Weed management systems were compared in bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton. A standard system of pendimethalin preplant incorporated (PPI), fluometuron preemergence (PRE), fluometuron plus MSMA early postemergence-directed (POST-DIR), and cyanazine plus MSMA late POST-DIR in combination with cultivation controlled broadleaf signalgrass, large crabgrass, common lambsquarters, jimsonweed, pitted morningglory, prickly sida, sicklepod, and smooth pigweed 98 to 100% late season. Weed control, cotton yield, and net returns were similar when pyrithiobac or bromoxynil plus MSMA postemergence (POST) replaced fluometuron plus MSMA POST-DIR. Fluometuron PRE had little to no effect in bromoxynil systems. Glyphosate POST to three- to four-leaf-stage cotton followed by cyanazine plus MSMA late POST-DIR and cultivation controlled weeds 96 to 100%. Glyphosate POST followed by glyphosate POST-DIR and cultivation controlled pitted morningglory and large crabgrass 89 to 90% and other species at least 94%. Yields and net returns at one location were similar for glyphosate applied twice or glyphosate POST followed by cyanazine plus MSMA POST-DIR and the standard system. Pendimethalin plus fluometuron in glyphosate systems did not increase yield or net returns. At a location severely infested with large crabgrass, pendimethalin plus fluometuron in glyphosate systems increased yield 37 to 44% and net returns 85 to 108%, respectively, when glyphosate was applied to cotton at the three-to four-leaf stage, but not if glyphosate was applied to cotton at the one-leaf stage. Yield and net returns were similar when bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton were treated using the standard system.

Glyphosate-resistant (GR) weeds have been a prime challenge to the sustainability of GR cotton-based production systems of the midsouthern United States. Barnyardgrass is known to be a high-risk species for evolving herbicide resistance, and a simulation model was developed for understanding the likelihood of glyphosate resistance evolution in this species in cotton-based systems. Under a worst-case scenario of five glyphosate applications in monoculture GR cotton, the model predicts resistance evolution in about 9 yr of continuous glyphosate use, with about 47% risk by year 15. A unique insight from this model is that management in response to GR Palmer amaranth in this system (a reactive response) provided a proactive means to greatly reduce the risks of glyphosate resistance evolution in barnyardgrass. Subsequent model analysis revealed that the risk of resistance is high in fields characterized by high barnyardgrass seedbank levels, seedling emergence, and seed production per square meter, whereas the risk is low in fields with high levels of postdispersal seed loss and annual seedbank loss. The initial frequency of resistance alleles was a high determinant of resistance evolution (e.g., 47% risk at year 15 at an initial frequency of 5e−8 vs. 4% risk at 5e−10). Monte Carlo simulations were performed to understand the influence of various glyphosate use patterns and production practices in reducing the rate and risk of glyphosate resistance evolution in barnyardgrass. Early planting and interrow cultivation are useful tools. Crop rotation is effective, but the diversity of weed management options practiced in the rotational crop is more important. Diversifying weed management options is the key, yet application timing and the choice of management option is critical. Model analyses illustrate the relative effectiveness of a number of diversified glyphosate use strategies in preventing resistance evolution and preserving the long-term utility of glyphosate in midsouthern U.S. cotton-based production systems.

A survey questionnaire was sent to cotton consultants of Arkansas and Mississippi through direct mail and Louisiana and Tennessee consultants through on-farm visits in fall of 2011. The survey was returned by a total of 22 Arkansas, 17 Louisiana, 10 Mississippi, and 11 Tennessee cotton consultants, representing 26, 53, 13, and 38% of total cotton planted in these states in 2011, respectively. Collectively, the area planted to glyphosate-resistant (Roundup Ready®, RR) cotton was 97%, glyphosate plus glufosinate-resistant (Widestrike® Flex, WRF) cotton was 30%, and glufosinate-resistant (Liberty Link, LL) cotton was 2.6% of the total cotton surveyed in 2011. Seventy percent of area in all states is still under continuous RR/WRF cotton. Average cost of herbicides in RR systems was $114 ha−1 and in LL systems was $137 ha−1. Across the states, cotton planted under no-tillage, conservation tillage, and conventional tillage was 31, 36, and 33%, respectively, of total scouted cotton. Area under conventional tillage increased and conservation tillage decreased in Arkansas compared with a previous survey conducted in 2006. Palmer amaranth, morningglories, and horseweed in the order of listing were the most problematic weeds of cotton across Arkansas, Mississippi, and Tennessee. In Louisiana, however, morningglories were the most problematic weed followed by Palmer amaranth and common waterhemp. Glyphosate-resistant (GR) Palmer amaranth infested only 13% of scouted cotton area in Louisiana compared with 75% in the remaining three states, and consequently, hand-weeding to control GR Palmer amaranth is practiced on only 2.5% of total scouted area of Louisiana and 49% of the scouted area of the remaining three states. Hand-weeding added an additional $12 to 371 ha−1 to weed-management costs. One-half (50%) of the cotton consultants emphasized the need for more research on residual herbicides that can control GR Palmer amaranth effectively.

