Rainfall amounts at test locations for 21 days following application of PRE herbicides.
Field studies were conducted to evaluate control of Amaranthus species and other weeds in corn and cotton. In corn, Palmer amaranth control was at least 90% with preemergence applications of fluthiacet‐methyl plus pyroxasulfone, atrazine plus either acetochlor, alachlor, dimethenamid‐P, S‐metolachlor, or S‐metolachlor plus mesotrione, saflufenacil plus dimethenamid‐P, and S‐metolachlor plus mesotrione. When using postemergence herbicides applied to Palmer amaranth less than 5 cm tall, atrazine, prosulfuron, and topramezone alone or the combinations of atrazine plus S‐metolachlor plus glyphosate, diflufenzopyr plus dicamba, dimethenamid plus glyphosate, halosulfuron‐methyl plus dicamba, mesotrione plus S‐metolachlor plus glyphosate, pyroxasulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione provided at least 91% control. In cotton, pyrithiobac applied preemergence resulted in no greater than 63% of control of Palmer amaranth and common waterhemp at the early season rating. Pendimethalin applied preemergence provided varied levels of control of common waterhemp. Trifluralin, applied preplant incorporated, consistently provided at least 86% or greater control of both species. A decreased level of control of both Palmer amaranth and common waterhemp was observed with pendimethalin applied preemergence followed by pyrithiobac‐applied early postemergence and followed by glufosinate applied mid‐post. Systems which included an early postemergence and mid‐postemergence application of glyphosate plus 2,4‐d choline provided at least 94% season‐long Palmer amaranth control.
- annual grasses
- broadleaf weeds
- weed efficacy
- crop response
- Amaranthus palmeri S. Wats
- Amaranthus rudis Sauer
During the past 20 years, the use of glyphosate‐resistant crop production systems has been adopted and used extensively in various regions of the USA . In 2009, nearly 61 million ha of soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L), and corn (Zea mays L.) contained a modified 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS) gene that confers resistance to glyphosate . The wide use of row crops with glyphosate‐resistance, the reduction of traditional herbicide and cultivation practices, and the use of intense management of weeds using glyphosate as the predominant control strategy has caused a shift in weed populations and created a selective advantage for glyphosate‐resistant weeds [3, 4].
The development of herbicide‐resistant crops allows weed control by nonselective postemergence (POST) herbicides, such as glyphosate and glufosinate, widening the array of weed management programs available to producers [5–7]. Both glyphosate and glufosinate control a wide range of weeds in herbicide‐resistant crops  with little, if any, crop injury [8, 9]. POST applications of glyphosate or glufosinate provide consistent and greater control of large‐seeded broadleaf weed species including velvetleaf (Abutilon theophrasti Medik.), giant ragweed (Ambrosia trifida L.), common cocklebur (Xanthium strumarium L.), and morningglory spp. (Ipomoea spp.) compared with preemergence (PRE) herbicides . Even though the performance of glyphosate and glufosinate is similar, glufosinate is less likely to succeed in a single POST application program since glufosinate is less effective on larger weeds, needs an increased spray volume, and a need for high humidity at application .
Glyphosate‐resistant weeds, specifically Amaranthus species, have become an issue across all the USA corn and cotton‐producing areas . Estimates are that more than 1.2 million ha of cropland in the USA are now affected by glyphosate‐resistant Amaranthus species . In cotton, Palmer amaranth (Amaranthus palmeri S. Wats.) has been shown to reduce lint yield by 57% when growing at a density of 10 plants per 9.1 m of row . Additionally, with Palmer amaranth growing at densities greater than six plants per 9.1 m of row, cotton may not be harvestable due to the potential for damage to harvest equipment . A study by Smith et al.  found that Palmer amaranth densities of 650–3260 plants ha−1 in dryland stripper‐harvested cotton increased harvesting time by 2‐ to 3.5‐fold.
Weed resistance to photosystem II (PSII)‐inhibiting herbicides, such as atrazine, has also been documented across many corn‐growing areas of the USA . Resistance to PSII inhibitors has been documented in 7 monocot and 17 dicot species in the corn‐producing regions . Also, populations of tall waterhemp [Amaranthus tuberculatus (Moq.) Sauer] have been identified with resistance to herbicides that inhibit acetolactate synthase (ALS), PSII, protoporphyrinogen oxidase (PPO), 5‐enolpyruvylshikimate‐3‐phosphate‐synthase (EPSPS), and 4‐hydroxyphenyl‐pyruvate‐dioxygenase (HPPD) in Illinois and Iowa, and Palmer amaranth populations resistant to ALS, PSII, and HPPD inhibitors have been identified in Kansas , indicating the continued need for alternative modes of action in corn to reduce the chance of herbicide resistance. The HPPD‐inhibiting herbicides have become popular among corn producers because of their broad‐spectrum weed control, flexible application timings, tank‐mix compatibilities, and crop safety [14–16].
Cotton growers have experienced more problems with weed resistance because of cotton’s slower emergence after planting and fewer registered herbicides compared with other major crops . The first documented cases of glyphosate‐resistant (GR) Palmer amaranth in cotton occurred in 2000 in Lauderdale County, TN  and in 2003 in Edgecombe County, NC . The first confirmed case of GR Palmer amaranth was documented in a biotype of Palmer amaranth growing in a Macon County, GA cotton field, where six‐ to eightfold levels of resistance to glyphosate were observed .
With the widespread adoption of glyphosate‐resistant cotton after its introduction in 1997, cotton weed management practices largely shifted away from the use of soil‐applied residual herbicides to POST herbicide programs based on glyphosate . Studies conducted in 2006 and 2007 by Legleiter and Bradley  confirmed glyphosate resistance in a biotype of common waterhemp (Amaranthusrudis Sauer) found in a Missouri soybean field following multiple glyphosate applications. Currently, glyphosate‐resistant Palmer amaranth and common waterhemp have been reported in 27 and 18 USA states, respectively . Through surveys sent to weed scientists across the USA, Culpepper  revealed that 50% of respondents indicated that weeds of the genus Amaranthus had increased significantly in cotton. The respondents also provided the following four recommendations for managing glyphosate‐induced weed species shifts: tank‐mix combinations of other herbicides with glyphosate for POST applications, rotating with non‐glyphosate‐resistant crops (though there was some disagreement among respondents), use of POST herbicides other than glyphosate, and using preplant‐incorporated (PPI) or (PRE) soil‐applied herbicides.
Amaranthus species are some of the most common weed species found in annual crop production throughout the USA . Palmer amaranth is now ranked as the most troublesome weed found in the USA . It is a common weed in many major crops around the world and is found in all areas of Texas . Up until the 1990s, its distribution in North America was the southern half of the USA ; however, since then, it has become established in every state with the exception of the northwestern USA, including Washington, Oregon, Montana, and North Dakota . In Texas, Palmer amaranth can be found in all areas of the state  and is one of the two Amaranthus species with confirmed resistant to glyphosate across Texas (common waterhemp is the other) . It is a dioecious, summer‐annual species that is native to the desert southwest region of the USA [28, 29]. Plants of the genus Amaranthus are often very problematic weeds in agronomic crops due to their ability to germinate under a wide range of conditions, grow rapidly, and produce large numbers of seed, all while competing with the crop for sunlight, moisture, and nutrients. Despite its origin, Palmer amaranth is able to survive in many diverse environments because of its biological characteristics [6, 30]. It has a lengthy germination window, robust growth habit, and is a prolific seed producer [31–33], and these characteristics make control of this weed difficult. Common waterhemp is an obligate outcrossing annual broadleaf weed that is capable of long‐distance pollen dispersal . It germinates optimally between 20/25 and 30/35°C , has an aggressive growth habit and may grow 1.6 mm per growing degree day , and is capable of producing more than 250,000 seeds per plant . These factors make it a strong competitor with most crops.
