Since its discovery and initial commercialization in the 1940s, 2,4-D has been an important tool for weed control in a wide variety of crop and noncrop uses. Work studying its chemistry, physiology, mode of action, toxicology, environmental behavior, and efficacy has not only helped elucidate the characteristics of 2,4-D but also provided basic methods that have been used to investigate the properties of hundreds of herbicides that followed it. Much of the information published by researchers over 60 yr ago is still pertinent to understanding the performance of 2,4-D today. Further, new studies continue to be published, especially regarding the mechanisms of 2,4-D action at the molecular level. New uses for 2,4-D, sometimes enabled by biotechnology, continue to be developed. This review strives to provide an overall understanding of 2,4-D activity in plants, plant sensitivity to 2,4-D, toxicological impacts, and current and future uses.

The objective of this research was to describe proportional differences across time and region in management practices among southern cotton farmers who experienced glyphosate-resistant (GR) weeds on their farms earlier than those who experienced them later and among farmers who were closest to one of four historical outbreak epicenters: Lauderdale County, TN; Macon County, GA; Edgecombe County, NC; and Terry County, TX. A mail survey was conducted with cotton farmers in 2012 from 13 southern, cotton-producing states. Survey responses on practices used by farmers were classified into three broad categories of labor, mechanical/tillage/chemical (MTC), and cultural. Proportions of respondents using practices from each category were identified by time and region; across which, proportional-difference tests were conducted. Results indicated respondents encountering GR weeds earlier were more likely than farmers who experienced them later to use the three broad-category practices (labor, 98 vs. 92%; MTC, 95 vs. 89%; and cultural, 86 vs. 76%) and specific practices, including hooded sprayers (76 vs. 58%), in-season herbicide change (83 vs. 60%), and field-border management (60 vs. 35%). Also, respondents closest to Lauderdale County were more likely than farmers closest to Edgecombe County to use broad-labor practices (99 vs. 91%) and specific practices, including hand hoeing (96 vs. 84%), hand spraying (49 vs. 31%), spot spraying (76 vs. 59%), wick applicator (13 vs. 11%), and field-border management (58 vs. 39%). Education programs on weed management can be developed and tailored according to the time and regional differences to provide effective information and communication channels to farmers.

Specialty crops, like flowers, herbs, and vegetables, generally do not have an adequate spectrum of herbicide chemistries to control weeds and have been dependent on hand weeding to achieve commercially acceptable weed control. However, labor shortages have led to higher costs for hand weeding. There is a need to develop labor-saving technologies for weed control in specialty crops if production costs are to be contained. Machine vision technology, together with data processors, have been developed to enable commercial machines to recognize crop row patterns and control automated devices that perform tasks such as removal of intrarow weeds, as well as to thin crops to desired stands. The commercial machine vision systems depend upon a size difference between the crops and weeds and/or the regular crop row pattern to enable the system to recognize crop plants and control surrounding weeds. However, where weeds are large or the weed population is very dense, then current machine vision systems cannot effectively differentiate weeds from crops. Commercially available automated weeders and thinners today depend upon cultivators or directed sprayers to control weeds. Weed control actuators on future models may use abrasion with sand blown in an air stream or heating with flaming devices to kill weeds. Future weed control strategies will likely require adaptation of the crops to automated weed removal equipment. One example would be changes in crop row patterns and spacing to facilitate cultivation in two directions. Chemical company consolidation continues to reduce the number of companies searching for new herbicides; increasing costs to develop new herbicides and price competition from existing products suggest that the downward trend in new herbicide development will continue. In contrast, automated weed removal equipment continues to improve and become more effective.

