in one out of two years: dicamba fb glufosinate + acetochlor controlled Palmer amaranth 98% 7 DASA, which was better than glufosinate + acetochlor fb dicamba (91%). For all other treatments and rating dates across 2018 and 2019, sequential order did not influence 10 to 20 cm Palmer amaranth control. Dry weight of Palmer amaranth that were 10 to 20 cm in height at initial application were reduced 86% 62 DASA from the dicamba fb dicamba treatment when compared to the non-treated control, which was similar to dicamba fb glufosinate (65%) (Table 2.3). Two applications of dicamba with acetochlor at either timing reduced dry weight 80 to 82% compared to the non-treated control. All treatments reduced Palmer amaranth dry weight compared to the non-treated control. No differences in Palmer amaranth density were observed 62 DASA (Table 2.4).

> 30 cm Palmer amaranth

In 2018, glufosinate controlled Palmer amaranth 51 to 57% 7 DAIA, which was greater than Palmer amaranth control following dicamba (39 to 43%) (Table 2.1). Dicamba + acetochlor fb glufosinate controlled Palmer amaranth 72% 7 DASA, which was greater than two applications of dicamba (50%) or glufosinate (60%). Twenty-one DASA, glufosinate fb dicamba controlled Palmer amaranth 55%, which was similar to dicamba + acetochlor fb glufosinate (54%) and greater than all treatments with dicamba alone in the initial application (50%). Palmer amaranth control ranged from 44 to 57% 30 DASA in 2018. In 2019, glufosinate alone provided similar Palmer amaranth control to glufosinate + acetochlor and dicamba + acetochlor, and greater control than dicamba alone (57%) 7 DAIA (Table 2.2). Palmer amaranth control following dicamba fb dicamba (66%) was similar to glufosinate + acetochlor fb dicamba (61%) 7 DASA. Two applications of glufosinate with or without acetochlor controlled Palmer amaranth 20% 21 DASA, while dicamba fb dicamba provided 79% control (Table 2.2). Dicamba fb glufosinate provided similar control (67%) to two applications of dicamba (79%) 21 DASA. Palmer amaranth was controlled < 69% following all treatments from 21 to 30

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DASA except dicamba + acetochlor fb dicamba, which maintained control above 70%. The challenge of reduced control of Palmer amaranth greater than 10 cm in size is well- documented. Reed (2012) reported applications of glufosinate alone controlled 5 to 10 cm Palmer amaranth greater than 15 to 20 cm Palmer amaranth 14 and 21 DAT. Whitaker et al. (2011) reported that although a second application of glufosinate initially improved Palmer amaranth control, many weeds survived and grew to heights too large to be controlled by a layby application when Palmer amaranth size at initial application was 5 to 8 cm. Thus, Palmer amaranth are best controlled before reaching 30 cm in height and later applications will not be effective. Treatments that included two applications of glufosinate did not reduce Palmer amaranth dry weight compared to the non-treated control (Table 2.3). Additionally, dicamba fb glufosinate + acetochlor did not reduce Palmer amaranth dry weight compared to the non-treated control. Glufosinate fb dicamba + acetochlor reduced Palmer amaranth dry weight by 60% compared to the non-treated control, which was similar to dicamba fb dicamba (72%). Acetochlor proved to impact the control of > 30 cm Palmer amaranth. Treatments that included acetochlor reduced subsequent weed flushes after application, reducing Palmer amaranth density 37 DASA compared to the non-treated control, except for treatments of dicamba fb dicamba + acetochlor and glufosinate + acetochlor fb dicamba (Table 2.4). The initial population of Palmer amaranth were relatively large and could have shaded out and out-competed emerging Palmer amaranth. Once treatments were applied and the canopy of Palmer amaranth was less dense, the shading and competition effect lessened and plots that were not treated with acetochlor could have experienced a new flush of Palmer amaranth. No efficacy difference was noted between the early postemergence or mid-postemergence application of acetochlor. The height of the Palmer amaranth likely plays a role in the importance of the addition of acetochlor to the herbicide system.

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Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals

Halfway

Three days after the initial application, dicamba treatments provided greater control (61%) than glufosinate treatments (43 and 53%) (Table 2.5). By 7 DAIA (4 DASA for the 3 day interval treatments, 7 and 10 day interval treatments had not been applied), glufosinate at 0.88 kg ai ha-1 controlled Palmer amaranth at 91 to 96%, which was better than all dicamba treatments (71 to 79%). Glufosinate at 0.88 kg ai ha-1 and glufosinate at 0.66 kg ai ha-1 at the 3 and 7 day interval provided greater Palmer amaranth control than all dicamba treatments 10 DAIA (7 DASA for the 3 day interval, 3 DASA for the 7 day interval, and the 10 day interval treatments had not been applied). In 2018, Palmer amaranth control ranged from 87 to 100% 30 DAIA (27 DASA for the 3 day interval, 23 DASA for the 7 day interval, and 20 DASA for the 10 day interval). Only one treatment resulted in an efficacy difference 30 DAIA in 2018. Glufosinate fb glufosinate at 0.88 kg ai ha-1 at the 3 day interval had 87% Palmer amaranth control, which was less than Palmer amaranth control at the 7 and 10 day intervals (94 and 100%, respectively). In 2019, dicamba treatments controlled Palmer amaranth 66 to 87% 30 DAIA, which was greater than all glufosinate treatments (27 to 37%). No efficacy differences were noted between the 3, 7, or 10 day sequential application timings for dicamba or glufosinate in 2019. The difference in Palmer amaranth control between years 30 DAIA could be due to increased rainfall in 2019, which created a favorable environment for regrowth of weeds that were not completely controlled and new weed flushes. Randell et al. (2019) reported 98 to 99% Palmer amaranth control with sequential glufosinate treatments at intervals of 1 to 7 days and 70 to 88% control with intervals of 10 to 14 days. All treatments reduced Palmer amaranth dry weight 36 DASA when compared to the non-treated control (Table 2.6). Dicamba fb dicamba at the 10 day interval reduced dry weight by 89% compared to the non-treated control, which was greater than all treatments of glufosinate at 0.66 kg ai ha-1 and glufosinate at 0.88 kg ai ha-1 at the 3 day interval. No differences in Palmer amaranth dry weight were noted based on sequential application timing interval.

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Palmer amaranth densities ranged from 3 to 33 m2 36 DASA and no differences among treatments were observed when compared to the non-treated control (Table 2.7). Palmer amaranth density was different between treatments. Treatments of glufosinate at 0.66 kg ai ha-1 at 3 and 7 day intervals had 33 and 28 Palmer amaranth per m2, respectively. Palmer amaranth density for dicamba treatments was 10 Palmer amaranth per m2 or fewer, with dicamba fb dicamba at the 10 day interval having 3 Palmer amaranth per m2. No differences in Palmer amaranth density were noted based on sequential application timing interval.

