Dicamba and 2,4-D are postemergence herbicides widely used to control broadleaf weed species in crop and non-crop areas in the United States. Currently, multiple formulations of 2,4-D and dicamba are available on the market. Even though the active ingredient is the same, the chemical formulation may vary, which can influence the volatility potential of these herbicides. Therefore, the objective of this study was to evaluate the response of soybean, cotton, and tobacco plants exposed to vapors of 2,4-D and dicamba formulations alone or mixed in humidomes for 24 h. Humidome studies were conducted in an open pavilion at the Lake Wheeler Turfgrass Field Lab of the North Carolina State University in Raleigh, NC. Dicamba and mixture treatments injured and caused a reduction in the height of soybean. Injury varied from 55% to 70%, and average plant height was 8.8 cm shorter compared with untreated control plants. Treatments with 2,4-D caused the least injury to soybean (≤21%), and differences among formulations were identified (dimethylamine > choline > dimethylamine-monomethylamine). However, soybean height was not affected by 2,4-D treatments. No differences between treatments were observed when herbicides were applied to cotton. The greatest injury to tobacco (23.3%) was caused by dicamba dimethylamine. Overall, the effect of 2,4-D and dicamba vapor was species-specific and formulation-dependent. Additionally, environmental conditions in the humidomes may have played a major role on the outcome of this study.

Herbicide resistance in weeds significantly threatens crop production in the United States. The introduction of dicamba-resistant soybean and cotton stacked with other herbicide tolerance traits has provided farmers with the flexibility of having multiple herbicide options to diversify their weed management practices and delay resistance evolution. XtendiMax® herbicide with VaporGrip® Technology is a dicamba formulation registered for use on dicamba-resistant soybean and cotton crops by the U.S. Environmental Protection Agency (EPA). One of the terms of its registration includes an evaluation of inquiries on reduced weed control efficacy by growers or users of XtendiMax for suspected weed resistance. A total of 3,555 product performance inquiries (PPIs) were received from 2018 to 2021 regarding reduced weed control efficacy by dicamba. Following the criteria recommended by EPA for screening of suspected resistance in the field, a total of 103 weed accessions from 63 counties in 13 states were collected for greenhouse testing over those 4 yr. Weed accessions for greenhouse testing were collected only in states where resistance to dicamba was not yet confirmed in the weed species under investigation. The accessions, which consisted primarily of waterhemp and Palmer amaranth, were treated with dicamba at rates of 560 g ae ha−1 and 1,120 g ae ha−1. All weed accessions, except for one accession each of Palmer amaranth and waterhemp, were controlled by ≥90% with dicamba at 21 d after treatment in the greenhouse.

Cotton and soybean growers were offered new technologies in 2016, expanding in-crop herbicide options to include dicamba or 2,4-D. Within 3 yr of commercialization, dicamba use in these crops increased 10-fold, and growers began to report Palmer amaranth escapes in dicamba-tolerant production systems in western Tennessee. In 2020, Palmer amaranth seed was collected from eight Tennessee locations where growers witnessed poor control following dicamba. Greenhouse experiments were conducted to evaluate the response of these Palmer amaranth populations to dicamba. In 2021, field experiments were conducted on two tentative dicamba-susceptible populations in Georgia, on three confirmed dicamba-resistant populations in Tennessee, and on a tentative dicamba-susceptible population in Texas to evaluate cotton response following dicamba and to examine if malathion insecticide (a cytochrome P450 inhibitor) would improve weed control and not reduce cotton yield when applied in conjunction with dicamba. Palmer amaranth populations collected in 2020 survived dicamba in the greenhouse at 1, 2, and 4 times the labeled rate. Five Palmer amaranth populations exhibited 15% to 26% survival to the labeled dicamba rate (560 g ha−1) in the greenhouse. These findings were reinforced in the field when research on three of those populations in 2021 showed 55% control with the labeled dicamba rate and 69% control with 2 times the labeled rate. This demonstrates that the dicamba resistance allele or alleles were passed between generations. This result was not consistent in the Macon County, GA, or Worth County, GA, locations, where malathion improved dicamba control of 15- to 38-cm-tall Palmer amaranth. Cotton injury was observed when malathion was applied in combination with dicamba. These results further document the evolution of dicamba-resistant Palmer amaranth in Tennessee. Moreover, the nonreversal of resistance phenotype by malathion may suggest that the resistance mechanism is something other than metabolism.

