New machine-vision technologies like the John Deere See & Spray™ could provide the opportunity to reduce herbicide use by detecting weeds and target-spraying herbicides simultaneously. Experiments were conducted for 2 yr in Keiser, AR, and Greenville, MS, to compare residual herbicide timings and targeted spray applications versus traditional broadcast herbicide programs in glyphosate/glufosinate/dicamba-resistant soybean. Treatments utilized consistent herbicides and rates with a preemergence (PRE) application followed by an early postemergence (EPOST) dicamba application followed by a mid-postemergence (MPOST) glufosinate application. All treatments included a residual at PRE and excluded or included a residual EPOST and MPOST. Additionally, the herbicide application method was considered, with traditional broadcast applications, broadcasted residual + targeted applications of postemergence herbicides (dual tank), or targeted applications of all herbicides (single tank). Targeted applications provided comparable control to broadcast applications with a ≤1% decrease in efficacy and overall control ≥93% for Palmer amaranth, broadleaf signalgrass, morningglory species, and purslane species. Additionally, targeted sprays slightly reduced soybean injury by at most 5 percentage points across all evaluations, and these effects did not translate to a yield increase at harvest. The relationship between weed area and targeted sprayed area also indicates that nozzle angle can influence potential herbicide savings, with narrower nozzle angles spraying less area. On average, targeted sprays saved a range of 28.4% to 62.4% on postemergence herbicides. On the basis of these results, with specific machine settings, targeted application programs could reduce the amount of herbicide applied while providing weed control comparable to that of traditional broadcast applications.

Palmer amaranth with resistance to dicamba, glufosinate, and protoporphyrinogen oxidase inhibitors has been documented in several southern states. With extensive use of these and other herbicides in South Carolina, a survey was initiated in fall 2020 and repeated in fall 2021 and 2022 to determine the relative response of Palmer amaranth accessions to selected preemergence and postemergence herbicides. A greenhouse screening experiment was conducted in which accessions were treated with three preemergence (atrazine, S-metolachlor, and isoxaflutole) and six postemergence (glyphosate, thifensulfuron-methyl, fomesafen, glufosinate, dicamba, and 2,4-D) herbicides at the 1× and 2× use rates. Herbicides were applied shortly after planting (preemergence) or at the 2- to 4-leaf growth stage (postemergence). Percent survival was evaluated 5 to 14 d after application depending on herbicide activity. Sensitivity to atrazine preemergence was lower for 49 and 33 accessions out of 115 to atrazine applied preemergence at the 1× and 2× rate, respectively. Most of the accessions (90%) were controlled by isoxaflutole applied preemergence at the 1× rate. Response to S-metolachlor applied preemergence indicated that 34% of the Palmer amaranth accessions survived the 1× rate (>60% survival). Eleven accessions exhibited reduced sensitivity to fomesafen applied postemergence; however, these percentages were not different from the 0% survivor group. Glyphosate applied postemergence at the 1× rate did not control most accessions (79%). Palmer amaranth response to thifensulfuron-methyl applied postemergence varied across the accessions, with only 36% and 28% controlled at the 1× rate and 2× rate, respectively. All accessions were controlled by 2,4-D, dicamba, or glufosinate when they were applied postemergence. Palmer amaranth accessions from this survey exhibited reduced susceptibility to several herbicides commonly used in agronomic crops in South Carolina. Therefore, growers should use multiple management tactics to minimize the evolution of herbicide resistance in Palmer amaranth in South Carolina.

Cover crop adoption is increasing among growers with the occurrence of herbicide-resistant weed species. A field study conducted at three sites from autumn 2021 through the crop harvest in 2022 in Alabama aimed to evaluate the combined effect of cover crop residue and herbicides for weed control and improved cotton lint yield. The experiment was conducted in split-plot design with main plots consisting of six cover crop treatments: cereal rye, crimson clover, oat, radish, cover crop mixture, and winter fallow. The subplots included four herbicide treatments: (i) preemergence, pendimethalin + fomesafen, (ii) postemergence, dicamba + glyphosate + S-metolachlor, (iii) preemergence followed by postemergence, and (iv) nontreated (NT) check. Cover crops, excluding radish, exhibited greater weed biomass reduction than winter fallow with corresponding herbicide treatments of either preemergence, postemergence, or preemergence + postemergence as compared to control (winter fallow and NT check). Considering preemergence + postemergence treatment, cereal rye, crimson clover, oat, and cover crop mixture provided >95% weed biomass reduction as compared to control. Looking at the overall effect of cover crop, cereal rye outperformed and showed greater weed biomass reduction than radish relative to control. Preemergence + postemergence herbicide treatment resulted in greater lint yield than other treatments. Cotton in cereal rye plots had a greater lint yield than in winter fallow at one out of three locations. In conclusion, integrating herbicides and incorporating high-residue cover crops such as cereal rye is an effective weed management strategy to control troublesome weeds.