Field studies were conducted in 2000 and 2001 at Rohwer, AR. Trifloxysulfuron (5.3 and 8 g ai/ha) and pyrithiobac (70 g ai/ha) were applied preemergence (PRE) and postemergence (POST) broadcast at the two- to three-leaf (EP) and three- to four-leaf (MP) cotton growth stages. Both materials were also applied POST in combination with glyphosate at 560 g ae/ha or bromoxynil at 560 g ai/ha at both growth stages. Trifloxysulfuron applied EP or MP at 8 g/ha provided greater control of sicklepod and pitted morningglory 28 d after application (DAA) than trifloxysulfuron at 5.3 g/ha or pyrithiobac at 70 g/ha; however, control of prickly sida was greater with pyrithiobac than with trifloxysulfuron at either rate. Glyphosate alone controlled sicklepod, prickly sida, and pitted morningglory greater than 80%. The addition of trifloxysulfuron at 8 g/ha and pyrithiobac at 70 g/ ha increased control of all species over glyphosate alone 28 DAA. Bromoxynil at 560 g/ha controlled pitted morningglory and hemp sesbania at all application timings; however, sicklepod and Palmer amaranth control was less than 50% with bromoxynil applied alone. When bromoxynil was applied in combination with trifloxysulfuron at either rate, control of sicklepod and Palmer amaranth increased to 80% or greater at all application timings. Trifloxysulfuron has the potential to complement both the glyphosate-resistant and bromoxynil-resistant weed control programs by providing control of less susceptible weeds and by providing residual control to both programs.

Field and greenhouse studies were conducted to investigate the compatibility of MSMA in a tank mixture with glyphosate or glufosinate for broadleaf and grass weed control. Glyphosate, glufosinate, and MSMA were evaluated at 0.5×, 1×, and 2× rates, with 1× rates of 0.84 kgae/ha, 0.5 kgai/ha, and 2.2 kgai/ha, respectively. Glyphosate and glufosinate provided similar levels of control for most weed species and were often more efficacious than MSMA alone. Glyphosate controlled Palmer amaranth better than glufosinate. Glufosinate controlled hemp sesbania, pitted morningglory, and ivyleaf morningglory better than glyphosate at one location. Weed control was not improved with the addition of MSMA to glyphosate or glufosinate when compared with either herbicide alone. MSMA antagonized glyphosate efficacy on barnyardgrass, browntop millet, hemp sesbania, Palmer amaranth, and redroot pigweed. MSMA antagonized glufosinate efficacy on browntop millet, hemp sesbania, ivyleaf morningglory, johnsongrass, Palmer amaranth, pitted morningglory, prickly sida, redroot pigweed, and velvetleaf. Antagonism of glyphosate or glufosinate by MSMA was often overcome by applying the 2× rate of either herbicide alone. MSMA is not a compatible tank-mixture partner with glyphosate or glufosinate for weed control in cotton.