Altus

Glufosinate controlled Palmer amaranth better than dicamba at either rate at the 3 and 7 day interval 10 DAIA (7 DASA for the 3 day interval, 3 DASA for the 7 day interval, and the 10 day interval treatments had not been applied) (Table 2.8). Twenty- three DAIA (20 DASA for the 3 day interval, 16 DASA for the 7 day interval, 13 DASA for the 10 day interval) glufosinate fb glufosinate at the 0.66 kg ai ha-1 at the 3 and 10 day interval provided 76 and 73% control, respectively, which was greater than the 7 day interval (60%). Glufosinate fb glufosinate at 0.88 kg ai ha-1 at the 3 and 10 day interval controlled Palmer amaranth 91% and 85% 23 DAIA, respectively, which was greater than at the 7 day interval (73%). Dicamba at the 7 day interval provided 65% Palmer amaranth control 23 DAIA, which was greater than at the 3 or 10 day interval (50%). Thirty-six DAIA (33 DASA for the 3 day interval, 29 DASA for the 7 day interval, and 16 DASA for the 10 day interval) dicamba treatments provided greater control (71 to 76%) than glufosinate treatments (45 to 63%), with the exception of glufosinate fb glufosinate at 0.88 kg ai ha-1 at the 3 day interval (70%). Glufosinate at 0.59 kg ai ha-1 provided greater control at the 3 day interval 36 DAIA (63%) than the 7 or 10 day intervals (45 and 53%, respectively). This trend was also observed in 48 DAIA (45 DASA for the 3 day interval 41 DASA for the 7 day interval, and 38 DASA for the 10 day interval). Glufosinate at 0.88 kg ai ha-1 at the 3 day interval controlled Palmer amaranth 58% 48 DAIA, which was greater than the 7 day interval (43%). Dicamba treatments controlled Palmer amaranth 70 to 76% 48 DAIA. No differences in efficacy based on sequential timing were noted for dicamba treatments 36 or 48 DAIA.

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Locational differences between Halfway, Texas and Altus, Oklahoma may be due to environmental differences at the time of application. Time of day, humidity, and plant health can impact success of application and herbicide penetration into the leaf cuticle. Palmer amaranth were not harvested or counted at the Altus location for density and dry weight measurements. Further research at the Altus location is required before recommendations based on observations at this location are made. Palmer amaranth control decreased as Palmer amaranth size at initial application increased in the sequential applications of glufosinate and dicamba with and without acetochlor study. Palmer amaranth < 10 cm in height at initial application were best managed with dicamba + acetochlor fb glufosinate, while Palmer amaranth > 10 cm can be controlled or suppressed with the same set of herbicides in either sequential order. The addition of acetochlor reduced new weed flushes and reduced density at harvest for < 10 cm and > 30 cm Palmer amaranth. Acetochlor should be included in the initial application to optimize reduction of new weed emergence, particularly for applications made to < 10 cm Palmer amaranth. Two applications of glufosinate alone provided minimal Palmer amaranth control. Two applications of dicamba may provide adequate Palmer amaranth control but is not advised because of increased selection pressure for herbicide-resistant weeds. If two applications of either herbicide are required, follow label guidelines for sequential application timing interval. Sufficient evidence was not obtained in these studies to alter sequential application timing interval specified on current herbicide labels. Glufosinate is an acceptable sequential application partner in a dicamba-based system.

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Literature Cited Anonymous (2018) Liberty® herbicide product label. Research Triangle Park, NC: Bayer CropScience. 27 p Berger ST, Ferrell JA, Rowland DL, Webster TM (2015) Palmer amaranth (Amaranthus palmeri) competition for water in cotton. Weed Sci 64:928-935. Corbett JL, Askew SD, Thomas WE, Wilcut JW (2004) Weed efficacy evaluations for bromoxynil, glufosinate, glyphosate, pyrithiobac, and sulfosate. Weed Technol 18:443-453. Culpepper AS, Grey TL, Vencill WK, Kichler JM (2006) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia. Weed Sci 54:620-626. 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 Sci Soc. Garetson R, Singh V, Singh S, Dotray P, Bagavathiannan M (2019) Distribution of herbicide-resistant Palmer amaranth (Amaranthus palmeri) in row crop production systems in Texas. Weed Technol 33:355-365. Heap I (2020) The international survey of herbicide resistant weeds. http://www.weedscience.org. Accessed March 12, 2020. Legleiter T, Johnson B (2013) Palmer amaranth biology, identification, and management. Purdue Extension. https://www.extension.purdue.edu/extmedia/WS/WS-51- W.pdf. Accessed March 12, 2020. 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. Randell TM, Hand LC, Vance JC, Culpepper AS (2019) Interval between sequential glufosinate applications influences weed control in cotton. Weed Technol 33:1-6. Reed J (2012) Palmer amaranth and ivyleaf morningglory management In GlyTol® + LibertyLink® cotton. Ph.D. dissertation. Lubbock, TX: Texas Tech University. 95 p. [USDA- NASS] United States Department of Agriculture- National Agricultural Statistics Service (2019) Annual cotton review. Southern Plains Regional Field Office. 4 p. [USDA-NRCS] United States Department of Agriculture- Natural Resource Conservation Service: Official soil series. https://soilseries.sc.egov.usda.gov/ Accessed October 28, 2020.

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Vann RA, York AC, Cahoon CW, Buck TB, Askew MC, Seagroves RW (2017) Glufosinate plus dicamba for rescue palmer amaranth control in XtendFlexTM cotton. Weed Technol 31:666-674. Webster TM, Grey TL (2015) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology, growth, and seed production in Georgia. Weed Sci 63:264- 272. Whitaker JR, York AC, Jordan DL, Culpepper AS (2011) Weed management with glyphosate- and glufosinate-based systems in PHY 485 WRF cotton. Weed Technol 25: 183-191. Young BG (2006) Changes in herbicide use patterns and production practices resulting from glyphosate-resistant crops. Weed Technol 20:301-307.

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Table 2.1 Palmer amaranth control following initial and sequential applications of glufosinate and dicamba in 2018.

Palmer amaranth size at application < 10 cm 10 to 20 cm > 30 cm Days after applicationa EPOSTb MPOST EPOST MPOST EPOST MPOST Initial Sequential 7c 7 14 21 30 7 7 14 21 30 7 7 14 21 30 application application ------%------Glufosinate 92 b 83 a 57 a Glufosinate 98 a 92 abcd 86 c 86 c 97 ab 82 d 69 bc 62 d 60 bcd 59 abc 52 ab 47 bc Glufosinate 92 d 83 e 76 d 77 d 96 ab 83 cd 66 c 62 d 67 ab 55 bc 47 c 44 c + acetochlor Dicamba 92 d 88 de 88 bc 87 bc 93 abc 86 abcd 74 ab 72 abc 65 ab 62 ab 55 a 53 a Dicamba 94 cd 92 abcd 92 abc 89 abc 95 abc 90 ab 74 ab 73 abc 63 bc 64 a 54 ab 53 a + acetochlor Dicamba 69 c 66 b 39 c Glufosinate 98 a 96 ab 93 ab 91 abc 95 abc 87 abcd 69 bc 71 abc 67 ab 60 abc 50 bc 50 ab Glufosinate 99 a 97 a 95 a 92 ab 98 a 88 abc 72 abc 70 bc 67 ab 61 ab 50 bc 57 bc + acetochlor Dicamba 90 e 93 abc 92 abc 92 ab 80 e 85 bcd 71 abc 74 ab 50 e 55 bc 50 bc 51 a Dicamba 93 d 91 cd 94 ab 93 a 90 cd 87 abcd 73 abc 73 ab 53 de 57 abc 50 bc 51 a + acetochlor Glufosinate 96 a 81 a 51 b + acetochlor Glufosinate 99 a 94 abc 91 abc 92 ab 98 a 90 a 72 abc 67 cd 57 cde 53 c 47 c 46 c Dicamba 96 b 94 abc 93 ab 90 abc 91 bcd 86 abcd 73 abc 75 ab 60 bcd 58 abc 51 ab 51 a Dicamba 73 c 64 b 43 c + acetochlor Glufosinate 98 a 96 ab 95 a 91 abc 98 a 91 a 78 a 71 abc 72 a 63 a 54 a 51 a Dicamba 95 cd 93 abc 94 ab 93 a 86 de 86 abcd 73 abc 77 a 55 de 57 abc 50 bc 50 ab a Abbreviations: EPOST, early postemergence application; MPOST, mid-postemergence application; b EPOST timing applied June 15, 2018; rating dates are given as days after initial application (DAIA). MPOST timing applied June 25, 2018; rating dates are given as days after sequential application (DASA). c Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05.