Traditional corn and cotton weed management programs have relied on PRE applications of a broadleaf and grass herbicide for residual season‐long weed control [36–41]. In corn, these PRE programs usually have included atrazine in combination for broad‐spectrum weed control. Atrazine is used in over 60% of the USA corn, and its doses have gotten lower with most doses of no more than 1.12 kg ha−1 with some growers applying no more than 0.84 kg ha−1 . Atrazine and 4‐hydroxylphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides are commonly used for weed control in corn and are effective in controlling glyphosate‐resistant weeds, including Palmer amaranth [36, 43, 44]. Atrazine can be applied PRE or POST alone or in tank‐mixtures with several herbicides .
Since POST herbicides are applied after the weed species and severity are known, this allows growers to assess the problem before making a herbicide application; therefore, POST herbicides are an essential component of an integrated weed management system to combat the herbicide‐resistant weeds . In addition, POST herbicides typically do not require rainfall for herbicide activation, making performance less dependent on environmental conditions . Also, POST herbicides can reduce the potential for water pollution . A Minnesota study showed reduced atrazine concentrations in runoff water when applied POST compared with soil‐applied applications because of the increased plant residue and cover, limiting the amount of herbicide reaching the soil .
Two new herbicide systems have recently become important in POST weed control in cotton [48–53]. Dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) is synthetic auxin herbicide that controls glyphosate‐resistant Palmer amaranth and other broadleaf weeds alone or in sequential combinations with glyphosate or glufosinate . An enzyme, dicamba O‐demethylase, was discovered in a soil bacterium (Pseudomonas maltophilia) that converts dicamba to 3,6‐dichlorosalicylic acid (DCSA) . The enzyme DCSA has no significant herbicidal properties. The gene responsible for this enzyme is known as DMO (dicamba monooxygenase). This gene was successfully inserted into mouse‐ear cress [Arabidopsis thaliana (L). Heynh.], tomato (Solanum lycopersicum L.), and tobacco (Nicotiana tabacum L.) and showed to provide these plants with effective tolerance to foliar applications of dicamba . Dicamba‐tolerant cotton, coupled with existing glyphosate‐ and glufosinate‐tolerant traits, was deregulated in the USA in 2015 and has since become significant portion of the cotton planted in the USA, comprising over 40% of the crop planted in 2016 [50, 51].
Enlist Duo herbicide, a premix formulation containing 195 g ae L−1 of 2,4‐d choline and 205 g ae L−1 of glyphosate dimethylamine, was developed for use in Enlist corn, cotton, and soybean. Resistance to 2,4‐d is conferred by the insertion of a gene that codes for the enzyme aryloxyalkanoate dioxygenase. Plants transformed to include this gene can metabolize 2,4‐d to a nonlethal form . Developed during World War II, 2,4‐D was the first selective herbicide widely used in agriculture . Since that time, researchers have demonstrated control of a large number of dicotyledonous weed species with 2,4-D [54–57].
The adoption of 2,4-D in Enlist crops will be influenced by yield potential of the crop, weed species infesting fields, and, most notably, the ability of growers to mitigate off‐target movement of 2,4‐D [58–60]. Although Enlist cotton is resistant to 2,4‐D , all other cotton cultivars, including cotton resistant to dicamba, are extremely sensitive to the herbicide, with reports of cotton injury due to 2,4‐D drift dating back to the time of development . Multiple studies showed that exposure to 2,4‐D resulted in cotton injury with sensitivity increasing at earlier growth stages and higher herbicide concentrations [63–65].
The prime strategy for managing herbicide resistance in weeds is to reduce the selection pressure for resistance evolution by any one selecting agent, while managing adequate weed control . Selection pressure has the greatest impact on herbicide‐resistance evolution and is a factor that growers can control. Selection pressure imposed by an herbicide is the product of efficacy and persistence in the soil . Herbicides applied in crop generally result in the greatest selection pressure compared with other application timings. Selection pressure against a weed population over time, resulting in increasing frequency of resistant individuals that collectively possess one or more resistance mechanisms, is a function of frequency of application . Herbicide sequences, rotations, or mixtures generally have the greatest effect in delaying resistance when the mechanism conferring resistance is target‐based, the weed species are highly self‐pollinated, and seed spread is restricted [68, 69] and herbicide mixtures may delay resistance longer than rotations .
The rapid increase in resistant weeds in corn and cotton and the concerns pertaining to the overuse of atrazine in corn, including detection in surface and groundwater, rotational crop injury, and the development of triazine‐resistant weeds, calls for the development of appropriate and effective management techniques. Also, growing questions about the renewed use of PRE and POST herbicides for early season and possibly season‐long weed control in corn and cotton have also become a major topic of discussion. Therefore, the objective of this research was to evaluate the effect of various PRE and POST herbicides alone and in combinations for crop tolerance and weed control efficacy in the Texas corn and cotton‐producing regions. In cotton, several herbicide programs in glyphosate‐, glufosinate‐, and dicamba‐tolerant cotton were evaluated for their efficacy on both Palmer amaranth and common waterhemp.
2. Materials and methods
2.1. Corn PRE studies in central and south Texas
These studies were conducted during the 2013 through 2015 growing season in central Texas near Taylor (30.5326° N, 97.4548° W) and in south‐central Texas near Ganado (29.0438° N, 96.4849° W). Study sites were located in different fields within the same general area of each year. Soils at the Taylor location were a Burleson clay (fine, montmorillonitic, and thermic Udic Pellusterts) with less than 1% organic matter and 7.6 pH, while soils at the Ganado location were a Houston Black clay (fine, montmorillonitic, and thermic Udic Pellusterts) with less than 1% organic matter and 7.4 pH.
Studies were arranged in a randomized complete block design with three replicates. Plot dimensions were two or four corn rows, wide spaced 76–97 cm apart, and 6.3 or 7.9 m long (depending on location). The corn hybrids BH 8846RR (2013), BH 8844VTTP (2014), and BH 8475SS (2015) were planted mid‐ to late February near Taylor and late February to early March near Ganado in each year to a depth of 2.5–3.5 cm at the rate of 54,000–65,500 seeds ha−1.
Herbicides were applied within 5–7 days after planting with a CO2‐pressurized backpack sprayer with TeeJet 11002 flat‐fan nozzles (Spraying Systems Co., North Avenue and Schmale Road, Wheaton, IL 60188) using a pressure of 180 kPa and calibrated to deliver 140 or 187 L ha−1 (depending on location). An untreated check was included for comparison at each location. All herbicide doses were based on the USA label dose with the exception of the acetochlor (74.8% formulation) dose which was applied at 2X of the labeled rate throughout the study by mistake. Once the error was realized, it was decided to maintain this dose throughout the study.
Weed populations varied from year to year and were from naturally occurring soil seed bank populations. At the Taylor location, browntop panicum [Panicum fasciculatum Sw. var. reticulatum (Torr.) Beal] populations in 2013 were moderate (3–4 plants/m2), while in 2014 populations were higher (6–8 plants/m2). Common barnyardgrass [Echinochloa crus‐galli (L.) P. Beauv.] populations in 2015 ranged from 4–8 plants/m2. At Ganado, Texas millet [Urochloa texana (Buckley) R. Webster] populations ranged from 6–10 plants/m2. Palmer amaranth populations varied from 4–8 plants/m2 at the Taylor location to 2–10 plants/m2 at the Ganado location. Hophornbeam copperleaf (Acalypha ostryifolia Riddell) populations at Taylor in both years were low to moderate (2–6 plants/m2), while common sunflower (Helianthus annuus L.) populations ranged from 2–6 plants/m2 depending on the year.