With the increase in hectares planted to auxin-resistant cotton, the number of preplant, at-plant, and postplant applications of dicamba and 2,4-D choline to aid in the control of troublesome broadleaf weeds, including glyphosate-resistant Palmer amaranth, has increased. More dicamba and 2,4-D choline applications mean an increased risk of off-target movement. Field studies were conducted in 2019 to 2021 at the Texas Tech University New Deal Research Farm to evaluate dicamba-resistant cotton response to various rates of 2,4-D choline when applied at four growth stages (first square [FS] + 2 wk, first bloom [FB], FB + 2 wk, and FB + 4 wk). Applications of 2,4-D choline were applied at 1,060 (1X), 106 (1/10X), 21 (1/50X), 10.6 (1/100X), 2.1 (1/500X), and 1.06 (1/1000X) g ae ha−1 to Deltapine 1822 XF cotton. Relative to the nontreated control, yield losses were observed in all years at FS + 2 wk and FB from rates of 2,4-D choline ≥ 1/100X. At the FB + 4 wk application, only the 1X rate of 2,4-D choline resulted in a yield reduction in all three years. Micronaire, fiber length, and uniformity were negatively influenced by the 1/10X and 1X rates of 2,4-D choline at various timings in 2019, 2020, and 2021. In addition, short fiber content, neps, and seed coat neps increased where micronaire, fiber length, and uniformity were negatively impacted.

The threat of herbicide-resistant weed species, such as Palmer amaranth, has driven the development of robust weed management programs that rely on more than chemicals for weed control. Previous research has shown that zero-tolerance weed thresholds, cover crops, deep tillage, and diverse herbicide programs are effective strategies for controlling Palmer amaranth. Unfortunately, research investigating the integration of all four of these weed management strategies in a system is lacking. To better leverage these integrated weed management strategies in cotton production systems, a long-term study was initiated in fall 2018 near Marianna, AR, with zero tolerance, deep tillage, a cereal rye cover crop, and either a dicamba or non-dicamba in-crop herbicide program as factors. Results found that total Palmer amaranth emergence was reduced 76% as the result of deep tillage in 2019 and, in the absence of a zero-tolerance strategy, 73% in 2020. In the absence of a zero-tolerance strategy, the combination of a non–cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 73%, while the combination of a cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 78% compared to the combination of a cover crop and non-dicamba herbicide program. Under a zero-tolerance strategy in 2019, tillage reduced cotton yield by 12% and partial returns by US$370 ha−1. In 2020, tillage reduced cotton yield by 14% and partial returns of US$371 ha−1 under a non-zero-tolerance strategy, while a 12% yield reduction and a US$260 ha−1 decrease in partial returns were observed under a zero-tolerance strategy. In 2019, the non-dicamba program resulted in greater partial returns than the dicamba in-crop program because of greater yield and lower program costs. However, in 2020, partial returns were greater for the dicamba in-crop herbicide program owing to greater yields achieved by this program.

Palmer amaranth has a long history of evolving resistance to herbicides to the point at which it has become a significant obstacle to row crop production. A survey of Palmer amaranth escapes in dicamba-resistant cotton and soybean fields in Tennessee was conducted in fall 2021 with the objective of determining whether poor control was due to environmental phenomena or the development of dicamba resistance. A greenhouse dicamba dose-response screen was conducted on 15 Tennessee accessions. Three accessions were identified with a relative resistance factor between 1.85 and 2.49, and one accession from Lauderdale County was found with a relative resistance factor of 14.25. The Lauderdale County 1 accession developed a higher dicamba resistance level than all others evaluated and can no longer be effectively controlled using dicamba. The history of Palmer amaranth escaping dicamba in the Lauderdale County 1 location from 2019 to 2021 in the field and in preliminary greenhouse screens would suggest that the dicamba resistance has passed between generations. This research documents the first findings of Palmer amaranth control failures in cotton and soybean fields due to the evolution of dicamba resistance.