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Table 2.2 Palmer amaranth control following initial and sequential applications of glufosinate and dicamba in 2019.

Palmer amaranth size at initial application < 10 cm 10- 20 cm > 30 cm Days after applicationa EPOSTb MPOST EPOST MPOST EPOST MPOST Initial Sequential 7 c 7 14 21 30 7 7 14 21 30 7 7 14 21 30 application application ------%------Glufosinate 90 a 90 a 67 a Glufosinate 99 a - 69 c 47 c - 91 a 77 ab 63 bcde 56 de 42 ab 20 e 10 e Glufosinate 97 a - 71 c 52 bc - 78 bcd 70 b 51 e 58 bcde 33 b 20 e 13 e + acetochlor Dicamba 91 bc - 57 d 48 c - 81 bcd 76 ab 64 abcde 55 de 45 ab 64 bcd 50 c Dicamba 89 bc - 57 d 50 c - 85 ab 82 a 68 abcde 18 f 52 ab 70 abc 57 b + acetochlor Dicamba 79 b 83 b 57 b Glufosinate 98 a - 83 b 58 bc - 85 ab 80 a 67 abcde 59 bcde 42 ab 67 abcd 37 d Glufosinate 99 a - 85 ab 64 ab - 84 abc 74 ab 58 cde 57 cde 50 ab 58 cd 35 d + acetochlor Dicamba 91 bc - 69 c 55 bc - 76 cd 82 a 80 ab 66 a 55 a 79 a 69 a Dicamba 89 bc - 74 c 58 bc - 78 bcd 83 a 81 a 64 ab 54 a 70 abc 69 a + acetochlor Glufosinate 89 a 92 a 63 ab + acetochlor Glufosinate 98 a - 83 b 59 bc - 90 a 81 a 71 abcd 55 e 38 ab 20 e 10 e Dicamba 89 bc - 70 c 55 bc - 86 ab 82 a 75 abc 61 abcd 48 ab 56 d 48 c Dicamba 78 b 82 b 58 ab + acetochlor Glufosinate 98 a - 91 a 75 a - 85 ab 78 ab 57 de 58 bcde 43 ab 62 bcd 40 d Dicamba 92 b - 91 a 73 a - 75 d 83 a 80 ab 63 abc 53 a 74 ab 72 a a Abbreviations: EPOST, early postemergence application; MPOST, mid-postemergence application; b EPOST timing applied July 1, July 9, and June 13 to Palmer amaranth < 10 cm, 10 to 20 cm, and > 30 cm in height at initial application, respectively; rating dates are given as days after initial application (DAIA). MPOST timing applied July 12, July 19, and June 24 to Palmer amaranth < 10 cm, 10 to 20 cm, and >30 cm in height at initial application, respectively; rating dates are given as days after sequential application (DASA). c Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05

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Table 2.3 Palmer amaranth dry weight 33 to 62 days after sequential applications of glufosinate and dicamba in 2019. Dry weightab Initial application Sequential application < 10 cm 10 to 20 cm > 30 cm ------g per m2------Non-treated control 679 a 685 a 547 a Glufosinate Glufosinate 275 bcd 307 b 520 a Glufosinate + acetochlor 188 de 278 bcd 544 a Dicamba 312 bc 292 bc 290 bcd Dicamba + acetochlor 357 b 220 bcde 220 cd Dicamba Glufosinate 293 bcd 241 bcde 323 bcd Glufosinate + acetochlor 205 cde 360 b 396 abc Dicamba 210 bcde 96 e 154 d Dicamba + acetochlor 160 de 134 cde 167 d Glufosinate + acetochlor Glufosinate 209 bcde 221 bcde 414 ab Dicamba 248 bcd 219 bcde 280 bcd Dicamba + acetochlor Glufosinate 90 e 317 b 315 bcd Dicamba 95 e 126 de 210 d a Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05 b Palmer amaranth harvested 33 days after sequential application for Palmer amaranth <10 cm at initial application, 62 days after sequential application for Palmer amaranth 10 to 20 cm at initial application, and 37 days for Palmer amaranth >30 cm at initial application.

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Table 2.4 Palmer amaranth density 33 to 62 days after sequential applications of glufosinate and dicamba in 2019. Palmer amaranth size at initial applicationa Initial application Sequential application < 10 cmb 10 to 20 cm > 30 cm ------No. per m2------Non-treated control 69 a 50 a 24 a Glufosinate Glufosinate 18 cdef 4 b 16 ab Glufosinate + acetochlor 13 def 8 b 9 b Dicamba 32 bc 14 b 20 ab Dicamba + acetochlor 42 b 13 b 8 b Dicamba Glufosinate 12 def 10 b 15 ab Glufosinate + acetochlor 8 ef 8 b 11 b Dicamba 28 bcd 4 b 20 ab Dicamba + acetochlor 20 cde 5 b 12 ab Glufosinate + acetochlor Glufosinate 3 f 3 b 10 b Dicamba 15 def 9 b 12 ab Dicamba + acetochlor Glufosinate 3 f 12 b 11 b Dicamba 8 ef 6 b 7 b a Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05 b Palmer amaranth counted 33 days after sequential application for Palmer amaranth <10 cm at initial application, 62 days after sequential application for Palmer amaranth 10 to 20 cm at initial application, and 37 days for Palmer amaranth >30 cm at initial application.

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Table 2.5 Palmer amaranth control following 3, 7, and 10 day sequential applications of glufosinate and dicamba in Halfway, Texas in 2018 and 2019.

Days after initial applicationa 2018b 2019 c Herbicide treatment Rate Initial application Sequential application 3 7 10 17 30 30 kg ai or ae ha-1 ------%------Glufosinate fb glufosinate 0.66 June 28 43 c July 1 95 a 95 a 95 abc 96 ab 33 b July 5 89 ab 94 a 95 abc 98 ab 27 b July 8 83 bc 83 b 90 cd 92 bc 37 b Glufosinate fb glufosinate 0.88 June 28 53 b July 1 96 a 97 a 98 a 87 c 27 b July 5 91 a 95 a 97 ab 94 ab 30 b July 8 91 a 93 a 98 a 100 a 32 b Dicamba fb dicamba 0.56 June 28 61 a July 1 79 cd 82 b 90 cd 97 ab 66 a July 5 71 e 79 b 88 d 99 ab 75 a July 8 74 de 79 b 92 bcd 100 a 87 a a Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05 b Data presented in these columns were pooled across both years (2018 and 2019) unless otherwise indicated c Rating dates are given as days after initial application (DAIA).

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Table 2.6 Palmer amaranth density and dry weight 36 days after the final sequential application of 3, 7, and 10 day sequential applications of glufosinate and dicamba in 2019.