Crop injury and weed control were estimated visually on a scale of 0–100 (0 indicating no control or injury and 100 indicating complete control or plant death). Crop injury consisted of plant stunting and early season (30 days after herbicide application) and late season (95–140 days after application) crop injury was recorded. Late season weed control ratings (95–140 days after herbicide application) are presented for all weeds with the exception of Palmer amaranth control at Ganado in 2015 where populations of this weed were low (<4 plants/m2) and somewhat inconsistent. Crop yield was determined by hand‐harvesting 3.8 m of each plot, shelling the kernels from the corn ear, and weighing the kernels. Crop weights were adjusted to 15% moisture.
Visual estimates of weed control and corn injury were transformed to the arcsine square root prior to analysis of variance but are expressed in their original form for clarity because the transformation did not alter interpretation. Means were compared with Fisher’s Protected LSD test at the 5% probability level . The non‐treated check was not included in the weed control analysis but was included in corn yield analysis.
2.2. Corn POST studies in central and south Texas
Field studies were conducted during the 2013 through 2015 growing season at two locations in central Texas including near Taylor (30.5326° N, 97.4548° W) and Beyersville (30.3036° N, 97.1947° W) and at three locations in south‐central Texas near Kendleton (29.44786° N, 95.99961° W), Ganado (29.0438° N, 96.4849° W), and Yoakum (29.1827° N, 97.0929° W). Where study sites were similar over years, these studies were located in different fields within the same general area. Soils at the central Texas locations near Taylor were a Burleson clay (fine, montmorillonitic, and thermic Udic Pellusterts) with less than 1% organic matter and 7.6 pH, while soils at the Beyersville location soils were a Houston Black clay (fine, smectitic, and thermic Udic Haplusterts) with less than 3% organic matter and 7.8 pH. Soils at the south‐central locations near Ganado were a Laewest clay (fine, montmorillonitic, and thermic Udic Pellusterts) with less than 1% organic matter and 7.4 pH, soils at Kendleton were a Bernard‐Edna complex (fine, smectitic, and hyperthermic Oxyaquic Vertic Argiudolls) with less than 3% organic matter and 6.8 pH. Soils at the Yoakum location were a Cuero sandy clay loam (fine, loamy, mixed, superactive, and thermic Pachic Argiustolls) with less than 2% organic matter and 7.2 pH.
Studies were arranged in a randomized complete block design with three replicates of treatments. Plot dimensions were either two or four rows (depending on location), spaced 76–97 cmapart by 6.3–7.9 m long. The corn varieties BH 8846RR (2013), BH 8844 VTTP (2014), and BH 8475 SS (2015) were planted from mid‐February to mid‐March depending on locations and environmental conditions to a depth of approximately 2.5–3.5 cm at the rate of 54,000–65,500 seeds ha−1.
Herbicides were applied POST with a CO2‐pressurized backpack sprayer using TeeJet 11002 flat‐fan nozzles (Spraying Systems Co., North Avenue and Schmale Road, Wheaton, IL 60188) with a pressure of 180 kPa and calibrated to deliver 140–187 L ha−1 (depending on location). An untreated check was included for comparison at each location. All herbicide doses were based on the USA label and included an adjuvant and either ammonium nitrate or sulfate per label requirements.
Weed populations varied from location to location and were from natural seed bank populations in the soil. At the Taylor location, browntop panicum populations in 2013 were sparse (3–4 plants/m2), while Texas millet populations at Beasley were extremely dense (16–18 plants/m2) and moderate at Beyersville (6–8 plants/m2). Common barnyardgrass pressure at Taylor was low to moderate (4–8 plants/m2). Palmer amaranth populations at the Yoakum and Ganado locations was dense (16–20 plants/m2), while populations at the Taylor locations were low (4–6 plants/m2).Pitted morningglory (Ipomoea lacunose L.), hophornbeam copperleaf, and Asiatic dayflower (Commelina communis L.) populations were low to moderate (4–8 plants/m2). Approximately 50% of the Palmer amaranth population at the Ganado location was glyphosate resistant.
Weed size at the time of treatment varied by location. Browntop panicum was no greater than 15 cm tall when treated, while Texas millet and common barnyardgrass were less than 20 cm tall at the time of herbicide application. Palmer amaranth at the Yoakum location was less than 5 cm tall at herbicide application, while at Taylor weed size was less than 20 cm. However, at the Ganado location, Palmer amaranth height varied from 40 to 60 cm due to rains which prevented entry into the field in a timely manner. Pitted morningglory length ranged from 5 to 20 cm, while hophornbeam copperleaf and Asiatic dayflower were less than 20 cm in height at the time of treatment. Corn height varied from location to location but was typically in the V4–V8 stage.
Crop injury and weed control were visually estimated on a scale of 0–100 (0 indicating no control or injury and 100 indicating complete control or plant death). Mid‐ to late season weed control ratings (31–98 days after herbicide application) are presented for all weeds. Crop yield was determined by hand‐harvesting 3.8 m of each plot, shelling the kernels from the corn ear, and weighing the kernels. Crop weights were adjusted to 15% moisture.
Visual estimates of weed control and corn injury were transformed to the arcsine square root prior to analysis of variance but are expressed in their original form for clarity because the transformation did not alter interpretation. Means were compared with Fisher’s Protected LSD test at the 5% probability level . The non‐treated check was not included in the weed control analysis but was included in corn yield analysis.
2.3. Cotton studies in south‐central Texas
Studies were conducted in Burleson County, TX (30.3257° N, 96.2615° W) at the Texas A&M AgriLife Research Farm in 2012 and 2013 to investigate management strategies for controlling Palmer amaranth and common waterhemp in cotton possessing glyphosate‐, glufosinate‐, and dicamba‐tolerant transgenic traits. Studies were in the same general area in each year. Soils at this site are characterized as a Westwood silty clay loam (fine, silty, mixed, superactive, and thermic Udifluventic Haplustepts) with 2% organic matter and 8.1 pH. The experiment included 12 treatments arranged as a randomized complete block design with 4 replications. Plots were four rows wide and 9.1 m in length with 102 cm row spacing. Buffers 4.5 m wide were maintained between blocks to facilitate lateral movement of equipment.
This experiment was conducted on a furrow‐irrigated field with large seed bank populations of both Palmer amaranth and common waterhemp. Both Palmer amaranth and common waterhemp were naturally occurring populations with 10–15 plants/m2. In 2012, none of the Palmer amaranth or common waterhemp populations were glyphosate resistant, while in 2013 approximately 10% of the Palmer amaranth population was resistant; however, none of the waterhemp populations were resistant. Treatments included preplant‐incorporated (PPI), PRE, and two POST application timings of an early POST (EPOST) and mid‐POST (MPOST). Plots receiving PPI applications of trifluralin were subjected to two passes of a rolling cultivator immediately following application to thoroughly incorporate the herbicide into the soil. Preemergence herbicide applications included fomesafen, pendimethalin, prometryn, pyrithiobac, and S‐metolachlor, while POST applications included acetochlor, dicamba, glufosinate, glyphosate, pyrithiobac, and trifloxysulfuron. Early postemergence applications in 2012 were made when weeds were approximately 12 cm tall and in 2013 when weeds were 10 cm in height, while MPOST treatments in 2012 were made when weeds were 25 cm tall and in 2013 when 15 cm in height. An untreated check was included in all studies.