Injury to quizalofop-resistant rice was reported in some fields following postemergence applications of quizalofop. Glyphosate-resistant (GR) corn, cotton, and soybean, and imidazolinone-resistant rice are grown near quizalofop-resistant rice. Herbicide drift from glyphosate and imazethapyr and the resulting crop injury and potential yield loss is a cause of concern for producers. Field experiments conducted near Colt, and Keiser, AR, in 2021 evaluated whether low rates of glyphosate or imazethapyr interact with sequential quizalofop applications to exacerbate injury to quizalofop-resistant rice compared to quizalofop applications alone. Herbicide treatments consisted of a low rate of glyphosate (90 g ae ha−1) or imazethapyr (10.7 g ai ha−1) applied 10, 7, 4, and 0 d before the 2-leaf growth stage of rice, and glyphosate or imazethapyr, at the same rate and timings, followed by quizalofop at 120 g ai ha−1 applied to 2-leaf rice. All plots treated with quizalofop received a subsequent application of the same herbicide and rate at the 5-leaf rice stage. At 28 d after final treatment (DAFT), glyphosate followed by quizalofop the same day to 2-leaf rice caused 77% injury compared with 58% when glyphosate was applied alone, regardless of location. Glyphosate followed by quizalofop the same day reduced rough rice grain yield by 67% compared with 33% when glyphosate was applied alone to 2-leaf rice at the Colt location. Application of imazethapyr followed by quizalofop the same day to 2-leaf rice caused more injury (63% and 19% injury at the Colt and Keiser locations, respectively) than imazethapyr alone (42% and 7% injury at the Colt and Keiser locations, respectively) at 35 DAFT. Overall, glyphosate and imazethapyr followed by quizalofop applications worsened injury compared to glyphosate, imazethapyr, and quizalofop applications alone. As the interval between exposure to a low rate of glyphosate or imazethapyr and quizalofop decreases, the detrimental effect of herbicide on rice likewise increases.

Rigid ryegrass is a problematic weed species in winter crops and winter fallow; however, recently, this weed species has been observed in summer crops and fallow. These observations warrant the evaluation of different postemergence herbicides for its control. Outdoor pot studies were conducted during the spring and summer of 2021 to 2022 to determine the performance of POST herbicides on two summer-emerging (S3 and S6) and two winter-emerging (W3 and W8) accessions of rigid ryegrass. Across all accessions, butroxydim, clethodim, paraquat, and paraquat + amitrole at the field rate provided complete control of rigid ryegrass. Both summer-emerging accessions were found to be resistant to the field rates of glufosinate (750 g ai ha−1), glyphosate (454 g ae ha−1), haloxyfop (52 g ai ha−1), and pinoxaden (30 g ai ha−1). The S6 accession had the highest dose required for a 50% reduction in biomass for these herbicides (glufosinate 1,120 g ai ha−1, glyphosate 1,210 g ae ha−1, haloxyfop 140 g ai ha−1, and pinoxaden 55 g ai ha−1). This summer-emerging accession (S6) was also resistant to iodosulfuron. All four accessions were found susceptible to imazamox + imazapyr (a commercial mixture) and mesosulfuron. The study provides the first evidence of poor control of summer-emerging accessions of rigid ryegrass with different herbicides. Multiple-herbicide-resistant summer-emerging rigid ryegrass accessions would be a challenge to the production of summer crops (e.g., cotton and sorghum) as well as winter crops that rely on weed-free summer fallows for soil moisture retention; therefore these accessions warrant diversified management strategies.