Multiple herbicide classes resistant (MHCR) kochia poses a serious concern for producers in the Central Great Plains, including western Kansas. Greenhouse and field experiments were conducted at Kansas State University Research and Extension Centers near Hays and Garden City, KS to evaluate pyridate-based postemergence (POST) herbicide mixtures for controlling MHCR kochia. One previously confirmed MHCR population (resistant to atrazine, glyphosate, dicamba, and fluroxypyr) and a susceptible (SUS) kochia population were tested in a greenhouse study. The kochia population at Hays field site was resistant to atrazine, dicamba, and glyphosate while kochia population at Garden City site was resistant to atrazine and glyphosate. Colby’s analysis revealed synergistic interactions when pyridate was mixed with atrazine, dicamba, dichlorprop-p, fluroxypyr, glyphosate, or halauxifen/fluroxypyr and resulted in ≥94% control and shoot dry biomass reduction of MHCR kochia in a greenhouse study. Similarly, synergistic interactions were observed for MHCR kochia control in fallow field studies at both sites when pyridate was mixed with glyphosate or atrazine. Kochia control was increased from 26% to 90% with the application of glyphosate + pyridate and from 28% to 95% with atrazine + pyridate at both sites as compared to separate applications of glyphosate or atrazine. This is the first report for such a strong synergistic effect for both glyphosate and atrazine mixtures with pyridate on a weed resistant to both. All other pyridate-based herbicide mixtures showed an additive interaction and resulted in better control of MHCR kochia (87 to 100%) as compared to their individual applications (23 to 92%) across both sites except 2,4-D. These results suggest that pyridate can play a crucial role in various POST herbicide mixtures for effective control of MHCR kochia.

A prelaunch survey of broadleaf weeds was conducted to predict the weed management efficacy of a novel genetically engineered sugar beet with resistance traits for glyphosate, dicamba, and glufosinate. We targeted problematic broadleaf weed species prevalent in sugar beet fields, including kochia, common lambsquarters, Palmer amaranth, and redroot pigweed in Colorado, Nebraska, and Wyoming. The results revealed that a significant percentage of kochia populations in Colorado, Nebraska, and Wyoming exhibited resistance to glyphosate (94%, 98%, and 75%, respectively) and dicamba (30%, 42%, and 17%, respectively). Palmer amaranth populations had resistance frequencies for glyphosate and dicamba of 80% and 20% in Colorado and 20% and 3% in Nebraska, respectively. No resistance to the tested herbicides was identified in common lambsquarters or redroot pigweed. Glufosinate resistance was not identified for any species. Kochia and Palmer amaranth populations from Colorado and Nebraska exhibited glyphosate resistance primarily through 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene amplification. However, one glyphosate-resistant kochia population from Wyoming lacked EPSPS gene amplification, indicating the presence of an alternative resistance mechanism. We identified the previously characterized IAA16 G73N substitution in a dicamba-resistant kochia population from Nebraska. However, dicamba-resistant kochia populations from Colorado did not possess this substitution, suggesting an alternative, yet-to-be-determined resistance mechanism. The widespread prevalence of glyphosate and dicamba resistance, coupled with the emergence of novel resistance mechanisms, poses a significant challenge to the long-term efficacy of this novel genetically engineered sugar beet technology. These findings underscore the urgent need for integrated weed management strategies that diversify effective herbicide sites-of-action and incorporate alternative weed management practices within cropping systems.