For the 2012 experiment, PPI applications were made on May 7, cotton was planted on May 22, EPOST applications were made on June 21, and MPOST applications were made on July 4. In 2013, PPI applications were made on May 8, cotton was planted on May 9, EPOST applications were made on June 7, and MPOST applications were made on June 16. The cotton variety was an experimental dicamba‐glyphosate tolerant entry from Monsanto. Herbicide applications were made with a CO2‐pressurized backpack sprayer calibrated to deliver 140 L ha−1 total spray volume. Preplant‐incorporated and PRE applications were made using TeeJet 11003 Drift Guard flat‐fan nozzles, while EPOST and MPOST applications were made with TeeJet 110015 Turbo TeeJet Induction flat‐fan nozzles (TeeJet Technologies, Wheaton, Illinois 60187).
Control of Palmer amaranth and common waterhemp was estimated visually at the time of the EPOST application, at the time of MPOST application, and 14 days after the MPOST application. These observations are reported as early, mid, and late, respectively. Plots were managed throughout the season according to standard crop management practices for this region. The center two rows of all plots were mechanically harvested and seed cotton yields were recorded. Means were compared with Fisher’s Protected LSD test at the 5% probability level .
2.4. Cotton studies in the High Plains of Texas
Field studies were conducted near New Deal (33.4413° N, 101.4358° W) and Halfway, TX (34.1881° N, 101.9522° W) during the 2015 and 2016 growing seasons to investigate management strategies for controlling Palmer amaranth in cotton possessing glyphosate‐, glufosinate‐, dicamba‐, and 2,4‐D choline‐tolerant transgenic traits. Soils at the New Deal site are characterized as a Pullman clay loam (fine, mixed, and superactive thermic Torrertic Paleustolls) with less than 1% organic matter and 7.9 pH, while soils at New Deal are a Olton clay loam (fine, mixed, and thermic Aridic Paleustoll) with less than 1% organic matter and a 7.4 pH. These experiments were conducted under center pivot irrigation at Halfway and 102 cm spacing of sub‐surface drip tape at New Deal with large populations of Palmer amaranth (8–10 plants/m2). These studies were conducted as a randomized complete block design with four replications. Plots were four rows wide and 9.1–12.7 m in length, with 102 cm row spacing.
Treatments for the glyphosate plus 2,4‐D choline study (Enlist Duo) included PPI treatments of trifluralin and EPOST and MPOST treatments of glyphosate, glufosinate, glyphosate plus 2,4‐D choline, S‐metolachlor, and 2,4‐D choline salt. Plots receiving PPI applications of trifluralin were subjected to two passes of a rolling cultivator immediately following application. Postemergence applications were made when Palmer amaranth was 15 cm or less in height. This study was conducted in 2016. The cotton variety was an experimental from Dow AgroSciences (9330 Zionsville Rd, Indianapolis, IN 46268) and was planted on May 26 at a seeding rate of 13.1 seeds m−1 of row.
Treatments for the glyphosate systems study included preplant applications of glyphosate plus either flumioxazin, fomesafen, the premix of rimsulfuron plus thifensulfuron‐methyl, or diruron, PRE applications of either flumeturon, pyrithiobac, acetochlor, or flumeturon plus paraquat, EPOST applications of glyphosate alone or plus either acetochlor, S‐metochlor, dimethenamid‐P, or pyrithiobac, MPOST applications of glyphosate alone or plus acetochlor, dimethenamid‐P, or S‐metochlor, and LPOST treatments of diruron plus MSMA. Postemergence applications were made when Palmer amaranth was 10 cm or less in height. This study was conducted in 2015 and 2016. Fibermax 2322GL was planted in both years with the same seeding rate as in the previous study.
Herbicide applications were made with a CO2‐pressurized backpack sprayer calibrated to deliver 93–140 L ha−1 total spray volume. PPI and PRE applications were made using TeeJet 11002 Drift Guard flat‐fan nozzles, while EPOST and MPOST applications were made with TeeJet 110015 Turbo TeeJet Induction flat‐fan nozzles. A Redball® tractor‐mounted hooded sprayer (Willmar Fabrication, LLC; Willmar, MN 56201) was used for LPOST herbicide applications.
Control of Palmer amaranth was estimated visually as in previous studies. Plots were managed throughout the season according to standard crop management practices for this region. The center two rows of all plots were mechanically harvested, and lint cotton yields were recorded. Means were compared with Fisher’s Protected LSD test at the 5% probability level .
3. Results and discussion
Although the primary emphasis in this chapter is the discussion on controlling Palmer amaranth and, to some extent, common waterhemp which have become troublesome weeds in corn and cotton, other weed species will be discussed since they are/can become problematic weeds as well.
3.1. Corn PRE studies
Since not all treatments were included in each year of the study, no attempt was made to combine results over years or locations. Also, rainfall amounts varied from site to site and year to year affecting herbicide response (Table 1). Rainfall during the 7 days after the application of PRE herbicide treatments occurred at all locations with the exception of Ganado in 2013 and 2014 when no rainfall occurred. Rainfall between 8 and 14 days after the PRE application varied from no rainfall at Ganado in 2013 to 78.2 mm at Ganado in 2015 (Table 1). Rainfall 15–21 days after the PRE application was low at Taylor in 2013 and Ganado in 2015, and no rainfall occurred at the other sites.
With respect to annual grasses, browntop panicum and Texas millet were present in 2013 and 2014 at the Taylor and Ganado sites, respectively. Common barnyardgrass was present at Taylor only in 2015. Broadleaf weeds were present at the Taylor and Ganado locations. Palmer amaranth was present in 2013 and 2015, while hophornbeam copperleaf and common sunflower were present in 2013 and 2014. Although this chapter discusses the control of herbicide‐resistant weeds, the control of other weeds will also be discussed since they are also a large part of the problem when providing effective weed control under normal growing conditions.
3.1.1. Annual grass control
Atrazine alone controlled common barnyardgrass 33%, while acetochlor (74.8%) or pendimethalin alone, acetochlor plus atrazine, S‐metolachlor plus mesotrione, or S‐metolachlor plus atrazine plus mesotrione provided 90–97% control (Table 2). The dinitroaniline herbicides, such as pendimethalin, are registered for use in over 40 crops . These herbicides usually provide excellent control of annual grasses [73–75].
|Browntop panicum||Texas millet||Barnyardgrass|
|Treatment||Kg ai ha−1||Taylor||Ganado||Taylor|
|Days after treatment|
|Fluthiacet‐methyl (FM)+pyroxasulfone (P)||0.006+0.2||‐||‐||‐||‐||58|
In 2013, pendimethalin alone, alachlor plus atrazine, S‐metolachlor plus mesotrione, or S‐ metolachlor plus atrazine plus mesotrione provided 96% or better browntop panicum control, while isoxaflutole, S‐metolachlor, and pyroxasulfone alone, and S‐metolachlor plus atrazine controlled this weed 80–88% (Table 2). In 2014, only the dose of acetochlor (74.8%) provided acceptable control (83%). The lack of effective control in 2014 can be attributed to greater plant populations at the test site in 2014 compared to 2013 and also the low rainfall amounts after the PRE application in 2014 (Table 1). Since many of the PRE herbicides can volatilize and photodecompose on the soil surface over time, these herbicides need to be mechanically incorporated or need rainfall or irrigation to move these herbicides into the weed seed zone [76–78], which explains the erratic control noted with these herbicides under the droughty conditions observed at Taylor in 2014.