A chloroacetamide herbicide by application timing factorial experiment was conducted in 2017 and 2018 in Mississippi to investigate chloroacetamide use in a dicamba-based Palmer amaranth management program in cotton production. Herbicides used were S-metolachlor or acetochlor, and application timings were preemergence, preemergence followed by (fb) early postemergence, preemergence fb late postemergence, early postemergence alone, late postemergence alone, and early postemergence fb late postemergence. Dicamba was included in all preemergence applications, and dicamba plus glyphosate was included with all postemergence applications. Differences in cotton and weed response due to chloroacetamide type were minimal, and cotton injury at 14 d after late postemergence application was less than 10% for all application timings. Late-season weed control was reduced up to 30% and 53% if chloroacetamide application occurred preemergence or late postemergence only, respectively. Late-season weed densities were minimized if multiple applications were used instead of a single application. Cotton height was reduced by up to 23% if a single application was made late postemergence relative to other application timings. Chloroacetamide application at any timing except preemergence alone minimized late-season weed biomass. Yield was maximized by any treatment involving multiple applications or early postemergence alone, whereas applications preemergence or late postemergence alone resulted in up to 56% and 27% yield losses, respectively. While no yield loss was reported by delaying the first of sequential applications until early postemergence, forgoing a preemergence application is not advisable given the multiple factors that may delay timely postemergence applications such as inclement weather.

Overreliance on herbicides for weed control has led to the evolution of herbicide-resistant Palmer amaranth populations. Farm managers should consider the long-term consequences of their short-term management decisions, especially when considering the soil weed seedbank. The objectives of this research were to (1) determine how soybean population and POST herbicide application timing affects in-season Palmer amaranth control and soybean yield, and (2) how those variables influence Palmer amaranth densities and cotton yields the following season. Soybeans were planted (19-cm row spacing) at a low-, medium-, and high-density population (268,000, 546,000, and 778,000 plants ha–1, respectively). Fomesafen and clethodim (280 and 210 g ai ha–1, respectively) were applied at the VE, V1, or V2 to V3 soybean growth stage. Nontreated plots were also included to assess the effect of soybean population alone. The following season, cotton was planted into these plots so as to understand the effects of soybean planting population on Palmer amaranth densities in the subsequent crop. When an herbicide application occurred at the V1 or V2 to V3 soybean stage, weed control in the high-density soybean population increased 17% to 23% compared to the low-density population. Economic return was not influenced by soybean population and was increased 72% to 94% with herbicide application compared to no treatment. In the subsequent cotton crop, Palmer amaranth densities were 24% to 39% lower 3 wk after planting when following soybean sprayed with herbicides compared to soybean without herbicides. Additionally, Palmer amaranth densities in cotton were 19% lower when soybean was treated at the VE stage compared to later stages. Thus, increasing soybean population can improve Palmer amaranth control without adversely affecting economic returns and can reduce future weed densities. Reducing the weed seedbank and selection pressure from herbicides are critical in mitigating resistance evolution.

Since the release of dicamba-tolerant cotton in 2016, preplant and POST applications of dicamba to control glyphosate-resistant Palmer amaranth have increased. With the increase in area treated with dicamba, the risk of off-target movement to nontarget crops has increased. A field study was conducted at the Texas Tech University New Deal Research Farm equipped with subsurface drip irrigation in 2017 and 2018 to evaluate non-dicamba tolerant cotton response to dicamba when applied at four crop growth stages [first square (FS) + 2 wk, first bloom (FB), FB + 2 wk, and FB + 5 wk]. Dicamba at 0.56 (1×), 0.056 (1/10×), 0.0112 (1/50×), 0.0056 (1/100×), and 0.00112 (1/500×) kg ae ha−1 was applied to ‘FM 1830GLT’ cotton. When applications were made at FS + 2 wk, a shift in boll nodal position was apparent following dicamba at the 1/50× rate in 2017 and at 1/10× in 2018 compared to the nontreated control (NTC). A shift in boll distribution from the 1/50× rate of dicamba was apparent at FB in 2017, but not in 2018. Dicamba applied at the 1× rate at FB + 2 wk resulted in reduced boll numbers. No change in boll number or boll position was apparent following any dicamba rate when applied at FB + 5 wk in both years. Dicamba applied at 1/500×, 1/100×, and 1/50× rates at all timings did not affect yield relative to the NTC. When dicamba was applied at the 1/10× rate, the greatest yield loss was observed at FS + 2 wk followed by FB and FB + 2 wk. Micronaire increased following dicamba applied at 1/10× at FS + 2 wk, FB, and FB + 2 wk in 2017. In 2018, micronaire decreased following dicamba applied at 1/10× at FB + 5 wk.