Twelve putative-resistant (R) redroot pigweed populations were collected in sunflower and soybean fields located in northeastern Greece, after repeated exposure to the acetolactate synthase (ALS)–inhibiting herbicides imazamox and tribenuron-methyl. Studies were conducted to determine the resistance status to these two ALS-inhibiting herbicides and evaluate alternative postemergence and preemergence herbicides for effective control. Two susceptible (S) populations were also included for comparison. Among the 12 putative-R populations studied in the whole-plant dose–response pot experiments, 11 were characterized as cross-resistant (R) to the imidazolinone imazamox and the sulfonylurea tribenuron-methyl. In contrast, the putative R5 and the two reference populations (S1, S2) populations were found to be susceptible. Sequencing of the ALS gene revealed that a point mutation (TGG to TTG at position 574) was selected in domain B, where in combination with domain A the majority of point mutations conferring resistance have been detected, resulting in an amino acid substitution from tryptophan (Trp) to leucine (Leu) in the 11 R populations. By contrast, all sequenced plants of the three susceptible populations were found with the wild-type allele encoding Trp574. The labeled rate of the postemergence herbicides tembotrione and dicamba provided fair to excellent control of the populations with ALS cross-resistance. In contrast, at this rate the preemergence herbicides S-metolachlor + terbuthylazine, isoxaflutole, aclonifen, metribuzin, and pendimethalin provided excellent control. These findings strongly suggest that 11 redroot pigweed populations have evolved cross-resistance to ALS-inhibiting herbicides, but viable options for chemical control of this weed still exist.

Yellow and knotroot foxtail are two common weed species infesting turfgrass and pastures in the southeastern region of the United States. Yellow and knotroot foxtail share morphological similarities and are frequently misidentified by weed managers, thus leading to confusion in herbicide selection. Greenhouse research was conducted to evaluate the response of yellow and knotroot foxtail to several turfgrass herbicides: pinoxaden (35 and 70 g ai ha−1), sethoxydim (316 and 520 g ai ha−1), thiencarbazone + dicamba + iodosulfuron (230 g ai ha−1), nicosulfuron + rimsulfuron (562.8 g ai ha−1), metribuzin (395 g ha−1), sulfentrazone (330 g ai ha−1), sulfentrazone + imazethapyr (504 g ai ha−1), and imazaquin (550 g ai ha−1). All treatments controlled yellow foxtail >87% with more than 90% reduction of the biomass. By comparison, only sulfentrazone alone controlled knotroot foxtail 90% and completely reduced aboveground biomass. Sethoxydim (520 g ai ha−1), metribuzin, and imazaquin controlled knotroot foxtail >70% at 28 d after application. In a rate response evaluation, nonlinear regression showed that yellow foxtail was approximately 8 times more susceptible to pinoxaden and 2 times more susceptible to sethoxydim than knotroot foxtail based on log (WR50) values, which were 50% reduction in fresh weight. Our research indicates that knotroot foxtail is more difficult to control across a range of herbicides, making differentiation of these two species important before herbicides are applied.

Deertongue is a perennial, warm-season grass, and is problematic in naturalized areas of golf courses due to limited control options. Research was conducted to evaluate several herbicides for deertongue control in naturalized areas consisting primarily of fine fescue. Greenhouse studies assessed 24 herbicide admixtures and indicated that fluazifop, glyphosate, imazapic, and thiencarbazone + iodosulfuron + dicamba (TID) reduced deertongue biomass by >80% at 10 wk after initial treatment (WAIT). Subsequent field trials were conducted on golf course naturalized areas. The first site was on a woodland edge and was partially shaded for 6 h each day, and the second trial site was 50 m away from the woodland edge and not subjected to more than 1 h of daily shade. At 9 WAIT, fluazifop at 420 g ai ha−1 applied once or three times at 3-wk intervals and topramezone at 37 g ai ha−1 applied thrice at 3-wk intervals injured fine fescue by ≤10% at both sites. Glyphosate applied at 1,120 g ae ha−1, imazapic at 105 g ai ha−1, and imazapic at 53 g ai ha−1 tank-mixed with glyphosate at 560 g ae ha−1 injured fine fescue by ≥50% under shaded conditions, whereas glyphosate alone did not injure fine fescue under sunny conditions. Fine fescue was completely recovered by 52 WAIT from injury following herbicide treatments, except for glyphosate-containing treatments at the shaded site and glyphosate + imazapic at both sites. At 52 WAIT, glyphosate-containing treatments and sequential applications of fluazifop controlled deertongue by ≥93% and reduced shoot density to ≤5 shoots m−2 averaged over both sites. Fluazifop at 420 g ha−1 applied thrice at 3-wk intervals selectively controls deertongue with excellent safety to fine fescue. Glyphosate also controls deertongue, but unacceptably injures fine fescue when managed under shaded conditions. Future research will assess how different light intensities influence fine fescue epicuticular wax deposits and associated response to glyphosate.