Field studies were conducted to assess two sulfur-containing additives for use with glyphosate applied postemergence to glyphosate-resistant cotton for the control of sicklepod and yellow nutsedge. Neither diammonium sulfate (AMS) nor ammonium thiosulfate (ATS), both applied at 2.24 kg/ha, increased control of either species. Effective control of both species was dependent on glyphosate (isopropylamine salt) rate alone, with optimum control at 1.26 kg ae/ha. Plant-mapping data further indicated that sulfur-containing additives generally had no effect on either cotton fruiting patterns or yield. However, applying glyphosate at any rate did increase seed cotton yield in 2 of 3 yr vs. no glyphosate. In addition, applying glyphosate at any rate resulted in an increase in the number of bolls vs. no glyphosate in the following plant-mapping responses: total number of bolls per plant, number of abcised bolls per plant, bolls at the top five sympodial nodes, and bolls at positions 1 and 2 on the sympodia. Glyphosate absorption and subsequent translocation, as influenced by the addition of the sulfur-containing additives, was evaluated using radiotracer techniques. Glyphosate absorption after 48 h was 86, 63, and 37% of amount applied in cotton, sicklepod, and yellow nutsedge, respectively. Absorption by sicklepod and yellow nutsedge was not affected by the addition of either of the additives. Absorption by cotton was reduced by ATS but was not affected by AMS. In yellow nutsedge and cotton, glyphosate concentration in the treated area and adjacent tissue was not affected by either additive. A greater portion of glyphosate was translocated away from the treated area in sicklepod with glyphosate plus AMS (32%) than with glyphosate plus ATS (21%). AMS and ATS may be used in glyphosate-resistant cotton without the risk of either crop injury or yield reduction. However, their use for increased control of annual weed species, such as sicklepod and yellow nutsedge, may not be warranted.

A simulation model is used to explore management options to mitigate risks of glyphosate resistance evolution in Palmer amaranth in glyphosate-resistant cotton in the southern United States. Our first analysis compares risks of glyphosate resistance evolution for seven weed-management strategies in continuous glyphosate-resistant cotton monoculture. In the “worst-case scenario” with five applications of glyphosate each year and no other herbicides applied, evolution of glyphosate resistance was predicted in 74% of simulated populations. In other strategies, glyphosate was applied with various combinations of preplant, PRE, and POST residual herbicides. The most effective strategy included four glyphosate applications with a preplant fomesafen application, and POST tank mixtures of glyphosate plus S-metolachlor followed by glyphosate plus flumioxazin. This strategy reduced the resistance risk to 12% of populations. A second series of simulations compared strategies where glyphosate-resistant cotton was grown in one-to-one rotations with corn or cotton with other herbicide resistance traits. In general, crop rotation reduced risks of resistance by approximately 50% and delayed the evolution of resistance by 2 to 3 yr. These analyses demonstrate that risks of glyphosate resistance evolution in Palmer amaranth can be reduced by reducing glyphosate use within and among years, controlling populations with diverse herbicide modes of action, and ensuring that population size is kept low. However, no strategy completely eliminated the risk of glyphosate resistance.

In fall 2011, cotton and soybean consultants from Arkansas, Louisiana, Mississippi, and Tennessee were surveyed through direct mail and on-farm visits, and rice consultants from Arkansas and Mississippi were surveyed through direct mail to assess the importance and level of implementation of herbicide resistance best management practices (HR-BMPs) for herbicide-resistant weeds. Proper herbicide timing, clean start with no weeds at planting, application of multiple effective herbicide modes of action, use of full labeled herbicide rates, and prevention of crop weed seed production with importance rating of ≥ 4.6 out of 5.0 were perceived as the most important HR-BMPs in all crops. Purchase of certified rice seed was on 90% of scouted hectares. In contrast, least important HR-BMPs as perceived by consultants with importance ratings of ≤ 4.0 in cotton, ≤ 3.7 in rice, and ≤ 3.8 in soybean were cultural practices such as manual removal of weeds; tillage including disking, cultivation, or deep tillage; narrow (≤ 50 cm)-row crops, cover crops, and altered planting dates. Narrow crop rows and cover crops in cotton; altered planting dates in cotton and soybean; and cleaning of farm equipment and manual weeding in rice and soybean is currently employed on ≤ 20% of scouted hectares. Extra costs, time constraints, adverse weather conditions, lack of labor and equipment, profitability, herbicide-related concerns, and complacency were perceived as key obstacles for adoption of most HR-BMPs. With limited adoption of most cultural practices that reduce risks of herbicide-resistant weeds, there are opportunities to educate growers concerning the proactive need and long-term benefits of adopting HR-BMPs to ensure sustainable weed management and profitable crop production.