Throughout eastern Arkansas, Palmer amaranth resistant to protoporphyrinogen oxidase (PPO)-inhibiting herbicides (Group 14 herbicides) has become widespread. Most PPO-resistant Palmer amaranth biotypes possess a target-site mutation, but a metabolic resistance mechanism to fomesafen (Group 14) has also been identified. Once metabolic resistance manifests, plants may also be tolerant to other herbicides and sites of action. To evaluate whether varying spray parameters affected control of PPO-resistant Palmer amaranth in dicamba-tolerant crops, field trials were conducted in 2017 and 2018 at the Lon Mann Cotton Research Station near Marianna, AR, and on-farm in Marion, AR. The experiment included split plot factors of dicamba rate, nozzle type, and carrier volume, with a whole plot factor of population. Dicamba was applied at 560 or 1120 g ae ha−1 through 110015 TTI or AirMix nozzles at 70 or 140 L ha−1 to PPO-resistant or PPO-susceptible Palmer amaranth. Palmer amaranth control 14 d after treatment (DAT) was influenced by an interaction between population and carrier volume. PPO-resistant Palmer amaranth control 14 DAT was 81% regardless of carrier volume, compared with 90% and 95% control at 70 and 140 L ha−1, respectively, of the PPO-susceptible population. An interaction between nozzle type and carrier volume influenced Palmer amaranth control 21 DAT, whereas AirMix nozzles at 140 L ha−1 controlled Palmer amaranth at a greater level (94%) than any other nozzle and carrier volume combination (≤90%). An interaction between population and dicamba rate influenced the relative density of Palmer amaranth 21 DAT. PPO-resistant Palmer amaranth density was less affected by dicamba at either rate than PPO-susceptible Palmer amaranth, relative to the nontreated check. Results concur with those of other research that suggest PPO-resistant Palmer amaranth is harder to control with dicamba. Otherwise, increasing carrier volume affected overall Palmer amaranth control to a greater degree than any other factor.

Proper management of glufosinate in glufosinate-resistant crop technologies is needed to mitigate the likelihood of resistance evolution. Antagonism may result from mixtures of glufosinate and other commonly used POST herbicides in soybean and cotton. Two experiments were conducted at the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, in 2015 and 2016 to evaluate mixtures of glufosinate + clethodim and glufosinate + glyphosate on barnyardgrass, broadleaf signalgrass, johnsongrass, and large crabgrass. Furthermore, droplet spectra analyses were conducted to determine if droplet size was associated with identification of herbicide interactions. Antagonism was dependent on the herbicide rates and the weed species. For barnyardgrass and large crabgrass control 4 wk after treatment, glufosinate + glyphosate was antagonistic at all rates evaluated. When large crabgrass was evaluated, some mixtures (e.g., 595 g ha–1 glufosinate + 76 g ha–1 clethodim) had a significant reduction in control relative to one of the herbicides applied alone. Glufosinate (451 and 595 g ai ha–1) + glyphosate (867 and 1,735 g ae ha–1) was antagonistic at all four possible rate combinations for broadleaf signalgrass control. Fewer instances of antagonism were observed for seedling johnsongrass control than for other species, but certain treatments were identified as antagonistic (e.g., glufosinate at 451 g ai ha–1 + clethodim at 76 g ai ha–1). Overall, antagonism was less likely and greater control was observed when the highest rates of both herbicides in a given mixture were used. The addition of glyphosate or clethodim to glufosinate can increase the volume median diameter and decrease the percentage volume of fines, compared to glufosinate alone. The droplet spectra analyses indicate that the glufosinate performance may be negatively affected by the addition of glyphosate or clethodim.