Introducing soybean cultivars resistant to 2,4-D and dicamba allowed for postemergence applications of these herbicides. These herbicides pose a high risk for off-target movement, and the potential influence on crops such as hemp is unknown. Two studies were conducted from 2020 through 2022 in controlled environments to evaluate hemp response to rates simulating off-target events of 2,4-D and dicamba. The objectives of these studies were to (1) determine the effects of herbicide (2,4-D and dicamba) and rate (1× to 1/100,000× labeled rate) on visible injury, height, and branching, and (2) determine the effect of 2,4-D rate (1× to 1/100,000× labeled rate) on visible injury, height, branching, and reproductive parameters. Herbicides were applied in the early vegetative stage, and evaluations took place 14 and 28 d after treatment (DAT) and at trial termination (42 DAT in the greenhouse trial and at harvest in the growth chamber trial). In the greenhouse study, 2,4-D and dicamba at the 1× rate, and the 1/10× rate of dicamba, caused 68%, 78%, and 20% injury 28 DAT, respectively. At the time of trial termination 42 DAT, plants treated with 1× rates of 2,4-D and dicamba, or 1/10× dicamba, were 19, 25, and 9 cm shorter than the nontreated control, respectively. Simulated off-target rates of 2,4-D and dicamba did not influence branching or plant weight at trial termination. In the growth chamber study, the 1× and 1/10× rates of 2,4-D caused 82% and 2% injury 28 DAT, respectively. Plant height, fresh weight, and cannabidiol (CBD) levels of plants treated with simulated off-target rates of 2,4-D were not different from the nontreated control. These studies suggest that hemp grown for CBD exposed to off-target rates of 2,4-D or dicamba in early vegetative stages may not have distinguishable effects 42 DAT or at harvest.

Mixing ammonium sulfate (AMS) can increase dicamba volatility. Therefore, AMS cannot be used with dicamba products in dicamba-resistant soybean. However, most dicamba products applied in corn are labeled to mix with AMS. The objectives of this study were to evaluate broadleaf weed control with dicamba (DiFlexx®) and dicamba/tembotrione (DiFlexx® DUO) applied alone or with AMS or AMS substitute and their effect on broadleaf weed density and biomass. Field experiments were conducted in Illinois, Missouri, and Nebraska in 2018 and 2019. In Illinois and Nebraska, mixing AMS + crop oil concentrate (COC) with dicamba applied at 1,120 g ae ha−1 increased the control of Palmer amaranth and waterhemp (Amaranthus species) from 78% to 92% and velvetleaf from 73% to 96% compared with dicamba applied alone 14 d after application (DAA); however, Missouri data showed no difference. Mixing AMS + COC with dicamba/tembotrione at 597 and 746 g ai ha−1 did not improve broadleaf weed control 14 DAA at any site compared with dicamba/tembotrione applied alone. Control of Amaranthus species was increased from 82% with dicamba applied at 840 g ae ha−1 to 96% when mixed with AMS + COC 28 DAA in Illinois; however, control was similar to dicamba applied at 1,120 g ae ha−1. Broadleaf weed control did not differ among dicamba or dicamba/tembotrione 28 and 56 DAA in Missouri and Nebraska. Broadleaf weed density decreased from 64 plants m−2 to 24 plants m−2 with dicamba at 1,120 g ae ha−1 with AMS + COC 14 DAA in Nebraska; however, no differences were observed in broadleaf weed density or biomass 56 DAA in any state. The results suggest that dicamba or dicamba/tembotrione can be applied without AMS or AMS substitute, especially at higher rates, without losing broadleaf weed control efficacy.