Herbicide treatmenta Rate Initial application Sequential application Palmer amaranthb kg ai or ae ha-1 ------No. per m2 ------g per m2------Non-treated control 23 abc 1106 a Glufosinate fb glufosinate 0.66 June 28 July 1 33 a 484 bc July 5 28 ab 474 bc July 8 20 abc 453 bc Glufosinate fb glufosinate 0.88 June 28 July 1 17 abc 504 b July 5 10 bc 290 bcd July 8 9 bc 392 bcd Dicamba fb dicamba 0.56 June 28 July 1 10 bc 305 bcd July 5 8 bc 187 cd July 8 3 c 118 d aAbbreviations: fb, followed by. bMeans within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05

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Table 2.7 Palmer amaranth control following 3, 7, and 10 day sequential applications of glufosinate and dicamba in Altus, Oklahoma in 2019. Days after initial application Herbicide treatmenta Rate Initial application Sequential application 10bc 23 36 48 kg ai or ae ha-1 ------%------Glufosinate fb glufosinate 0.66 June 18 June 21 88 ab 76 bc 63 def 49 de June 25 90 a 60 ef 45 h 28 g June 28 50 d 73 cd 53 gh 34 fg Glufosinate fb glufosinate 0.88 June 18 June 21 91 a 91 a 70 abcd 58 cd June 25 94 a 73 cd 58 fg 43 ef June 28 58 cd 85 ab 61 ef 50 de Dicamba fb dicamba 0.56 June 18 June 21 50 d 50 f 71 abc 70 ab June 25 53 d 65 de 76 a 76 a June 28 50 d 50 f 74 ab 73 a aAbbreviations: fb, followed by. bMeans within columns followed by the same letter are not significantly different according to differences of means using Fisher’s Protected LSD in SAS at P < 0.05. c Rating dates are given as days after initial application (DAIA).

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CHAPTER III

THE INFLUENCE OF GLUFOSINATE IN 2,4-D BASED HERBICIDE SYSTEMS Abstract Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern region of the United States and for decades has been one of the most common weeds in West Texas. The management of Palmer amaranth has changed since the discovery of glyphosate resistant populations in 2005. Weed managers are searching for control options that mitigate further herbicide resistance. One such system that was introduced in 2016 is 2,4-D tolerant cotton that utilizes EnlistTM. To best utilize this system, research was needed to better understand the optimal timing of glufosinate and 2,4-D. Two field studies were conducted in a non-crop environment in Lubbock, Texas in 2018 and 2019 to determine the influence of sequential order when applying 2,4-D choline and glufosinate alone and in tank-mixtures on Palmer amaranth control across different growth stages. The addition of a soil residual herbicide, pyrithiobac, to sequential applications also was evaluated. The population of Palmer amaranth at this location averaged over 70 plants per m2. In the study sequential applications of glufosinate and 2,4-D choline, 2,4-D choline + glyphosate followed by glufosinate controlled 7 to 15 cm Palmer amaranth 100% 10 days after sequential application in 2018 and 97% 21 days after sequential application in 2019. When applied to 25 to 30 cm Palmer amaranth, 2,4- D choline + glyphosate followed by glufosinate controlled Palmer amaranth 98% 21 days after the sequential application. The addition of pyrithiobac did improve Palmer amaranth control in 2019, with 2,4-D choline + pyrithiobac followed by glufosinate providing greater control (96%) than 2,4-D alone followed by glufosinate (86%). 2,4-D choline followed by 2,4-D choline provided the least Palmer amaranth control at multiple visual rating events in both years. In the study 2,4-D choline tank-mix applications, 2,4-D choline tank-mixed with glufosinate followed by glufosinate controlled 7 to 15 cm Palmer amaranth 70% 17 days after sequential application. 2,4-D choline + glyphosate followed by 2,4-D choline tank-mixed with glufosinate provided 70% control of 25 to 30 cm Palmer amaranth 7 days after sequential application. Palmer amaranth < 30 cm in

Texas Tech University, Grace Flusche Ogden, December 2020 height at initial application can be controlled with 2,4-D choline + glyphosate followed by glufosinate. 2,4-D choline alone applied sequentially failed to control Palmer amaranth and is not recommended. Treatments of 2,4-D choline + glyphosate followed by glufosinate incorporate 3 modes of action into the herbicide system, helping to maintain the integrity of this new system and provide Palmer amaranth control when used as a sequential application system.

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Introduction The High Plains of West Texas is home to 970,000 hectares of cotton (Gossypium hirsutum L.) (USDA- NASS 2019), where producers must manage limited resources to grow their crops. One management concern in the region is glyphosate resistant Palmer amaranth (Amaranthus palmeri S. Watson). Palmer amaranth is native to the southwestern United States and is a common weed in West Texas. Palmer amaranth can produce more than half a million seeds and use twice as much water as cotton (1.2 g of water cm-2 d-1) (Berger et al. 2015; Legletier and Johnson 2013; Webster and Grey 2015). Morgan et al. (2001) reported a 54% reduction in cotton yield with 10 Palmer amaranth per 9 m of row. When glyphosate resistant crops were introduced in 1997, glyphosate use increased (Young 2006). Over-reliance on glyphosate created an environment that aided in the development of glyphosate resistance, complicating the management of Palmer amaranth (Garetson et al. 2015). In the wake of glyphosate resistance, weed managers are searching for new options to control weeds. Cotton varieties tolerant to 2,4-D choline that utilize EnlistTM technology were introduced in 2016 and offer the opportunity to use a herbicide in cotton that previously was not an option. This line of 2,4-D tolerant cotton also is tolerant to glufosinate and glyphosate, allowing for other postemergence applications once the cotton has emerged. Glyphosate, glufosinate, and 2,4-D have different modes of action from one another and can be tank-mixed or sequentially applied with each other or a variety of soil residual herbicides or gramacides for full spectrum weed control. Using multiple modes of action in a herbicide system slows the development of herbicide resistance and can prolong the sustainability of the 2,4-D tolerant cotton system. In order to best utilize the 2,4-D tolerant cotton system, research is required to evaluate the influence of sequential order, tank-mixes, and Palmer amaranth size when applying glufosinate and 2,4-D choline or 2,4-D choline + glyphosate. The first study, sequential applications of glufosinate and 2,4-D choline, aimed to determine the influence of sequential spray order on efficacy of 2,4-D and glufosinate when applied 10 days apart on 7 to 15 and 25 to 30 cm Palmer amaranth. Additionally, pyrithiobac was added to two sequential treatments to determine the benefit of a soil residual herbicide in the system. In the second study, 2,4-D tank-mix applications were used to evaluate the

Texas Tech University, Grace Flusche Ogden, December 2020 efficacy of sequential applications of tank-mixes of 2,4-D choline and glufosinate on 7 to 15 and 25 to 30 cm Palmer amaranth. The information from this research could serve to improve Palmer amaranth control on the High Plains and provide information on factors that may contribute to the success of 2,4-D choline herbicide systems.