In 2013, isoxaflutole alone, thiencarbazone‐methyl plus isoxaflutole, acetochlor plus atrazine, or saflufenacil plus dimethenamid‐P controlled Texas millet at least 92% (Table 2). Pendimethalin or saflufenacil alone, atrazine plus either S‐metolachlor, alachlor, or dimethenamid‐P, and the three‐way combination of S‐metolachlor plus atrazine plus mesotrione provided 81–89% control. In 2014, acetochlor, pendimethalin, or pyroxasulfone alone or S‐metolachlor plus mesotrione controlled this weed at least 95%, while isoxaflutole or mesotrione alone and atrazine plus either acetochlor or S‐metolachlor controlled 83–89% (Table 2). In the two years, S‐metolachlor alone provided 75–78% Texas millet control compared with 75–99% control with pyroxasulfone. Typically, S‐metolachlor alone provides poor control of this weed [79, 80]. With high populations of Texas millet, Grichar et al.  reported less than 70% control with 1.7 and 3.4 kg ha−1 of metolachlor in dryland peanut (Arachishypogaea L.) and 25–76% control under irrigated conditions. Steele et al.  reported that pyroxasulfone, at a 10‐fold lower use rate than S‐metolachlor, controlled Texas millet 84–96%, while S‐metolachlor provided 75–85% control when rated 9 weeks after treatment. They attributed the results to the longer residual activity of pyroxasulfone .
3.1.2. Broadleaf weed control
At Taylor in 2013, under moderate weed pressure (4 plants m2), all herbicides, with the exception of atrazine (73%), provided at least 97% Palmer amaranth control, while in 2015 under increased populations (8 plants m2), atrazine controlled Palmer amaranth 79%, while isoxaflutole, mesotrione, or saflufenacil provided no better than 71% control (Table 3). All other herbicide treatments provided at least 96% control. At the Ganado location, in 2013 and 2015, control was more erratic than at the Taylor location. This may be due to the greater weed populations noted in 2013 (10 plants m2) and variable populations in 2015. In 2013, either atrazine or isoxaflutole alone, acetochlor, alachlor, S‐metolachlor, or dimethenamid-P plus atrazine, or the three‐way combination of S‐metolachlor plus atrazine plus mesotrione provided 97–100% control, while mesotrione, dimethenamid‐P, or acetochlor (33%) alone and rimsulfuron plus mesotrione controlled this weed 61% or less (Table 3). In 2015, acetochlor (74.8%) alone, dimethenamid‐P plus atrazine, fluthiacet‐methyl plus pyroxasulfone, and saflufenacil plus dimethenamid‐P controlled Palmer amaranth at least 95%, while isoxaflutole, mesotrione, S‐metolachlor, and pendimethalin alone and rimsulfuron plus thifensulfuron‐methyl controlled this weed less than 70%.
|Kg ai ha−1||Days after treatment|
|Fluthiacet‐methyl (F)+pyroxasulfone (P)||0.006 + 0.2||‐||‐||99||98|
|(F)+(P)+(A)||0.004 + 0.2 + 1.3||‐||‐||40||93|
|(S)+(A)||1.4 + 1.8||100||99||99||92|
|Alachlor+(A)||2.5 + 1.5||100||99||‐||‐|
|Thiencarbazone‐methyl+isoxaflutole||0.02 + 0.06||100||92||99||83|
|(S)+(A)+(M)+bicyclopyrone||0.4 + 0.8||‐||‐||99||72|
|0.09 + 0.02|
|Acetochlor+(A)||2.1 + 1.3||100||100||99||93|
|Dimethenamid‐P+(A)||1.6 + 3.2||100||100||99||95|
|Rimsulfuron+(M)||0.02 + 0.2||100||27||‐||‐|
|Rimsulfuron+thifensulfuron‐methyl||0.02 + 0.02||99||90||98||37|
|(S)+(A)+(M)||1.5 + 1.5 + 0.2||100||97||99||90|
|Saflufenacil+dimethenamid‐P||0.08 + 0.7||100||95||99||100|
|(S)+(M)||2.8 + 0.3||100||91||100||94|
In previous research, mesotrione applied PRE controlled smooth pigweed (Amaranthus hybridus L.), but control of morningglory species (Ipomoea spp.) and common lambsquarter (Chenopodium album L.) was inconsistent and dependent upon a timely rainfall following application [38, 82]. Armel et al.  reported improved weed control with mixtures of mesotrione plus acetochlor or atrazine over that of mesotrione alone. As seen in this study, the combination of mesotrione with metolachlor plus atrazine has enhanced weed control in other studies .
In 2013, thiencarbazone‐methyl plus isoxaflutole provided perfect control (100%) of hopho-rnbeam copperleaf while acetochlor (33%), saflufenacil or pyroxasulfone alone, alachlor plus atrazine, rimsulfuron plus thifensulfuron‐methyl, S‐metolachlor plus atrazine plus mesotrione, saflufenacil plus dimethenamid‐P, and S‐metolachlor plus mesotrione controlled this weed at least 92% (Table 4). Atrazine and mesotrione alone and rimsulfuron plus mesotrione provided unacceptable control (<60%). In 2014, either acetochlor (74.8%), isoxaflutole, saflufenacil, or pyroxasulfone alone controlled hophornbeam copperleaf at least 93% (Table 4). The combinations of S‐metolachlor plus either atrazine or mesotrione and saflufenacil plus dimethenamid‐P controlled this weed 90–98%, while rimsulfuron plus either mesotrione or thifensulfuron‐methyl and acetochlor (33%) provided 67–70% control.
|Hophornbeam copperleaf||Common sunflower|
|Days after treatment|
|Kg ai ha−1||%|
|(S)+(A)||1.4 + 1.8||76||90||100||85|
|Alachlor+(A)||2.5 + 1.5||93||‐||98||‐|
|Thiencarbazone‐methyl+(I)||0.02 + 0.06||100||77||98||100|
|Acetochlor+(A)||2.1 + 1.3||79||‐||99||‐|
|(D)+(A)||1.7 + 3.2||80||72||97||93|
|Acetochlor (74.8% formulation)||6.8||‐||99||‐||97|
|Rimsulfuron (R)+(M)||0.02 + 0.2||60||67||97||93|
|(R)+thifensulfuron‐methyl||0.02 + 0.02||98||70||87||99|
|(S)+(A)+(M)||1.5 + 1.5 + 0.2||98||74||100||87|
|(Sa)+(D)||0.08 + 0.7||99||98||100||97|
|Acetochlor (33% formulation)||1.7||92||63||100||72|
|(S)+(M)||2.8 + 0.3||92||93||99||98|
In 2013, under low common sunflower pressure (2–3 plants m2), all herbicides, with the excep-tion of atrazine alone (73%) and rimsulfuron plus thifensulfuron‐methyl (87%), controlled this weed at least 95% (Table 4). In 2014, under slightly greater common sunflower populations (4–6 plants m2), control was more variable. Acetochlor (74.8%) alone, thiencarbazone‐methyl plus isoxaflutole, rimsulfuron plus thifensulfuron‐methyl, saflufenacil plus dimethenamid‐P, and S‐metolachlor plus mesotrione controlled this weed at least 97%. Mesotrione, pendimethalin, or pyroxasulfone alone provided unacceptable control (<60%). The development of ALS‐resistant common sunflower has limited the options for growers having to control common sunflower with POST herbicides [83, 84]. Results from this study are consistent with previous findings which found that common sunflower control with herbicide systems containing isoxaflutole was at least 85% in most instances [84, 85].
3.1.3. Corn injury and yield
Grain yields were obtained only in 2013 at both locations and in 2015 at Taylor. Early season crop injury consisted of stunting and was never more than 3% with any herbicide treatment (data not shown). Corn recovered from the slight early season stunting and typically by harvest no differences in corn plant growth between the untreated check and any herbicide treatments were noted (data not shown). Although no appreciable crop injury was noted in these studies, this is not always true. Instances of isoxaflutole phytotoxicity in corn have been documented [85, 86] and attributed to several factors, including application timing , increased use dose , and varied susceptibility of corn hybrids to isoxaflutole . Environmental factors (cool and wet) and soil characteristics  can also lead to corn injury by isoxaflutole. Johnson et al.  reported that PPI herbicide applications resulted in greater injury than PRE applications, and this was probably due to increased amount of precipitation. Armel et al.  reported that acetochlor, atrazine, or mesotrione combinations did cause 11–18% corn stunting when followed by 32 mm of rainfall, but that the corn recovered quickly and by 4 weeks after treatment injury did not exceed 2%.