Field experiments were conducted in Alabama during 1999 and 2000 to test the hypothesis that any glyphosate-induced yield suppression in glyphosate-resistant cotton would be less with irrigation than without irrigation. Yield compensation was monitored by observing alterations in plant growth and fruiting patterns. Glyphosate treatments included a nontreated control, 1.12 kg ai/ha applied POST at the 4-leaf stage, 1.12 kg/ha applied DIR at the prebloom stage, and 1.12 kg/ha applied POST at 4-leaf and postemergence directed (DIR) at the prebloom cotton stages. The second variable, irrigation treatment, was established by irrigating plots individually with overhead sprinklers or maintaining them under dryland, nonirrigated conditions. Cotton yield and all measured parameters including lint quality were positively affected by irrigation. Irrigation increased yield 52% compared to nonirrigated cotton. Yield and fiber quality effects were independent of glyphosate treatments. Neither yield nor any of the measured variables that reflected whole plant response were influenced by glyphosate treatment or by a glyphosate by irrigation interaction.

Greenhouse and field studies were conducted to evaluate potential interactions between glyphosate and trifloxysulfuron on barnyardgrass, browntop millet, hemp sesbania, seedling johnsongrass, pitted morningglory, prickly sida, sicklepod, and velvetleaf control as well as cotton injury and yield. In the greenhouse, glyphosate at 840 g ae/ha controlled all weed species 62 to 99%, which was better than trifloxysulfuron at 2.5 or 5 g ai/ha. Control of four-leaf pitted morningglory and hemp sesbania was 80 to 88% when glyphosate and trifloxysulfuron were mixed compared with 62 to 66% control with glyphosate alone. Mixing trifloxysulfuron with glyphosate did not affect control of other species compared with glyphosate alone. In the field, glyphosate controlled barnyardgrass, prickly sida, sicklepod, seedling johnsongrass, and velvetleaf 68 to 100%. Trifloxysulfuron controlled hemp sesbania, seedling johnsongrass, and sicklepod 65 to 88%. All other species were controlled 36 to 72% with glyphosate and 10 to 60% with trifloxysulfuron. Combinations of glyphosate (840 g/ha) and trifloxysulfuron (5 g/ha) were applied postemergence over-the-top and postemergence-directed to three-, six-, and nine-leaf glyphosate-resistant cotton in the field. Cotton injury at 2 wk after treatment (WAT) was less than 13% for all herbicide treatments and less than 5% by 3 WAT. Herbicides did not affect the percent of open bolls or nodes per plant. Seed cotton yield ranged from 1,430 to 1,660 kg/ha, and only the sequential over-the-top applications of trifloxysulfuron reduced cotton yield compared with the weed-free, nontreated cotton.

A 3-yr dryland study was initiated in 1999 at Munday, TX, on an Altus fine sandy loam to determine the most appropriate cotton growth stage or stages at which to apply glyphosate. The objectives were: (1) to evaluate control of silverleaf nightshade in the cotton and (2) to determine the effect on yield. Treatments were: (1) control (C; two cultivations), (2) early glyphosate at the four-leaf stage (E), (3) early glyphosate followed by a midseason application 21 d later (E+M), (4) two cultivations plus late glyphosate at 20% open bolls (L), (5) treatment E+L, and (6) treatment E+M+L. In fall 1999 and 2001, silverleaf nightshade stem numbers decreased in the plots sprayed early and midseason, and increased in plots receiving only late or no applications when compared with the counts in the spring of those years. However, in fall 2000, nightshade numbers were less than in early spring 2000 regardless of treatment, probably because of hot and dry weather. In fall 2001, silverleaf nightshade populations had increased 13-fold and twofold for the C and L treatments, respectively, when compared with populations at the beginning of the study. Other treatments had population decreases of 10 to 90%. Three-year average lint yields were higher with early or early plus midseason applications. Lint yields were similar with early or early plus midseason sprays. Lint yields were higher when nightshade/cotton biomass competition was lower. Early application of glyphosate can effectively control silverleaf nightshade populations and can increase yield when compared to no application or a late application.