The anticipated release of EnlistTM cotton, corn, and soybean cultivars likely will increase the use of 2,4-D, raising concerns over potential injury to susceptible cotton. An experiment was conducted at 12 locations over 2013 and 2014 to determine the impact of 2,4-D at rates simulating drift (2 g ae ha−1) and tank contamination (40 g ae ha−1) on cotton during six different growth stages. Growth stages at application included four leaf (4-lf), nine leaf (9-lf), first bloom (FB), FB + 2 wk, FB + 4 wk, and FB + 6 wk. Locations were grouped according to percent yield loss compared to the nontreated check (NTC), with group I having the least yield loss and group III having the most. Epinasty from 2,4-D was more pronounced with applications during vegetative growth stages. Importantly, yield loss did not correlate with visual symptomology, but more closely followed effects on boll number. The contamination rate at 9-lf, FB, or FB + 2 wk had the greatest effect across locations, reducing the number of bolls per plant when compared to the NTC, with no effect when applied at FB + 4 wk or later. A reduction of boll number was not detectable with the drift rate except in group III when applied at the FB stage. Yield was influenced by 2,4-D rate and stage of cotton growth. Over all locations, loss in yield of greater than 20% occurred at 5 of 12 locations when the drift rate was applied between 4-lf and FB + 2 wk (highest impact at FB). For the contamination rate, yield loss was observed at all 12 locations; averaged over these locations yield loss ranged from 7 to 66% across all growth stages. Results suggest the greatest yield impact from 2,4-D occurs between 9-lf and FB + 2 wk, and the level of impact is influenced by 2,4-D rate, crop growth stage, and environmental conditions.

Flumioxazin and fomesafen are commonly used to control glyphosate-resistant Palmer amaranth in cotton and other crops, thus increasing risk to select for Palmer amaranth biotypes resistant to protoporphyrinogen oxidase (PPO) inhibitors. A field experiment was conducted to determine the potential for fluridone and acetochlor to substitute for soil-applied PPO inhibitors in a Palmer amaranth management system with glufosinate applied twice POST and diuron plus MSMA POST-directed in conservation tillage cotton. Fluridone and flumioxazin applied preplant 23 to 34 d prior to planting were similarly effective. Fluridone and acetochlor plus diuron applied PRE controlled Palmer amaranth as well as fomesafen plus diuron PRE. All systems with preplant and PRE herbicides followed by glufosinate POST and diuron plus MSMA layby controlled Palmer amaranth well. Cotton yield did not differ among herbicide treatments. This research demonstrates that fluridone and acetochlor can substitute for soil-applied PPO-inhibiting herbicides in management systems for Palmer amaranth.

Palmer amaranth is a very problematic weed that has evolved resistance to several classes of herbicides, including 5-enolypyruvylshikimate-3-phosate synthase–inhibiting herbicides and photosystem II–inhibiting herbicides. In recent years, corn producers have had difficulty controlling large Palmer amaranth (> 20 cm) in corn > 30 cm whether it be due to environmental conditions or management failures. Palmer amaranth management in corn this tall is made even more challenging because atrazine is not labeled POST in corn > 30 cm tall. Therefore, a study was conducted in 2013 and 2014 in Jackson, TN, to evaluate herbicide programs in corn > 30 cm tall for the control of glyphosate-resistant Palmer amaranth > 20 cm tall. Treatments consisted of herbicides applied alone and in mixtures with dicamba plus diflufenzopyr. Herbicides were applied POST to corn between the V5 and V6 growth stages. Dicamba plus diflufenzopyr 28 d after application controlled Palmer amaranth > 87%. The herbicides alone or in combinations applied as tank mixtures did not improve control (< 76%) over dicamba plus diflufenzopyr alone. There were no grain-yield differences among treatments because of Palmer amaranth control. This was likely due to the Palmer amaranth competition having already affected corn yield by the V5 to V6 corn growth stages.