In 2013 at the Taylor location, atrazine, isoxaflutole, and pyroxasulfone alone, S‐metolachlor plus atrazine and/or mesotrione produced grain yields that were greater than the untreated check (Table 5). Although not significant, all herbicide treatments resulted in a numerical increase in grain yield over the untreated check. At the Ganado location, grain yields from the herbicide treatments were not significantly different from the untreated check; however, all yields from the herbicide treatments were numerically greater than the untreated check with the exception of S‐metolachlor plus mesotrione which produced a 10% decrease in yield from the untreated check. No reason for this reduction can be determined.
|Kg ai ha−1||Kg ha−1|
|Fluthiacet‐methyl (FM)+pyroxasulfone (P)||0.006+0.2||‐||‐||9342|
In 2014, no significant differences between the untreated check and any herbicide treatments were noted, although several herbicide treatments produced numerically greater yields than the untreated check (Table 5). Dimethenamid‐P and pyroxasulfone alone, fluthiacet‐methyl plus pyroxasulfone, thiencarbazone‐methyl plus isoxaflutole, dimethenamid‐P plus atrazine, S‐metolachlor plus atrazine plus mesotrione, and saflufenacil plus dimethenamid‐P produced grain yields that were 14–21% greater than the untreated check.
With glyphosate‐resistant pigweed becoming more widespread throughout the state, the use of soil‐applied herbicides can not only control resistant weed species in glyphosate‐resistant corn production systems but can also reduce the risk of new herbicide‐resistant weed species occurring. In general, many treatments with two or three herbicide modes of action provided better weed control than one herbicide alone, and the chance of corn injury appears to be minimal with any herbicide combinations under normal growing conditions. Our results indicate that in a year with little or no rainfall within 7–14 days after PRE herbicide application, any combination of PRE herbicides may need to be followed by POST herbicides for control of escaped weeds.
3.2. Corn POST Studies
3.2.1. Annual grass control
Limited control of browntop panicum was noted when using POST herbicides. Glyphosate and tembotrione alone provided 99% browntop panicum control, while the combinations of atrazine plus S‐metolachlor plus glyphosate, mesotrione plus S‐metolachlor plus glyphosate, and thiencarbazone‐methyl plus tembotrione provided 96–98% control (Table 6). Mesotrione and topramezone alone and the combination of primisulfuron‐methyl plus pyroxasulfone controlled this weed 77–83%; however, no other herbicides provide better than 68% control. Stephenson et al.  noted that thiencarbazone plus tembotrione controlled browntop millet (Urochloa ramose L.) 93% which was greater than tembotrione, atrazine, or glufosinate alone. They also noted that the co‐application of atrazine, glufosinate, or glyphosate with thiencarbazone plus tembotrione did not increase browntop millet control.
|Dose||Browntop panicum||Texas milletg||Barnyardgrass|
|Herbicide treatment||Kg ai or ae ha−1||Taylor||Bea||Beyersh||Taylor|
|Days after treatment|
|Fluthiacet‐methyl (FM)a, c||0.07||43||‐||53||39|
|Glyphosate (G)||1.5 ae||99||25||63||85|
|Halosulfuron‐methyl (HM) a, c||0.07||49||‐||72||20|
|Mesotrione (M)a, c||0.1||79||‐||57||58|
|Primisulfuron‐methyl (PM)b, d||0.04||68||89||‐||‐|
|(A)+S‐metolachlor (S)+(M)+ bicyclopyronea, c||0.7+1.5+ 0.17+0.04||‐||‐||‐||93|
|(A)+(S)+(G)a, c||1.8+1.5+0.8 ae||98||56||73||99|
|Diflufenzopyr+dicamba (D)a, c||0.06+0.1 ae||59||‐||‐||50|
|(PM)+(P)a,c||0.03 + 0.01||83||74||70||43|
In 2014 at Beasley only nicosulfuron, primisulfuron‐methyl, and topramezone alone or the combination of pyroxasulfone plus glyphosate provided acceptable Texas millet control (>84%), while at Beyersville only the combinations of mesotrione plus S‐metolachlor plus glyphosate and fluthiacet‐methyl plus pyroxasulfone plus atrazine controlled this weed at least 81% (Table 6). Prostko et al.  found that glyphosate applied sequentially was more effective at controlling Texas millet than either nicosulfuron or foramsulfuron. Again, the added control noted with pyroxasulfone can be attributed to the extended residual activity of this herbicide .
The combinations of atrazine plus S‐metolachlor plus mesotrione plus bicyclopyrone, atrazine plus S‐metolachlor plus glyphosate, dimethenamid plus glyphosate, fluthiacet‐methyl plus pyroxasulfone plus glyphosate, mesotrione plus S‐metolachlor plus glyphosate, pyroxasulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione controlled barnyardgrass at least 93% (Table 6). Lamore et al.  reported that tembotrione at 92 g ha−1 provided greater than 90% control, which is similar to the results in this study. Stephenson et al.  reported that thiencarbazone plus tembotrione or tembotrione alone provided equivalent control of barnyardgrass to atrazine plus either glufosinate or glyphosate.
3.2.2. Broadleaf weed control
In 2014 at Yoakum, under dense Palmer amaranth populations, atrazine, prosulfuron, and topramezone alone or the combinations of atrazine plus S‐metolachlor plus glyphosate, diflufenzopyr plus dicamba, dimethenamid plus glyphosate, halosulfuron‐methyl plus dicamba, mesotrione plus S‐metolachlor plus glyphosate, pyroxasulfone plus glyphosate, and thiencarbazone‐methyl plus tembotrione provided at least 91% control (Table 7). Armel et al.  reported improved weed control with mixtures of mesotrione plus acetochlor or atrazine.
|Herbicide treatment||Kg ai or ae ha−1||Yoakum||Taylor||Ganadog|
|Days after treatment|
|Fluthiacet‐methyl (FM)a, c||0.07||58||54||33|
|Glyphosate (G)||1.5 ae||68||100||59|
|Halosulfuron‐methyl (HM)a, c||0.07||53||85||13|
|Mesotrione (M)a, c||0.1||83||100||55|
|(A)+S‐metolachlor(S)+(M) +bicyclopyronea, c||0.8+1.5+0.2+0.04||‐||99||75|
|(A)+(S)+(G)a, c||1.8+1.5+0.8 ae||100||99||73|
|Diflufenzopyr+dicamba (D)a, c||0.06+0.14 ae||91||99||65|
|(FM)+pyroxasulfone (P)+(G)d||0.004+0.15+1.3 ae||‐||100||69|
|(HM)+(D)a, c||0.07+0.3 ae||99||100||60|
|(M)+(S)+(G)a, c||0.1+1.1+1.1 ae||100||100||60|
|Thiencarbazone‐methyl+tembotrionea, c||0.02 + 0.07||100||100||58|
At the Taylor location in 2015, under low populations, only carfentrazone‐ethyl, fluroxypyr, fluthiacet‐methyl, and primisulfuron‐methyl plus pyroxasulfone failed to provide at least 85% Palmer amaranth control. At the Ganado location, only pyroxasulfone plus glyphosate controlled this weed at least 80%, and this general lack of control was probably due to weed height (40–60 cm) at the time of herbicide application. Herbicide application to weeds 10–15 cm tall can result in corn grain yields equal to those in weed‐free plots , but POST applications when weeds are greater than 15 cm tall provided inconsistent season-long weed control when compared with applications when weeds are less than 15 cm tall . Stephenson et al.  reported that atrazine alone provided 96% control of this weed, while thiencarbazone plus tembotrione or tembotrione, glufosinate, and glyphosate alone provided 92% or less control.