The introduction of 2,4-D–resistant soybean and cotton provided growers a new POST active ingredient to include in weed management programs. The technology raises concerns regarding potential 2,4-D off-target movement to sensitive vegetation, and spray droplet size is the primary management factor focused on to reduce spray particle drift. The objective of this study was to investigate the droplet size distribution, droplet velocity, and particle drift potential of glyphosate plus 2,4-D choline pre-mixture (Enlist Duo®) applications with two commonly used venturi nozzles in a low-speed wind tunnel. Applications with the TDXL11004 nozzle had larger DV0.1 (291 µm), DV0.5 (544 µm), and DV0.9 (825 µm) values compared with the AIXR11004 nozzle (250, 464, and 709 µm, respectively), and slower average droplet velocity (8.1 m s−1) compared with the AIXR11004 nozzle (9.1 m s−1). Nozzle type had no influence on drift deposition (P = 0.65), drift coverage (P = 0.84), and soybean biomass reduction (P = 0.76). Although the TDXL11004 nozzle had larger spray droplet size, the slower spray droplet velocity could have influenced the nozzle particle drift potential. As a result, both TDXL11004 and AIXR11004 nozzles had similar spray drift potential. Further studies are necessary to understand the impact of droplet velocity on drift potential at field scale and test how different tank solutions, sprayer configurations, and environmental conditions could influence the droplet size and velocity dynamics and consequent drift potential in pesticide applications.

Cover crops can provide many benefits to peanut and cotton crops planted in rotation including suppressing weeds, conserving soil moisture after termination, increasing soil organic matter, and reducing soil erosion. However, herbicide carryover can affect cover crop establishment. The objective of this study was to investigate the responses of 6 cover crops (daikon radish, cereal rye, oat, crimson clover, winter wheat, and common vetch) to 12 soil residual herbicides. A multiyear (2016–2018), multilocation study was conducted in Macon and Henry counties, Alabama. Herbicide treatments included S-metolachlor, acetochlor, pyroxasulfone, diclosulam, imazapic, chlorimuron-ethyl, bentazon plus acifluorfen, pyrithiobac-sodium, trifloxysulfuron-sodium, diuron, prometryn, and flumioxazin, each applied at 10% of the full-labeled rate. At 42 to 52 and 145 to 149 d after planting (DAP), cover crop plant heights and stand counts were evaluated, as was biomass at 145 to 149 DAP. Treatments varied from year to year but not locations. In 2016, significant stand reductions (P ≤ 0.10) of 36% to 43% in rye and 44% to 75% in wheat were observed at 48 to 52 DAP for S-metolachlor, acetochlor, pyroxasulfone, imazapic, and bentazon plus acifluorfen compared with nontreated plants. Vetch had stand reductions ranging from 14% to 80% for all treatments 50 DAP except for plants treated with prometryn. S-metolachlor, pyroxasulfone, and acetochlor reduced stands of rye, wheat, and vetch more than any other herbicides. In 2017, at 147 to 149 DAP, clover stands were reduced by 29% with diclosulam and by 38% with trifloxysulfuron-sodium. Similarly, radish stands were reduced by 64% with diclosulam treatment. No significant biomass reductions were observed for any cover crop species either year. Oat showed the most tolerance with no treatments reducing any growth parameters either year. Although initial injury and stunting may occur, biomass at termination of cover crops were not affected by herbicide residues evaluated in this study.