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Materials and Methods Two separate trials were conducted at the Texas A&M AgriLife Research and Extension Center near Lubbock, Texas (33°69’N, -101°82’W) to evaluate Palmer amaranth control following applications of glufosinate, 2,4-D choline, and 2,4-D choline + glyphosate. The sequential applications study evaluated applications of glufosinate and 2,4-D choline or 2,4-D choline + glyphosate during the 2018 and 2019 growing seasons. A tank-mix applications study evaluated tank-mixes of glufosinate and 2,4-D choline or 2,4-D choline + glyphosate in 2019. Both trials were conducted in a fallow field with a dense population Palmer amaranth (approximately 70 m2). Plot size was 4.05 m by 9.14 m. The soil was an Acuff Loam with 1% organic matter and a pH of 7.5. In-furrow irrigation was used to promote weed emergence, but no supplemental irrigation was used during the duration of the trial. Rainfall during the duration of the trials amounted to 100 mm occurring from June 1 to August 31 in 2018 and 302 mm during the same period in ® -1 2019. Pendimethalin (Prowl H2O ) at 0.86 kg active ingredient (ai) ha was applied preplant and incorporated twice to a depth of 5 to 8 cm using a rolling cultivator immediately after application on May 3, 2018 and April 25, 2019 to lessen Palmer amaranth density. Pendimethalin was applied using a tractor mounted three-point sprayer equipped with Turbo TeeJet 11002 nozzles calibrated to deliver 140 L ha-1 at 207 kPa. -1 A handheld 1.93 m CO2-pressurized backpack calibrated to deliver 140 L ha was used to apply all treatments. Application speed was 4.8 km hour-1. Turbo TeeJet 11002 nozzles were used for all glufosinate treatments whereas Turbo TeeJet Induction 11002 nozzles were used for all 2,4-D treatments. All glufosinate treatments included ammonium sulfate at 2.86 kg ha-1. Treatments were applied to 7 to 15 cm or 25 to 30 cm Palmer amaranth. Weed size was based on height at initial application. Palmer amaranth control was estimated from 0 to 100, with 0 indicating no control and 100 indicating complete necrosis (Frans et al. 1986). Palmer amaranth were counted and harvested from one m2 within each plot and placed in mesh bags once weed control was less than 50% for the majority of plots. Fresh weights were recorded. Palmer amaranth were dried in a plant dryer at 35°C for one to two weeks, and dry weights recorded.

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Sequential Applications of Glufosinate and 2,4-D Choline

This study was conducted to evaluate Palmer amaranth control with sequential applications of glufosinate-ammonium, 2,4-D choline (2,4-Dichlorophenoxyacetic acid, choline salt), 2,4-D choline + glyphosate (2,4-Dichlorophenoxyacetic acid, choline salt + N-(phosphonomethyl)glycine, dimethylammonium salt) and pyrithiobac-sodium (Sodium 2-chloro-6-[(4,6-dimethoxy pyrimidin- 2-yl)thio] benzoate). Treatments included a non-treated weedy check for each weed size, 2,4-D choline + glyphosate fb 2,4-D choline + glyphosate, 2,4-D choline + glyphosate fb glufosinate, 2,4-D choline + glyphosate fb 2,4-D choline, 2,4-D choline fb 2,4-D choline, 2,4-D choline fb 2,4-D choline + glyphosate, 2,4-D choline fb glufosinate, 2,4-D choline + pyrithiobac fb glufosinate, glufosinate fb glufosinate, glufosinate fb 2,4-D choline + glyphosate, glufosinate fb 2,4-D choline, and glufosinate + pyrithiobac fb glufosinate. Treatments were applied to 7 to 15 cm Palmer amaranth on May 25, 2018 with sequential applications on June 4, 2018 and on June 7, 2019 with sequential application on June 17, 2019. Treatments were applied to 25 to 30 cm Palmer amaranth on June 4, 2018 with sequential applications following on June 14, 2018 and on June 13, 2019 with sequential applications on June 24, 2019. In 2018, the experiment was performed with treatments arranged in a randomized complete block design within weed size while in 2019 it was performed as a factorial design arranged in a randomized complete block with three replications. Glufosinate rate was dependent upon the initial herbicide treatment. If glufosinate was applied in sequence, the second application rate was 0.59 kg ai ha-1 due to restrictions per the 2018 label (Anonymous 2018). Initial applications of glufosinate and those applied sequentially after 2,4-D or 2,4-D + glyphosate were applied at 0.88 kg ai ha-1, 2,4-D + glyphosate (Enlist DuoTM with Colex D Technology TM) at 1.62 kg acid equivalent (ae) ha-1, 2,4-D choline (Enlist OneTM with Colex D TechnologyTM) was applied at 0.80 kg ae ha-1, and pyrithiobac-sodium (Staple® LX) was applied at 0.073 kg ai ha-1. Treatments that contained pyrithiobac included concentrated crop oil at 0.5% volume product per volume mix basis. Palmer amaranth control was evaluated 7 to 10 days after initial application (DAIA) and 10 to 15, 21, and 36 days after sequential application (DASA). The number

Texas Tech University, Grace Flusche Ogden, December 2020 of Palmer amaranth were counted and harvested from one m2 within each plot for dry weight and density 43 and 36 days after the last sequential application for 7 to 15 cm and 25 to 30 cm Palmer amaranth, respectively. Data were subjected to ANOVA using the PROC GLIMMIX procedure of SAS 9.1 (SAS Institute Inc., SAS Campus Drive, Cary, NC 27513). Random variables were year, weed size, and replication. A year by treatment interaction was observed. Means of treatment effects were separated by year, weed size, and at an alpha level of P=0.05.

2,4-D Choline Tank-Mix Applications

This study was conducted to evaluate Palmer amaranth control with sequential, tank-mixed applications of glufosinate, 2,4-D choline and/or 2,4-D choline + glyphosate. Treatments consisted of an non-treated weedy check for both weed sizes, 2,4-D choline + glyphosate fb 2,4-D choline + glufosinate, 2,4-D choline + glufosinate fb 2,4-D choline + glyphosate, 2,4-D choline + glufosinate fb glufosinate, 2,4-D choline + glufosinate fb 2,4-D choline + glufosinate, and glufosinate fb 2,4-D choline + glufosinate. Applications were made to 7 to 15 cm Palmer amaranth on July 1, 2019 and sequential applications on July 12, 2019. Applications were made to 25 to 30 cm Palmer amaranth on June 13 with sequential applications made on June 24, 2019. Treatments were arranged as a randomized complete block design within weed size and replicated three times. Glufosinate rate was dependent on the time of sequential treatment order. If glufosinate was applied in sequence, the second application rate was 0.59 kg ai ha-1 due to restrictions per the 2018 label (Anonymous 2018). Initial applications of glufosinate and those applied sequentially after a tank-mix with 2,4-D choline or 2,4-D choline + glyphosate were applied at 0.88 kg ai ha-1, 2,4-D choline at 0.80 kg ae ha-1, and 2,4-D choline + glyphosate at 1.62 kg ae ha-1. Percent weed control was estimated 8 DAIA and 7, 15 to 17, and 23 to 28 DASA. Palmer amaranth density were counted and harvested from one m2 within each plot for dry weight and density 34 and 37 days after the last sequential application for 7 to 15 cm and 25 to 30 cm Palmer amaranth, respectively. Data were subjected to ANOVA using the PROC GLIMMIX procedure of SAS 9.1 (SAS Institute Inc., SAS Campus Drive,

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Cary, NC 27513). Random variables were weed size and replication. Means of treatment effects were separated at an alpha level of 0.05.