Field experiments were conducted to evaluate weed control and cotton response to glyphosate or glufosinate applied alone or with residual herbicides applied in the last POST-directed application (LAYBY) in glyphosate- and glufosinate-resistant cotton. Glyphosate (0.86 kg ae/ha) or glufosinate (0.47 kg ai/ha) were applied alone over the top of glyphosate- or glufosinate-resistant cotton early POST (EPOST) followed by (fb) late POST (LPOST) fb one of the herbicides applied either alone or with a residual herbicide at LAYBY. Glyphosate- and glufosinate-based treatments were applied only to glyphosate- and glufosinate-resistant cotton, respectively. Residual herbicides evaluated included prometryn (1.12 kg ai/ha), fluometuron (1.12 kg ai/ha), diuron (1.12 kg ai/ha), oxyfluorfen (1.12 kg ai/ha), pendimethalin (1.0 kg ai/ha), prometryn + trifloxysulfuron (1.33 kg ai/ha + 12 g ai/ha), or linuron + diuron (0.56 + 0.56 kg ai/ha). Glyphosate-and glufosinate-based weed management systems with and without residual LAYBY herbicides resulted in little to no injury to cotton. Three applications of glyphosate or glufosinate alone provided better full-season control of most species when compared to two applications of either herbicide. The addition of a residual herbicide to glyphosate or glufosinate at LAYBY did not improve cotton yields, but did improve overall control of barnyardgrass and yellow nutsedge and reduced weed dry biomass present at time of cotton harvest when compared to three applications of glyphosate or glufosinate alone.

Field studies were conducted at the Alabama Agricultural Experiment Station near Shorter, AL, from 1998 to 2000 to evaluate six commercial spray adjuvants and their effects on glyphosate applied to cotton with an air-assisted sprayer. Each adjuvant was evaluated with a conventional sprayer calibrated to deliver 94 L/ha solution and a low-volume, air-assisted sprayer calibrated to deliver 19 L/ha solution. Glyphosate was applied to two-leaf pitted and entireleaf morningglory growing in glyphosate-resistant cotton. Visual control of pitted and entireleaf morningglory was more dependent on the rate of herbicide application than on a particular adjuvant or sprayer, with glyphosate at 0.42 kg ae/ha providing 70% control 21 d after treatment. Pitted and entireleaf morningglory biomass measurements generally reflected visual control data. Ammonium sulfate and formulated glyphosate consistently gave the highest visual control and the greatest biomass reduction. Glyphosate application rate was more important than adjuvant addition or sprayer type, with the higher rates of application providing greater control. No differences in weed control were observed between spray systems; therefore, air-assisted sprayers may be used on a field-scale basis with consistent and adequate results.

New weed management tools and growth regulators make production of ultra narrow row (UNR) cotton possible. Weed control, cotton yield, fiber quality, and net returns were compared in UNR bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton. Weeds included broadleaf signalgrass, carpetweed, common cocklebur, common lambsquarters, common ragweed, goosegrass, jimsonweed, large crabgrass, Palmer amaranth, pitted morningglory, prickly sida, sicklepod, smooth pigweed, and tall morningglory. Pendimethalin preplant incorporated (PPI) in conventional-tillage or preemergence (PRE) in no-till systems plus fluometuron PRE did not adequately control many of these weeds. Pyrithiobac plus MSMA early postemergence (POST) often was more effective than pyrithiobac alone. Pendimethalin plus fluometuron at planting followed by pyrithiobac plus MSMA early POST controlled sicklepod 82%, goosegrass 89%, Palmer amaranth 92%, and the other species at least 95% late season. Pyrithiobac at mid-POST did not improve control. Bromoxynil plus MSMA early POST was more effective than bromoxynil alone only on sicklepod. Pendimethalin plus fluometuron at planting followed by bromoxynil plus MSMA early POST controlled sicklepod 62%, Palmer amaranth 81%, goosegrass 83%, and all other species at least 95%. Glyphosate early POST did not adequately control many species due to sustained weed emergence. Glyphosate early POST followed by glyphosate late POST (after last effective bloom date) controlled all species except pitted morningglory and tall morningglory at least 93%. Pendimethalin plus fluometuron followed by glyphosate early POST was the most effective glyphosate system overall, and it controlled sicklepod 88%, pitted morningglory 90%, and other species at least 93%. Glyphosate late POST did not increase control in systems with pendimethalin plus fluometuron at planting followed by glyphosate early POST. Yields and net returns were similar with all herbicide/cultivar systems at two of five locations. At other locations, yields and net returns were similar with systems of pendimethalin plus fluometuron at planting followed by pyrithiobac plus MSMA early POST, pendimethalin plus fluometuron followed by bromoxynil plus MSMA early POST, and glyphosate early POST. Greatest yields and net returns were obtained with pendimethalin plus fluometuron at planting followed by glyphosate early POST. Herbicide systems did not affect fiber quality.