Field trials were conducted near Lubbock, TX, in 2013, 2014, and 2015 to evaluate non–2,4-D–resistant cotton response to low rates of glyphosate plus 2,4-D choline. Cotton was treated with five rates of glyphosate plus 2,4-D choline (0.0183, 0.183, 1.83, 18.3, and 183 g ae ha−1) at two application timings (nine leaf and first bloom). These rates correspond to contamination rates of 0.0008%, 0.008%, 0.08%, 0.8%, and 8%, respectively. Visual cotton injury, boll retention, lint yield, and fiber properties were recorded. When averaged over contamination rates, visual injury after applications made to nine-leaf cotton was greater than for first-bloom cotton in three of 3 yr and yield loss was greater when applications were made to nine-leaf cotton when compared with first-bloom cotton in two of 3 yr. Averaged over application timing, lint yield in 2013, 2014, and 2015 after glyphosate plus 2,4-D choline contamination rates of 0.0008% and 0.008% were not different than that of the nontreated control, whereas contamination rates of 0.08%, 0.8%, and 8% decreased yield by 3% to 20%, 45% to 58%, and 80% to 96%, respectively. Contamination rates of 0.0008%, 0.008%, 0.08%, and 0.8% rarely affected fiber quality; however, a contamination rate of 8% frequently decreased micronaire, fiber length, fiber length uniformity, and fiber strength. This decrease in fiber quality also resulted in a reduction in cotton loan value and potential financial return. Although decreases in fiber quality parameters were not observed with the 0.8% contamination rate, significant reductions in financial return occurred due to yield loss caused by injury from glyphosate plus 2,4-D choline.

Residual herbicides applied to summer cash crops have the potential to injure subsequent winter annual cover crops, yet little information is available to guide growers’ choices. Field studies were conducted in 2016 and 2017 in Blacksburg and Suffolk, Virginia, to determine carryover of 30 herbicides commonly used in corn, soybean, or cotton on wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed cover crops. Herbicides were applied to bare ground either 14 wk before cover crop planting for a PRE timing or 10 wk for a POST timing. Visible injury was recorded 3 and 6 wk after planting (WAP), and cover crop biomass was collected 6 WAP. There were no differences observed in cover crop biomass among herbicide treatments, despite visible injury that suggested some residual herbicides have the potential to effect cover crop establishment. Visible injury on grass cover crop species did not exceed 20% from any herbicide. Fomesafen resulted in the greatest injury recorded on forage radish, with greater than 50% injury in 1 site-year. Trifloxysulfuron and atrazine resulted in greater than 20% visible injury on forage radish. Trifloxysulfuron resulted in the greatest injury (30%) observed on crimson clover in 1 site-year. Prosulfuron and isoxaflutole significantly injured rapeseed (17% to 21%). Results indicate that commonly used residual herbicides applied in the previous cash crop growing season result in little injury on grass cover crop species, and only a few residual herbicides could potentially affect the establishment of a forage radish, crimson clover, or rapeseed cover crop.

S-Metolachlor is commonly used by soybean and cotton growers, especially with POST treatments for overlapping residuals, to obtain season-long control of glyphosate- and acetolactate synthase (ALS)–resistant Palmer amaranth. In Crittenden County, AR, reports of Palmer amaranth escapes following S-metolachlor treatment were first noted at field sites near Crawfordsville and Marion in 2016. Field and greenhouse experiments were conducted to confirm S-metolachlor resistance and to test for cross-resistance to other very-long-chain fatty acid (VLCFA)–inhibiting herbicides in Palmer amaranth accessions from Crawfordsville and Marion. Palmer amaranth control in the field (soil <3% organic matter) 14 d after treatment (DAT) was ≥94% with a 1× rate of acetochlor (1,472 g ai ha–1; emulsifiable concentrate formulation) and dimethenamid-P (631 g ai ha–1). However, S-metolachlor at 1,064 g ai ha–1 provided only 76% control, which was not significantly different from the 1/2× and 1/4× rates of dimethenamid-P and acetochlor (66% to 85%). In the greenhouse, Palmer amaranth accessions from Marion and Crawfordsville were 9.8 and 8.3 times more resistant to S-metolachlor compared with two susceptible accessions based on LD50 values obtained from dose–response experiments. Two-thirds and 1.5 times S-metolachlor at 1,064 g ha–1 were the estimated rates required to obtain 90% mortality of the Crawfordsville and Marion accessions, respectively. Data collected from the field and greenhouse confirm that these accessions have evolved a low level of resistance to S-metolachlor. In an agar-based assay, the level of resistance in the Marion accession was significantly reduced in the presence of a glutathione S-transferase (GST) inhibitor, suggesting that GSTs are the probable resistance mechanism. With respect to other VLCFA-inhibiting herbicides, Marion and Crawfordsville accessions were not cross-resistant to acetochlor, dimethenamid-P, or pyroxasulfone. However, both accessions, based on LD50 values obtained from greenhouse dose–response experiments, exhibited reduced sensitivity (1.5- to 3.6-fold) to the tested VLCFA-inhibiting herbicides.