Off-target movement of growth regulator herbicides can cause severe injury to susceptible plants. Apart from not spraying on windy days or at excessive boom heights, making herbicide applications using nozzles that produce large droplets is the preferred method for reducing herbicide drift. Although large droplets maintain a higher velocity and are more likely to reach the leaf surface in windy conditions, their ability to remain on the leaf surface is poorly understood. Upon impact with the leaf surface, droplets may shatter, bounce, roll off, or be retained on the leaf surface. We examined how different nozzles, pressures, and adjuvants impact spray droplet adsorption on the leaf surface of common lambsquarters and soybean. Plants were grown in a greenhouse and sprayed in a spray chamber. Three nozzles (XR, AIXR, and TTI) were evaluated at 138, 259, and 379 kPa, respectively. Dicamba (0.14 kg ae ha−1) was applied alone and with methylated seed oil (MSO), a non-ionic surfactant, silicone-based adjuvant, crop oil concentrate, or a drift reduction adjuvant. A 1,3,6,8-pyrene tetra sulfonic acid tetra sodium salt was added as a tracer. Dicamba spray droplet adsorption when using the XR nozzle, which produced the smallest spray droplets, was 1.75 times greater than when applied with the TTI nozzle with the largest spray droplets. Applying dicamba with MSO increased adsorption on leaf surfaces nearly 4 times the amount achieved without an adjuvant. The lowest application pressure (138 kPa) increased dicamba spray volume adsorbed more than 10% compared to the higher pressures of 259 and 379 kPa. By understanding the impacts of these application parameters on dicamba spray droplet adsorption, applicators can select application parameters, equipment, and adjuvants that will maximize the amount of dicamba spray volume retained on the target leaf surface while minimizing dicamba spray drift.

Pink purslane is often ranked as one of the most troublesome weeds in vegetable production systems in Georgia. Pink purslane encroachment along field edges and in-field of agronomic crops has recently increased. Postemergence herbicides are an effective component of agronomic crop weed management. However, little research has addressed pink purslane control in agronomic crops. Therefore, greenhouse and field studies were conducted from 2022 to 2023 in Tifton, Georgia, to evaluate the response of pink purslane to postemergence herbicides commonly used in agronomic crops. Greenhouse screening provided preliminary evidence whereby 13 of the 21 postemergence herbicides evaluated provided ≥80% aboveground biomass reductions. These 13 herbicides were then used for field studies. Results from the field studies, pooled across two locations, indicated that only three of the 13 herbicides provided aboveground biomass reductions of ≥70% compared to the nontreated control. Those herbicides included atrazine at 1,682 g ai ha−1, glufosinate at 656 g ai ha−1, and lactofen at 219 g ai ha−1 with 79%, 70%, and 83% biomass reduction, respectively (P < 0.05). This research suggests that many of the postemergence herbicides used on agronomic crops will not effectively control pink purslane. Thus, when trying to manage pink purslane with postemergence herbicides in agronomic crops, growers should plant crops or cultivars that are tolerant of either atrazine, glufosinate, lactofen, or a combination of these.

In mid-southern, southeastern, and northeastern U.S. soybean production regions, the evolution of herbicide-resistant weeds has become a significant management challenge for growers. The rising herbicide costs for managing herbicide-resistant weeds are also a growing concern, leading to the utilization of cover crops as an integrated weed management strategy for addressing these challenges. Field experiments were conducted at two locations in Alabama in 2022 to evaluate winter cereal cover crops, including a mixture and herbicide system integration in soybean. Treatments included five cover crops: oats, cereal rye, crimson clover, radish, and a cover crop mixture. Cover crops were evaluated for their weed-suppressive characteristics compared to a winter fallow treatment. Additionally, four herbicide treatments were applied: a preemergence (PRE) herbicide, a postemergence (POST) herbicide, PRE plus POST herbicides, and a nontreated (NT) check. The PRE herbicide was S-metolachlor; the POST treatment contained a mixture of dicamba and glyphosate. The PRE plus POST system contained the PRE application followed by POST application. Results show that cereal rye and the cover crop mixture provided weed biomass reduction compared to all cover crop treatments across both locations. Furthermore, we observed greater soybean yield following the cereal rye cover crop than following the winter fallow treatment at one location. POST and PRE plus POST herbicide treatment resulted in greater weed biomass reduction and improved soybean yield than the PRE herbicide treatment alone and the NT check at both locations.