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Results and Discussion Sequential Applications of Glufosinate and 2,4-D Choline

7 to 15 cm Palmer amaranth

In 2018, glufosinate alone controlled 7 to 15 cm Palmer amaranth 95% 10 days DAIA, which was greater than all treatments containing 2,4-D choline (Table 3.1). Ten DASA, only treatments of 2,4-D choline + glyphosate fb 2,4-D choline and 2,4-D choline fb 2,4-D choline controlled Palmer amaranth < 92%. Palmer amaranth was controlled 100% by 2,4-D choline + glyphosate fb glufosinate 10 DASA and 97% 21 DASA in 2018. Pyrithiobac provided an increase Palmer amaranth control 21 DASA when used with 2,4-D choline and fb glufosinate (96%), compared to 2,4-D choline alone fb glufosinate (86%). Sequential application order influenced treatments of 2,4-D choline fb glufosinate, with 2,4-D choline in the initial application providing greater Palmer amaranth control than vice versa 21 DASA. In 2019, 2,4-D choline + glyphosate controlled 7 to 15 cm Palmer amaranth 92% 7 DAIA, which was greater than all other treatments (Table 3.2). Treatments that included 2,4-D choline + glyphosate in the initial application controlled Palmer amaranth 90 to 93% 10 DASA, while treatments with 2,4-D alone in the initial application controlled Palmer amaranth 62 to 75%. Two applications of 2,4-D choline + glyphosate controlled Palmer amaranth 80% 21 DASA, which was similar to 2,4-D choline fb glufosinate (79 to 80%). 2,4-D choline fb 2,4-D choline + glyphosate controlled Palmer amaranth greater than vice versa 21 DASA in both years. Sequential order did not influence the efficacy of 2,4-D choline + glyphosate and glufosinate in either year. Culpepper et al. (2011) reported sequential 2,4-D herbicide systems controlled up to 7 cm Palmer amaranth 84%. Stephenson et al. (2011) reported up to 10 cm Palmer amaranth control was greatest following 2,4-D + glyphosate when compared to other treatments of 2,4-D alone or co-applied with glufosinate or glyphosate. There were no differences among treatments for measurements of Palmer amaranth density or dry weight (Tables 3.3 and 3.4).

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25 to 30 cm Palmer amaranth

In 2018, 25 to 30 cm Palmer amaranth control ranged from 65 to 85% 10 DAIA (Table 3.1). 2,4-D choline + glyphosate, glufosinate + pyrithiobac, and glufosinate alone controlled Palmer amaranth better than 2,4-D choline alone or with pyrithiobac 10 DAIA. 2,4-D choline + glyphosate applied twice, 2,4-D choline + glyphosate fb glufosinate, and glufosinate fb 2,4-D choline + glyphosate controlled Palmer amaranth 95 to 98% 11 DASA, which was greater than all other treatments. Glufosinate + pyrithiobac fb glufosinate controlled Palmer amaranth 55% 11 DASA and 35% 21 DASA. 2,4-D choline + glyphosate fb glufosinate controlled Palmer amaranth 98% 11 and 21 DASA. Treatments containing an application of 2,4-D choline alone controlled Palmer amaranth ≤ 70% 21 DASA, except for 2,4-D choline + glyphosate fb 2,4-D choline (91%). In 2019, 2,4-D choline + glyphosate controlled Palmer amaranth 80% 7 DAIA, while other treatments controlled Palmer amaranth 58 to 66% (Table 3.2). Palmer amaranth control ranged from 25 to 65% 15 DASA. 2,4-D choline + glyphosate fb 2,4-D choline or glufosinate controlled Palmer amaranth > 60% 15 DASA. Palmer amaranth control was ≤ 30% for 9 out of 11 treatments 36 DASA in 2019. Treatments with 2,4-D choline + glyphosate and glufosinate applied in either order were among the treatments with the greatest Palmer amaranth control 21 DASA in 2018 and 15 DASA in 2019. 2,4- D choline + glyphosate fb glufosinate provided successful control of both sizes of Palmer amaranth at multiple rating dates across both years. 2,4-D choline fb glufosinate provided greater weed control than glufosinate fb 2,4-D choline in both years. Merchant et al. (2014) reported 93 to 94% control of 20 cm Palmer amaranth following 2,4-D fb glufosinate applications made 10 days apart. There were no differences among treatments for measurements of Palmer amaranth density or dry weight (Tables 3.3 and 3.4).

2,4-D Choline Tank-Mix Applications

7 to 15 cm Palmer amaranth

Seven DAIA, 2,4-D choline + glufosinate controlled 7 to 15 cm Palmer amaranth 92%, which was greater than 2,4-D choline + glyphosate (88%) (Table 3.5). Palmer amaranth control ranged from 96 to 99% 7 DASA. In the 10 days following, more Palmer

Texas Tech University, Grace Flusche Ogden, December 2020 amaranth emerged and grew quickly. Palmer amaranth control ranged from 57 to 70% 17 DASA. 2,4-D choline + glufosinate fb glufosinate controlled Palmer amaranth 70% 17 DASA. Palmer amaranth control was 20% for all treatments 28 DASA. A thick, uniform blanket of 10 cm Palmer amaranth was observed 28 DASA. Initial herbicide efficacy was quickly overshadowed by lack of emerging weed prevention. With the initial weeds Meyer and Norsworthy (2019) reported 97% control of 10 cm Palmer amaranth by tank- mixes of 2,4-D choline and glufosinate when evaluated 5 weeks after treatment. All treatments decreased 7 to 15 cm Palmer amaranth dry weight 46 to 52% when compared to the non-treated control; however, there were no differences between treatments (Table 3.6). Palmer amaranth dry weight was decreased 47% by 2,4-D choline + glufosinate fb glufosinate when compared to the non-treated control. No treatments decreased Palmer amaranth per m2 compared to the non-treated control (Table 3.7).

25 to 30 cm Palmer amaranth

2,4-D choline + glyphosate controlled Palmer amaranth 78% 7 DAIA, which was greater than 2,4-D choline + glufosinate (70%) or glufosinate alone (72%) (Table 3.5). Seven DASA, 2,4-D choline + glyphosate fb 2,4-D choline + glufosinate controlled Palmer amaranth 70%, which was greater than all other treatments and remained greater than all other treatments for the duration of the study. Palmer amaranth control ranged from 37% to 63% 23 DASA. Meyer and Norsworthy (2019) reported greater Palmer amaranth control of 30 cm Palmer amaranth following 2,4-D + glufosinate than 2,4-D or glufosinate alone. Chafin et al. (2010) reported 91% or greater Palmer amaranth control of slightly smaller (17 to 22 cm) Palmer amaranth following tank-mixes of glufosinate + 2,4-D. Palmer amaranth 20 cm in height were controlled 99% at layby following two sequential applications of 2,4-D + glufosinate made 10 days apart (Merchant et al. 2014). Treatments containing an application of glufosinate alone did not decrease 25 to 30 cm Palmer amaranth dry weight compared to the non-treated control (Table 3.6). Palmer amaranth dry weight was reduced 58% following 2,4-D choline + glyphosate fb 2,4-D choline + glufosinate when compared to the non-treated control. No treatments decreased Palmer amaranth per m2 when compared to the non-treated control (Table 3.7).