Glyphosate alone provided 100% pitted morningglory control, while mesotrione plus S‐ meto-lachlor plus glyphosate controlled this weed 82% (Table 8). Typically, glyphosate provides inadequate control of pitted morningglory when applied alone at normal label use doses [92–94]. However, greater than 90% late season control of tall morningglory (Ipomoea purpurea L.), ivyleaf morningglory (I. hederacea L.), and entireleaf morningglory (I. hederacea var. integriuscula Gray) in the field has been documented with 1.12 kg ha−1 of glyphosate applied to plants with six true leaves or less . However, sequential in‐season glyphosate applications are often required to provide similar levels of pitted morningglory control [96, 97]. No other herbicides provided better than 68% control. Bararpour et al.  observed 90–100% control of entireleaf and pitted morningglory with the combination of thiencarbazone plus tembotrione plus either atrazine, glufosinate, or glyphosate, while Stephenson et al.  observed 85–88% control with thiencarbazone plus tembotrione alone.
|Herbicide treatment||Dose||Pitted morningglory||Hophornbeam copperleaf||Asiatic dayflower|
|Kg ai or ae ha−1||Days after treatment|
|Fluthiacet‐methyl (FM)a, c||0.07||7||91||55|
|Glyphosate (G)||1.5 ae||100||79||62|
|Halosulfuron‐methyl (HM) a, c||0.07||13||73||‐|
|Mesotrione (M)a, c||0.1||47||64||84|
|Primisulfuron-methyl (PM)b, d||0.04||50||65||‐|
|A + S‐metolachlor (S)+(M)+ bicyclopyronea, c||0.8+1.5+0.2+0.04||‐||‐||93|
|(A)+(S)+(G)a, c||1.8+1.5+0.8 ae||63||99||93|
|Diflufenzopyr+(D)a, c||0.06+0.14 ae||30||62||79|
|(FM)+pyroxasulfone (P)+ (A)d||0.004+0.15+1.3||‐||‐||70|
|(HM)+(D)a, c||0.07+0.3 ae||20||40||83|
|(M)+(S)+(G)a, c||0.1+1.1+1.1 ae||82||92||81|
Fluthiacet‐methyl and tembotrione alone and the combinations of atrazine plus S‐metolachlor plus glyphosate and mesotrione plus S‐metolachlor plus glyphosate provided at least 90% hophornbeam copperleaf control, while atrazine, carfentrazone‐ethyl, and prosulfuron alone and the combination of thiencarbazone‐methyl plus tembotrione provided 81–86% control (Table 8).
Glufosinate ammonium alone controlled Asiatic dayflower 90%, while the combinations of atrazine plus S‐metolachlor plus mesotrione plus bicyclopyrone and atrazine plus S‐metolachlor plus glyphosate provided 93% control (Table 8).
3.2.3. Corn injury and yield
Crop injury consisted of stunting with some leaf chlorosis and necrosis and was never more than 8% with any herbicide treatment (data not shown). Corn recovered from the slight early season stunting and typically by harvest no differences in corn plant growth between the untreated check and any herbicide treatments were noted (data not shown). Although no appreciable crop injury was noted in these studies this is not always true. Other studies have reported corn injury more than 50% with isoxaflutole, imazethapyr, imazapic, and prosulfuron in field or sweet corn [99–101]. In addition, herbicides such as halosulfuron and dicamba plus diflufenzopyr have been reported to cause as much as 25 and 15% injury, respectively [102, 103]. Corn phytotoxicity has been attributed to several factors, including application timing , increased use doses , and varied susceptibility of corn hybrids to different herbicides .
Corn yield was combined over locations due to a lack of treatment by location interaction. Yields were likely affected more by weed control than any other factor (rainfall, etc.) in any year. Pyroxasulfone plus glyphosate produced the greatest yield while halosulfuron alone and the untreated check produced the least yield (Table 9). Treatments that contained the combination of atrazine plus glyphosate resulted in yields that were greater than 5200 kg ha−1.
|Kg ai or ae ha−1||Kg ha−1|
|Glyphosate (G)||1.5 ae||4413|
|Halosulfuron‐methyl (HM) a,c||0.07||942|
|(A) + S‐metolachlor + (M) + bicyclopyronea,c||0.8+1.5+0.2+ 0.04||5248|
|(A) + (S) + (G)a,c||1.8+1.5+ 0.8 ae||5587|
|Diflufenzopyr + (D)a,c||0.06+0.14||1601|
|Dimethenamid‐P + (G)||0.8+1.5 ae||4425|
|(FM) + (M)a,c||0.09+0.09||4909|
|(FM) + pyroxasulfone (P) + (A)d||0.004+0.15+1.3||2178|
|(HM) + (D)a,c||0.07+0.3 ae||3490|
|(M) + (S) + (G)a,c||0.1+1.1+1.1 ae||4149|
|(PM) + (P)a,c||0.03 + 0.01||1506|
|(P) + (G)||0.1+1.5 ae||5781|
|Thiencarbazone‐methyl + tembotrionea,c||0.02+0.07||4281|
Some research suggests that timely POST control can be an effective alternative to soil‐applied herbicides in corn [45, 105, 106]. The use of POST herbicides only is generally considered a greater risk and requires careful management [45, 105, 106]. Also, weed density and application timing are factors in weed efficacy with POST herbicides. Halford et al.  reported a reduction in yield when weeds remained beyond V6 corn. In addition, Gower et al.  found that subsequent emergence and competition after early glyphosate applications was likely responsible for corn yield reductions. Also, late POST applications can reduce corn grain yields, although weed control was nearly perfect [105, 108].
With glyphosate‐resistant pigweed becoming more widespread throughout the state, the use of POST herbicide combinations, which may or may not contain glyphosate, can not only control resistant weed species in glyphosate‐resistant corn production systems but can also reduce the risk of new herbicide‐resistant weed species occurring. In general, many treatments with two or three herbicides with different modes of action provided better weed control than one herbicide alone, and the chance of corn injury appears to be minimal with any herbicide combination under normal growing conditions.
3.3. Cotton studies
3.3.1. South‐central Texas
A significant year‐by‐treatment interaction existed for all weed control and cotton yield data, thus data were analyzed separately by year. Weed control data required arcsine transformation in order to meet the assumption of homogeneity of variances for ANOVA; however, the non‐transformed means are reported in the (Table 10–13).
|Herbicide and application timinga||Palmer amaranth||Common waterhemp||Seed cotton|
|Herbicide and application timinga||Palmer amaranth||Common waterhemp||Seed cotton|
|Pyr||[A] + D+Ace||63||99||100||60||99||100||4034|
|T||Gluf + D||[A]+D||99||100||100||98||100||100||4207|
|Herbicide and applicationa||Palmer amaranth control||Cotton injury||Lint yield|
|Herbicide and application timinga|
|Gly+Flumi||Flume+Par||Gly + Ace||Gly+Ace||D+MSMA||89||506|
|Gly+Ace||Flume+Par||Gly + S||Gly+S||D+MSMA||93||794|
|Gly+Ace||Ace+Flume+Par||Glu + Pyr||Gly||D+MSMA||99||801|
|Gly+Rim+Thi||Pyr+Par||Gly + Dim||Gly+Dim||D+MSMA||68||609|
|Gly+D||Ace+Par||Gly + Pyr||Gly+S||D+MSMA||91||770|
|Gly+Flumi||Ace+Flume+Par||Gly + Ace||Gly||D+MSMA||89||753|
In 2012, control of Palmer amaranth ranged from 29 to 97% while common waterhemp control ranged from 55 to 100% prior to EPOST applications (Table 10). At that timing, control of Palmer amaranth was lowest with pyrithiobac applied PRE. Similar results were seen for common waterhemp control, where pyrithiobac applied PRE provided only 55% control. After EPOST and MPOST applications, no differences in Palmer amaranth control were detected among treatments, with means ranging from 93 to 100%. After the EPOST application timing, control of common waterhemp with S‐metolachlor plus fomesafen applied PRE was lower than control provided by trifluralin applied PPI followed by either glyphosate plus dicamba‐ or glufosinate plus dicamba applied EPOST, and all treatments that included glyphosate plus dicamba plus acetochlor applied EPOST. After MPOST applications, no differences in common waterhemp control among herbicide treatments were observed. Seed cotton yields of treated plots ranged from 3581 to 4002 kg ha−1, which were all greater than the non‐treated check (1823 kg ha−1).