Waterhemp has evolved resistance to Group 2, 5, 9, 14, and 27 herbicides in Ontario, Canada, making control of this challenging weed even more difficult. Acetochlor is a Group 15, chloroacetanilide herbicide that has activity on many small-seeded annual grasses and some small-seeded annual broadleaf weeds, including waterhemp. The objective of this study was to ascertain if acetochlor mixtures with broadleaf herbicides (dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin), applied preemergence (PRE), increase multiple-herbicide-resistant (MHR) waterhemp control in soybean. Five trials were conducted over 2 yr (2021 and 2022). The acetochlor mixtures caused ≤7% soybean injury, except acetochlor + flumioxazin, which caused 19% soybean injury. Acetochlor applied PRE controlled MHR waterhemp 89% at 4 wk after application (WAA). Dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin controlled MHR waterhemp 59%, 67%, 58%, 64%, and 86%, respectively, at 4 WAA. Acetochlor applied in a mixture with dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin provided good to excellent control of MHR waterhemp; control ranged from 91% to 98% but was similar to acetochlor applied alone. Acetochlor alone reduced MHR waterhemp density and biomass 98% and 93%; acetochlor + flumioxazin reduced waterhemp density and biomass by an additional 2% and 7%, respectively. This research concludes that acetochlor applied in a mixture with flumioxazin was the most efficacious mixture evaluated for MHR waterhemp control.

Waterhemp is a summer annual, broadleaf weed with high fecundity, short seed longevity in the soil, and wide genetic diversity. Populations have evolved resistance to five herbicide modes of action (Groups 2, 5, 9, 14, and 27), which are present across southern Ontario; this has increased the challenge of controlling this competitive weed species in corn, the most important grain crop produced worldwide and the highest-value agronomic crop in Ontario. Acetochlor is a Group 15 soil-applied residual herbicide that has activity on many grass and broadleaf weeds but has yet to be registered in Canada. The objective of this study was to ascertain whether mixtures of acetochlor with flumetsulam, dicamba, atrazine, isoxaflutole/diflufenican, or mesotrione + atrazine applied preemergence would increase the control of multiple-herbicide-resistant (MHR) waterhemp in corn. Five field trials were conducted between 2022 and 2023. No corn injury was observed. Acetochlor applied alone controlled MHR waterhemp 97% 12 wk after application (WAA). All herbicide mixtures controlled MHR waterhemp similarly at ≥98% 12 WAA; there were no differences among herbicide mixtures. Flumetsulam, dicamba, and atrazine provided lower MHR waterhemp control than all other herbicide treatments and did not reduce density or biomass. Acetochlor reduced waterhemp density 98%, while the acetochlor mixtures reduced density similarly at 99% to 100%. This study concludes that the acetochlor mixtures evaluated provide excellent waterhemp control; however, control was not greater than acetochlor alone. Herbicide mixtures should be used as a best management practice to mitigate the evolution of herbicide resistance.

Palmer amaranth, which is resistant to glyphosate and protoporphyrinogen oxidase inhibitors, remains a threat to cotton and soybean production in Tennessee. This is partly due to the recent evolution of dicamba-resistant Palmer amaranth in western Tennessee, which further complicates weed management. Experiments were conducted in 2021 and 2022 to determine the best timing between sequential applications and the order in which 2,4-D or dicamba should be used with glufosinate to control resistant Palmer amaranth. Palmer amaranth control increased when the interval between postemergence herbicide applications decreased from 21 to 7 d. At the 7-d interval in a dicamba-based system, the order of herbicides did not affect Palmer amaranth control. However, in a 2,4-D-based system, the greatest control was achieved when 2,4-D was applied first, followed by either 2,4-D or glufosinate. While weed height at the time of application had a significant effect on Palmer amaranth control with auxin herbicides, control was still unacceptable in the field at the labeled rates of dicamba or 2,4-D when applied to weeds that were <10 cm tall (48% and 53%, respectively). Neither dicamba nor 2,4-D provided acceptable control of the Palmer amaranth populations evaluated in this study. Sequential applications separated by 7 d provided better weed control than those separated by 21 d. Given that the better 7-d sequential treatments provided less than 90% control and resulted in more than 64,000 surviving Palmer amaranth plants per hectare suggests that relying solely on these herbicides for Palmer amaranth control is not a sustainable weed management strategy.