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Palmer amaranth < 30 cm in height at initial application can be controlled or suppressed with 2,4-D choline + glyphosate fb glufosinate. Herbicide efficacy decreased as Palmer amaranth size at initial application increased, making timely applications to smaller weeds important for success of this system. Sequential order influenced a few treatments. 2,4-D choline provided greater control when used in the initial application when paired with glufosinate across both years and both weed sizes. 2,4-D choline fb 2,4- D choline + glyphosate controlled 7 to 15 cm Palmer amaranth greater than vice versa 21 DASA in both years. Tank-mixing 2,4-D choline with glufosinate provided effective short-term Palmer amaranth control when used in a sequential application system; however, further research to find a soil residual herbicide that performs well in this system is required in order for it to be successful more than 7 DASA. 2,4-D choline alone applied sequentially failed to control Palmer amaranth and is not recommended.

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Literature Cited Anonymous (2018) Liberty ® herbicide product label. Research Triangle Park, NC: Bayer CropScience. 27 p Berger ST, Ferrell JA, Rowland DL, and Webster TM (2015) Palmer amaranth (Amaranthus palmeri) competition for water in cotton. Weed Sci 64:928-935. Culpepper AS, Richburg JS, York AC, Steckel LE, Braxton LB (2011) Managing glyphosate-resistant Palmer amaranth using 2,4-D systems in DHT cotton in GA, NC, and TN. Pages 1543-1544 in the 2011 Beltwide Cotton Conferences proceedings. Atlanda, GA. Chafin JE, Culpepper AS, Braxton LB (2010) Palmer amaranth, benghal dayflower, carpetweed, pitted morningglory, and broadleaf signalgrass response to glufosinate applied alone or mixed ith 2,4-D or dicamba. Pages 1551-1552 in 2010 Beltwide Cotton Conferences proceedings. New Orleans, LA. 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 Sci Soc. Garetson R, Singh V, Singh S, Dotray P, Bagavathiannan M (2019) Distribution of herbicide-resistant Palmer amaranth (Amaranthus palmeri) in row crop production systems in Texas. Weed Technol 33:355-365. Legleiter T, Johnson B (2013) Palmer amaranth biology, identification, and management. Purdue Extension. https://www.extension.purdue.edu/extmedia/WS/WS-51- W.pdf. Accessed on October 28, 2019. Merchant RM, Culpepper AS, Eure PM, Richburg JS, Braxton LB (2014) Salvage Palmer amaranth programs can be effective in cotton resistant to glyphosate, 2,4-D, and glufosinate. Weed Technol 28: 316-322. Meyer CJ and Norsworthy JK (2019) Influence of weed size on herbicide interactions for EnlistTM and Roundup Ready® technologies. Weed Technol 33:569-577. 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. Stephenson DO, Bond JA, Siebert J, Walton L (2011) Control of varioius weeds with 2,4- D alone or co-applied with glufosinate or glyphosate. P 1546 in 2011 Beltwide Cotton Conferences proceedings. Atlanta, GA. [USDA-NASS] United States Department of Agriculture- National Agricultural Statistics Service (2019) Annual cotton review. Southern Plains Regional Field Office. 4 p.

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Webster TM and Grey TL (2015) Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) morphology, growth, and seed production in Georgia. Weed Sci 63:264- 272. Young BG (2006) Changes in herbicide use patterns and production practices resulting from glyphosate-resistant crops. Weed Technol 20:301-307.

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Table 3.1 Palmer amaranth control following initial and sequential applications of glufosinate and 2,4-D choline in 2018. Palmer amaranth size at initial application 7 to 15 cma 25 to 30 cm EPOSTb MPOST EPOST MPOST Initial application Sequential application 10c 10 21 10 11 21 ------%------2,4-D choline + glyphosate 91 b 85 a 2,4-D choline + glyphosate 97 bc 91 abc 95 a 96 a 2,4-D choline 87 e 78 de 88 b 91 ab glufosinate 100 a 97 a 98 a 98 a 2,4-D choline 69 c 54 d 2,4-D choline + glyphosate 92 d 90 abc 77 d 53 de 2,4-D choline 73 f 63 f 63 f 62 cd glufosinate 95 c 86 bcd 78 d 70 bcd 2,4-D choline + pyrithiobac 67 c 65 c glufosinate 98 ab 96 a 84 c 77 abc Glufosinate 95 a 79 b 2,4-D choline + glyphosate 99 ab 92 abc 95 a 86 ab 2,4-D choline 92 d 73 ef 68 e 48 de glufosinate 98 ab 83 cd 89 b 84 abc Glufosinate + pyrithiobac 92 ab 82 ab glufosinate 98 bc 93 ab 55 g 35 e a Abbreviations: EPOST, early postemergence application; MPOST, mid-postemergence application; DAIA, days after initial application; DASA, days after sequential application. b EPOST timing applied May 25 for 7 to 15 cm, June 4 for 25 to 30 cm; rating dates are given as days after initial application (DAIA). MPOST timing applied June 4 for 7 to 15 cm, June 14 for 25 to 30 cm; rating dates are given as days after sequential application (DASA). c Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05

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Table 3.2 Palmer amaranth control following initial and sequential applications of glufosinate and 2,4-D choline in 2019. Palmer amaranth size at initial application 7 to 15 cma 25 to 30 cm EPOSTb MPOST EPOST MPOST Initial application Sequential application 7c 10 21 7 15 36 ------%------2,4-D choline + glyphosate 92 a 80 a 2,4-D choline + glyphosate 91 ab 80 a 59 ab 35 b 2,4-D choline 90 abc 51 bc 65 a 30 bc glufosinate 93 a 79 a 64 a 22 cde 2,4-D choline 40 d 58 b 2,4-D choline + glyphosate 75 de 66 ab 58 ab 50 a 2,4-D choline 62 f 32 d 33 cd 20 def glufosinate 73 de 35 cd 46 bc 22 cde 2,4-D choline + pyrithiobac 55 c 65 b glufosinate 82 bcd 52 bc 50 b 27 bcd Glufosinate 79 b 66 b 2,4-D choline + glyphosate 89 abc 67 ab 54 ab 22 cde 2,4-D choline 67 ef 32 d 25 d 7 gh glufosinate 78 d 40 cd 27 d 12 fg Glufosinate + pyrithiobac 83 b 66 b glufosinate 80 bcd 42 cd 32 d 13 efg a Abbreviations: EPOST, early postemergence application; MPOST, mid-postemergence application; DAIA, days after initial application; DASA, days after sequential application. b EPOST timing applied June 7 for 7 to 15 cm, June 13 for 25 to 30 cm; rating dates are given as days after initial application (DAIA). MPOST timing applied June 17 for 7 to 15 cm, June 24 for 25 to 30 cm; rating dates are given as days after sequential application (DASA). c Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05

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Table 3.3 Palmer amaranth density 36 to 43 days after sequential applications of glufosinate and 2,4-D choline in 2019. Palmer amaranth size at intial application Initial application Sequential application 7 to 15 cma 25 to 30 cm ------No. per m2------Untreated 11 11 2,4-D choline + glyphosate 2,4-D choline + glyphosate 14 14 2,4-D choline 2 15 glufosinate 13 11 2,4-D choline 2,4-D choline + glyphosate 22 7 2,4-D choline 7 13 glufosinate 8 8 2,4-D choline + pyrithiobac glufosinate 6 13 Glufosinate 2,4-D choline + glyphosate 8 11 2,4-D choline 5 13 glufosinate 10 8 Glufosinate + pyrithiobac glufosinate 5 10 a Palmer amaranth density means were not significantly different according to Fishers Protected LSD in SAS at P < 0.05