Palmer amaranth accessions resistant to protoporphyrinogen oxidase (PPO), 5-enolpyruvyl-shikimate-3-phosphate synthase, and acetolactate synthase (ALS)-inhibitor herbicides are widespread in the Midsouth, making control difficult. Field experiments were conducted in Marion and Crawfordsville, AR, in 2016 and 2017 to assess PRE and POST herbicides labeled for use in corn, cotton, or soybean for control of multiresistant Palmer amaranth. Accessions at both locations were resistant to glyphosate and ALS inhibitors and segregating for both the R128 and ΔG210 PPO resistance mechanisms. Of the 15 herbicide treatments tested, only atrazine (1,120 g ai ha−1), pyroxasulfone (149 g ha−1), and flumioxazin (144 g ha−1) provided 85% or greater Palmer amaranth control 14 days after treatment (DAT). Visible control ratings at 35 DAT declined sharply, with no treatment providing more than 84% control, suggesting POST applications should be made no later than 28 DAT. Glufosinate (594 and 818 g ha−1), dicamba (560 g ae ha−1), 2,4-D plus glyphosate (784 g ae ha−1 plus 834 g ae ha−1), and paraquat (700 g ha−1) applied POST to 7- to 10-cm plants reduced Palmer amaranth density 83% or more 14 DAT. Both glyphosate (1,266 g ha−1) and pyrithiobac sodium (73 g ha−1) provided less than 7% Palmer amaranth control. Although flumioxazin alone at a labeled rate controlled Palmer amaranth 82% in the PRE experiment, PPO inhibitors by themselves applied POST provided no more than 37% control at 14 DAT. Effective foliar herbicides applied POST, including residual herbicides, should be made when Palmer amaranth are less than 10-cm tall for optimal control of these multiresistant Palmer amaranth accessions.

Auxin herbicides are used in combinations to control glyphosate-resistant horseweed preplant burndown. Herbicide labels for 2,4-D–containing products require a 30-d rotation interval for planting cotton cultivars not resistant to 2,4-D. Dicamba labels require an accumulation of 2.5 cm of rain plus 21 d per 280 g ae ha–1 rotation interval for planting cotton cultivars not resistant to dicamba. Previous research has shown that cotton injury caused by dicamba applied 14 d before planting was transient with little effect on cotton yield, whereas 2,4-D has little effect on cotton when applied 7 d prior to planting. Injury caused by dicamba and 2,4-D is inversely related to rainfall received between herbicide application and cotton planting. Experiments were conducted to evaluate cotton tolerance to halauxifen-methyl, a new Group 4 herbicide, applied at intervals shorter than labeled requirements. Experiments were established near Painter and Suffolk, VA, and Belvidere, Clayton, Eure, Lewiston, and Rocky Mount, NC, during the 2017 and 2018 growing seasons. Herbicide treatments included halauxifen, dicamba, and 2,4-D applied 4, 3, 2, 1, and 0 wk before planting (WBP). Visible estimates of cotton growth reduction and total injury were collected 1, 2, and 4 wk after cotton emergence (WAE). Cotton stand and percentage of plants with distorted leaves were recorded 2 and 4 WAE. Cotton plant heights were recorded 4 and 8 WAE. Halauxifen was less injurious (9%) than dicamba (26%) or 2,4-D (21%) 2 WAE when herbicides were applied 0 WBP. Cotton stand reduction 2 WAE by halauxifen was less than 2,4-D and dicamba when applied 0 WBP. Injury observed from herbicides applied 1, 2, 3, and 4 WBP was minor, and no significant differences in cotton stand were observed. Early-season cotton injury was transient, and seed cotton yield was unaffected by any treatment.