In 2013, Palmer amaranth control prior to EPOST application ranged from 63 to 100%, while control of common waterhemp ranged from 60 to 100% (Table 11). Similar to 2012, pyrithiobac applied PRE resulted in the lowest control of both Palmer amaranth and common waterhemp (63 and 60%, respectively). Treatments that included pendimethalin‐applied PRE provided reduced control of Palmer amaranth (88–90%) when compared with many other treatments at the early rating. A similar pattern was observed with common waterhemp, where control was numerically lower from treatments of pendimethalin‐applied PRE than many other treatments, though this was not always significant. Prior to the MPOST application, control of Palmer amaranth with S‐metolachlor applied PRE was less than that of all other treatments except for pendimethalin applied PRE followed by pyrithiobac applied EPOST, which itself was lower than treatments other than S‐metolachlor plus prometryn applied PRE. Control of common waterhemp prior to the MPOST application was lowest with S‐metolachlor applied PRE (81%) and pendimethalin applied PRE followed by pyrithiobac applied EPOST (86%). At the last rating, control of Palmer amaranth and common waterhemp was reduced with pendimethalin applied PRE followed by pyrithiobac applied EPOST followed by glufosinate applied MPOST (92 and 93%, respectively) when compared to all other herbicide treatments. Mean yield of the non‐treated control was 254 kg ha−1, which was lower than that of all herbicide treatments (3526–4209 kg ha−1).
Pyrithiobac applied PRE has been shown to provide satisfactory control of Amaranthus weeds [109, 110]; however, the opposite was observed in this experiment, where pyrithiobac applied PRE resulted in decreased levels of control of both Palmer amaranth and common waterhemp. The reasons for this lack of control are unknown, as the treatment was applied at the recommended rate and timing . Pendimethalin applied PRE provided varied levels of control of common waterhemp, particularly in 2012. This may be due to the utilization of furrow irrigation for herbicide incorporation rather than overhead irrigation, which is recommended on the product label . Trifluralin applied PPI consistently provided the best levels of control of both species. This is likely due in large part to the thorough mechanical incorporation of herbicide into the soil, which has been observed to affect the efficacy of trifluralin [113, 114].
In 2013, a decreased level of control of both Palmer amaranth and common waterhemp later in the season was observed with pendimethalin applied PRE followed by pyrithiobac‐applied EPOST followed by glufosinate applied MPOST. This is attributed to a failure of glufosinate to control weeds that survived pyrithiobac EPOST and grew to a size larger than that recommended for control with glufosinate . This weed size effect on glufosinate performance was observed by Craigmyle et al. , where control of common waterhemp was found to decrease with increasing plant height. Pendimethalin applied PRE followed by glyphosate EPOST provided excellent control of both species; however, in the presence of a glyphosate‐resistant population, this treatment would likely not provide acceptable levels of control. In addition, this reliance on glyphosate as the single POST herbicide mechanism of action is not recommended due to the potential for selection of glyphosate‐resistant plants . Excellent control of both Palmer amaranth and common waterhemp was achieved in both years in treatments that included glyphosate plus dicamba plus acetochlor‐applied EPOST. In addition to providing successful levels of weed control, this tank‐mix would likely be very resilient against selecting resistant biotypes as suggested by Evans et al. , who found that the presence of glyphosate‐resistant biotypes of common waterhemp was much less common in fields that received applications of mixed herbicide mechanisms of action.
3.3.2. High Plains studies
In the glyphosate plus 2,4‐D choline study, trifluralin alone failed to control Palmer amaranth with only 20% control early season and no control late season, while systems which include POST applications of either glyphosate or glufosinate alone controlled this weed 23–53% (Table 12). Systems which included an EPOST and MPOST application of glyphosate plus 2,4‐D choline provided at least 94% season‐long control of this weed. Chahal and Johnson  reported that the addition of 2,4‐D to glyphosate provided 99% control of glyphosate‐resistant horseweed [Conyza canadensis (L.) Cronq.] compared to only 12% with glyphosate alone. In a similar study, 2,4‐D added to glufosinate provided an increased level of common waterhemp control compared to herbicide treatments consisting of glufosinate only . Miller and Norsworthy  reported that the addition of a residual herbicide, such as trifluralin, would provide an additional effective herbicide mode of action for managing resistant Palmer amaranth. Applications of 2,4‐d, glyphosate, and glufosinate alone or tank‐mixed represent broad‐spectrum POST herbicides that have the potential to control 9 of the 10 most problematic weeds in the southern cotton and soybean production .
Cotton injury was greatest (19%) with trifluralin‐applied PPI followed by S‐metochlor plus glufosinate‐applied EPOST followed by glyphosate plus 2,4-D choline applied MPOST (Table 12). Cotton lint yields were greatest with herbicide treatments which provided greater than 90% Palmer amaranth control with the exception of trifluralin applied PPI followed by glyphosate plus 2,4‐D choline applied EPOST and MPOST.
In the glyphosate herbicide systems study, a late season rating suggests that the herbicide system which included glyphosate plus the pre‐mix of rimsulfuron plus thifensulfuron‐methyl applied preplant controlled Palmer amaranth less than 70%, while all other systems which included glyphosate plus either flumioxazin, fomesafen, or diruron applied preplant provided 89–99% control (Table 13). Diverse herbicide programs for controlling resistant Palmer amaranth and common waterhemp is an important herbicide‐resistant management strategy . Additionally, full labeled‐use doses should always be used to achieve the greatest level of possible control and reduce the likelihood for the evolution of resistance. These results further displayed the high level of weed control this new technology is capable of providing. Also, emphasis should be placed on a zero‐tolerance weed threshold , herbicides should also be applied at or less than the recommended weed height, and programs should not begin with an EPOST or MPOST application but rather start prior to planting with the application of residual herbicides.
While a few late emerging Palmer amaranth plants may be considered as being harmless, previous research has reported that late season Palmer amaranth seedlings are capable of seed production within 30 days after emergence . Previous research also has shown that weeds left in the field at the time of harvest have the potential to enter harvesting machinery and be distributed across the field . Thus, leaving weeds in the field prior to harvest can result in spreading viable weed seeds across the field. This practice will not only lead to increasing weed populations in that field but will also negatively impact sustainable weed management .
Also, a major challenge in managing weeds is minimizing the return of weed seed to the soil seed bank . Menges  reported that maintaining fields weed free for 6 years reduced the soil seed bank of Palmer amaranth by 98%; however, 18 million seeds ha−1 remained in the soil. Palmer amaranth seed viability decreased when buried below the depth of optimal germination for at least 36 months . Given that Palmer amaranth has become the most challenging weed to manage in corn and cotton , understanding population dynamics of this weed may help lead to strategies that more effectively manage this weed.