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Table 3.4 Palmer amaranth dry weight 36 to 43 days after sequential applications of glufosinate and 2,4-D choline in 2019. Dry weight Initial application Sequential application 7 to 15 cma 25 to 30 cm ------g per m2------Untreated 489 489 2,4-D choline + glyphosate 2,4-D choline + glyphosate 211 151 2,4-D choline 365 260 glufosinate 172 293 2,4-D choline 2,4-D choline + glyphosate 310 203 2,4-D choline 344 364 glufosinate 311 324 2,4-D choline + pyrithiobac glufosinate 321 338 Glufosinate 2,4-D choline + glyphosate 316 318 2,4-D choline 472 339 glufosinate 468 337 Glufosinate + pyrithiobac glufosinate 315 407 a Palmer amaranth dry weight means were not significantly different according to Fishers Protected LSD in SAS at P < 0.05

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Table 3.5 Palmer amaranth control following initial and sequential tank-mix applications of glufosinate and 2,4-D choline in 2019. Palmer amaranth size at initial application 7 to 15 cma 25 to 30 cm EPOSTb MPOST EPOST MPOST Initial application Sequential application 7c 7 17 28 7 7 15 23 ------%------2,4-D choline + 92 a 70 b glufosinate 2,4-D choline + 96 b 57 b 20 a 45 c 56 b 50 b glyphosate glufosinate 99 a 70 a 20 a 47 bc 50 b 41 c 2,4-D choline + 99 a 57 b 20 a 45 c 50 b 40 c glufosinate 2,4-D choline + 88 b 78 a glyphosate 2,4-D choline + 98 ab 57 b 20 a 70 a 66 a 63 a glufosinate Glufosinate 90 ab 72 b 2,4-D choline + 97 b 57 b 20 a 50 b 50 b 37 d glufosinate a Abbreviations: DAIA, days after initial application; DASA, days after sequential application. b EPOST applications made July 1 and June 13 for 7 to 15 cm and 25 to 30 cm Palmer amaranth, respectively; rating dates are given as days after initial application (DAIA). MPOST applications made July 12 and June 24 to 7 to 15 cm and 25 to 30 cm Palmer amaranth, respectively; rating dates are given as days after sequential application (DASA). c Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P< 0.05

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Table 3.6 Palmer amaranth dry weight 34 to 37 days after sequential tank-mix applications of glufosinate and 2,4-D choline in 2019.

Dry weight Initial application Sequential application 7 to 15 cma 25 to 30 cm ------grams------Untreated 604 a 582 a 2,4-D choline + glufosinate 2,4-D choline + glyphosate 287 b 391 c glufosinate 317 b 512 ab 2,4-D choline + glufosinate 324 b 450 bc 2,4-D choline + glyphosate 2,4-D choline + glufosinate 320 b 244 d Glufosinate 2,4-D choline + glufosinate 322 b 492 abc a Means within a column followed by a common letter were similar according to Fishers Protected LSD in SAS at P < 0.05

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Table 3.7 Palmer amaranth density 35 days after sequential tank-mix applications of glufosinate and 2,4-D choline in 2019.

Palmer amaranth size at initial application Initial application Sequential application 7 to 15 cma 25 to 30 cm ------No. per m2------Untreated 70 93 2,4-D choline + glufosinate 2,4-d choline + glyphosate 72 114 glufosinate 52 111 2,4-D choline + glufosinate 63 94 2,4-D choline + glyphosate 2,4-D choline + glufosinate 55 65 Glufosinate 2,4-D choline + glufosinate 57 114 a Palmer amaranth density means were not significantly different according to Fishers Protected LSD in SAS at P < 0.05

Texas Tech University, Grace Flusche Ogden, December 2020

SUMMARY AND CONCLUSIONS Palmer amaranth control decreased as Palmer amaranth size at initial application increased the sequential applications of glufosinate and dicamba with and without acetochlor study. A difference in efficacy due to application order was observed when applications were made to < 10 cm Palmer amaranth, where glufosinate fb dicamba was less effective than dicamba fb glufosinate in both years. For > 10 cm Palmer amaranth, no difference in efficacy based on herbicide order was observed.

Acetochlor reduced newly emerging weed flushes and reduced > 30 cm Palmer amaranth density at harvest in 6 out of 8 treatments and were the only treatments that reduced > 30 cm Palmer amaranth density. Acetochlor reduced < 10 cm Palmer amaranth density when used in the initial application of dicamba + acetochlor fb glufosinate compared to glufosinate fb dicamba + acetochlor. Acetochlor should be included in the initial application to optimize reduction of weed emergence, particularly for applications made to < 10 cm Palmer amaranth.

Palmer amaranth < 10 cm in height at initial application were best managed with dicamba + acetochlor fb glufosinate, while Palmer amaranth > 10 cm can be controlled or suppressed with the same set of herbicides in either sequential order. Dicamba + acetochlor fb glufosinate controlled Palmer amaranth better than dicamba fb dicamba during at least one rating event across all weed sizes. Two applications of glufosinate provided minimal Palmer amaranth control. Two applications of dicamba may provide effective Palmer amaranth control but does not provide enough herbicide mode of action diversity to prevent against the development of herbicide resistant weeds. If two applications of either herbicide are required, follow label guidelines for sequential application timing interval. Sufficient evidence was not obtained in these studies to alter sequential application timing interval recommendations from current label restrictions.

Herbicide efficacy decreased as Palmer amaranth size at initial application increased in the 2,4-D choline and glufosinate systems trials; therefore, timely applications to smaller weeds was critical for success of this system. Sequential application order did influence efficacy in a few treatments. 2,4-D choline fb glufosinate provided greater control than glufosinate fb 2,4-D choline across both years and weed

Texas Tech University, Grace Flusche Ogden, December 2020 sizes. 2,4-D choline fb 2,4-D choline + glyphosate controlled 7 to 15 cm Palmer amaranth greater than 2,4-D choline + glyphosate fb 2,4-D choline 21 days after sequential application in both years. The addition of pyrithiobac did improve Palmer amaranth control in 2019, with 2,4-D choline + pyrithiobac followed by glufosinate providing greater control (96%) than 2,4-D alone followed by glufosinate (86%). Palmer amaranth < 30 cm in size at initial application can be controlled or suppressed with 2,4-D choline + glyphosate fb glufosinate. Tank-mixing 2,4-D choline with glufosinate provided effective short-term Palmer amaranth control when used in a sequential application system; however, the addition of a soil residual should extend the weed control to delay the need for the second postemergence application. 2,4-D choline alone applied sequentially failed to control Palmer amaranth and is not recommended.

Treatments of 2,4-D choline + glyphosate fb glufosinate incorporate 3 modes of action and successfully provided control of Palmer amaranth in 2,4-D based production systems. Subsequent rains and warm weather provided favorable conditions for Palmer amaranth germination and created a thick, uniform mat of weeds in the 7 to 15 cm Palmer amaranth portion of the 2,4-D sequential tank-mix applications trial. This series of events, although not always common in the semi-arid location of Lubbock, Texas, showcased the importance and potential benefit of including a soil residual herbicide.

Herbicide treatments that incorporate multiple modes of action offer the greatest potential to reduce the risk of selecting for herbicide resistant weeds when using chemical control methods. Glufosinate complimented both auxin-based systems and provided Palmer amaranth control when paired in sequential applications with either auxinic herbicide. Applications should be made to actively growing plants that are 10 cm or less in height with a thin cuticle, as this allows for best herbicide penetration and efficacy.