The herbicide dicamba has injured millions of hectares of sensitive plant species in the United States since 2017. This injury has coincided with the commercialization of dicamba-resistant soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.). We quantified atmospheric deposition and mass flux of dicamba in 12 soybean production regions of Missouri. Dicamba was routinely detected in weekly deposition samples collected during agriculturally intensive spray periods. Observed concentrations were indicative of both local (<1 km) and long-distance transport (>1 km) of airborne dicamba. High-deposition events (>100 µg m−2) occurred annually in southeast Missouri, and peak dicamba deposited at these sites (12.5 to 84.0 µg m−2) was sufficient to injure non–dicamba resistant soybean. Adoption rate of dicamba-resistant crops and atmospheric stability explained much of the variance, and it is difficult for a herbicide product label to address these variables. Overall, these results demonstrated that dicamba was commonly deposited from the atmosphere during the growing season, and observed concentrations and fluxes were strongly related to the timing and magnitude of rainfall events and the amount of dicamba usage near collection sites.

Palmer amaranth can grow 4.2 mm in height per degree day; hence, delays of a few days in weed control deployment can result in applications of herbicides to weeds that are larger than those for which the herbicide label recommends. Therefore, it is critically necessary to understand the effect of plant size at the time of herbicide application in conjunction with herbicide spray solution and nozzle type pairings on the effectiveness of weed management programs in the Enlist E3 and XtendFlex production systems. Field experiments were conducted in 2020, in no-crop conditions, at two locations in Arkansas, to evaluate the influence of Palmer amaranth size on its control with glufosinate, dicamba, and 2,4-D applied alone and in mixture with specific nozzle pairings as mandated by label requirements. Also, a laboratory experiment was conducted to evaluate the droplet size and velocity of the spray solutions and nozzles used in the field experiments. A 5- and 10-percentage point reduction in control was observed when dicamba (66%) and 2,4-D (63%) were applied alone, respectively, compared with those herbicides mixed with glufosinate (71% and 73%, respectively). Palmer amaranth density increased to 55, 73, 100, 115, and 140 plants m−2 when plants were sprayed at heights of 15, 25, 41, 61, and 76 cm, respectively, compared with plants that were sprayed when they were 5 cm tall (9 plants m−2). Nozzle type did not affect weed control or density. The percentage of driftable fines increased when a mixture of glufosinate and 2,4-D were used compared with 2,4-D alone. Effective short-term and long-term chemical control of Palmer amaranth will require growers to correctly time their weed management practices and overlay residuals, and expect the need for sequential applications.

Two low-dose dicamba exposure trials were conducted on container-grown peach trees in Fayetteville, AR. Peach trees were ‘July Prince’ scions grafted onto ‘Guardian’ rootstock, were transplanted into 19-L containers, and received experimental dicamba treatments in each year. Container trials were initiated in 2020 and repeated on new trees in 2021. In the repeated application trial, dicamba was applied at 5.6 g ae ha−1 (1/100X field rate) in five sequences: an untreated control receiving no herbicide, one treatment receiving only an initial application, and three treatments receiving an initial application plus sequential applications at the same rate occurring at 14 d, 28 d, and 14 d + 28 d after initial treatment (DAT). A separate trial assessed peach tree responses to dicamba applied at 11.2 g ae ha−1 (1/50X field rate) using a selection of nozzles with differing droplet spectrum characteristics: Turbo TeeJet® induction nozzle TTI11002, air induction turbo TwinJet® nozzle AITTJ60-11002, air induction extended-range (XR) TeeJet® nozzle AIXR11002, XR TeeJet® flat-fan nozzle XR11002, and XR TeeJet® flat-fan nozzle XR1100067. Peach tree height, tree cross-sectional area, and leaf chlorophyll content were not reduced in response to any sequence of dicamba application or nozzle selection. Repeated applications of dicamba at a 1/100X rate did not increase peach injury after 28 DAT. By 84 DAT, no effect of nozzle type on peach tree injury was discernable, and all treatments caused below 4% injury. No dicamba or dicamba metabolites were observed in leaf samples collected at 14, 69, or 85 DAT from trees treated with XR1100067 or in untreated controls. While peach tree injury was observed throughout the experiment, dicamba residues were detected consistently only in 2020 from leaf samples of trees treated with dicamba at a 1/50X rate using TTI1102, AITTJ60-11002, AIXR11002, and XR11002 nozzles.

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.