The herbicide fluridone is a soil-residual herbicide that should provide effective control of several problematic agronomic weeds, but because of herbicide persistence, injury to rotational crops is possible. In this experiment, multiple rates of fluridone were applied PRE to cotton at four irrigated locations across Arkansas to determine the risk of fluridone persisting and injuring subsequently planted wheat, corn, soybean, rice, grain sorghum, and sunflower. The multiple rates of fluridone were compared with fluometuron and evaluated for percentage of crop injury, crop density, and potential yield loss for each crop at the end of the subsequent growing season. Regardless of the location, wheat exhibited the greatest injury with 13 to 26% at Fayetteville (silt loam), 8 to 15% at Pine Tree (silt loam), 2 to 7% at Keiser (silty clay), and 3 to 8% at Rohwer (silty clay). Along with high levels of injury to wheat, fluridone at 900 g ai ha−1 caused loss of wheat stands to 29 plants m−1 row compared with fluometuron, which had stands of 49 plants m−1 row. Although injury occurred in wheat at all locations, no rate of fluridone reduced wheat yields compared with fluometuron. Injury to grain sorghum ranged from 5 to 10% from all rates of fluridone at Pine Tree. Fluridone at 900 g ha−1 (11 plants m−1 row) also reduced grain sorghum stands at Pine Tree over that of fluometuron (19 plants m−1 row). A decrease in grain sorghum yield was also observed from fluridone at 448, 673, and 900 g ha−1 compared with fluometuron at Pine Tree. At Keiser, rice exhibited significant levels of injury (1 to 13%) from fluridone 393 d after treatment. In conclusion, injury to a wheat rotational crop is more likely following an application of fluridone in cotton than is injury to other rotational crops, but yield reductions are not expected for most rotational crops when fluridone is applied to cotton at an anticipated labeled rate of 224 g ha−1.

Fall panicum is the most troublesome annual grass weed in sugarcane in Florida. The critical timing of fall panicum removal in sugarcane or the maximum amount of early season interference that sugarcane can tolerate before it suffers irrecoverable yield loss is not known. Field studies were conducted from 2012 to 2015 in Belle Glade, FL to determine the critical timing of fall panicum removal and season-long interference in sugarcane. The effect of season-long fall panicum interference and critical timing of removal based on 5 and 10% acceptable yield loss (AYL) levels were determined by fitting a log-logistic equation to percentage millable stalk, cane, and sugar yield loss data. Millable stalks, cane, and sucrose yield decreased as the duration of fall panicum interference increased. Season-long interference of fall panicum resulted in 34 to 60%, 34 to 62%, and 44 to 60% millable stalk, cane, and sucrose yield loss, respectively. The critical timing of fall panicum removal based on 5 and 10% AYL for millable stalks was 5 to 9 wk after sugarcane emergence (WAE). At 5 and 10% AYL, the critical timing of fall panicum removal ranged from 5 to 9 WAE and 6 to 8 WAE for cane and sucrose yield loss, respectively. These results show that fall panicum is competitive with sugarcane early in the season, demonstrating the need for timely early-season control to reduce negative effect on yield.

Carrier water pH, hardness, coapplied foliar fertilizer, water conditioning agents, and plant height are critical considerations for optimum herbicide performance. Field studies were conducted to evaluate the effect of carrier water pH (4, 6.5, and 9) and zinc (Zn) or manganese (Mn) foliar fertilizer on mesotrione for horseweed and Palmer amaranth control. Additionally, effect of carrier water pH and foliar fertilizer was evaluated on 7.5-, 12.5-, and 17.5-cm tall horseweed. Greenhouse treatments consisted of carrier water pH and foliar fertilizer (Zn, Mn, or without fertilizer); or water hardness (0 to 1,000 mg L−1) in the presence or absence of ammonium sulfate (AMS) for mesotrione control of giant ragweed, horseweed, and Palmer amaranth. Mesotrione activity was greater on horseweed with carrier water pH 6.5 compared to pH 4 or 9. Coapplied Zn fertilizer reduced mesotrione activity on Palmer amaranth in the field study in 2014 and horseweed in the greenhouse study. Mesotrione efficacy was greatly influenced by horseweed height. Percent control ranged from 96 to 99%, 75 to 89%, or 61 to 64% with mesotrione applied on 7.5-, 12.5-, or 17.5-cm tall horseweed, respectively, and results were similar for plant height and dry weight reduction. Increasing carrier water hardness from 0 to 1,000 mg L−1 reduced mesotrione efficacy 28, 18, and 18% (or greater) on giant ragweed, horseweed, and Palmer amaranth, respectively. The addition of AMS enhanced mesotrione efficacy 9, 6, or 9% (or greater) for giant ragweed, horseweed, and Palmer amaranth control, respectively. Mesotrione should be applied at near neutral carrier water pH, hardness < 200 mg L−1, and with AMS for achieving optimum weed control.

Carrier water quality is an important consideration for herbicide efficacy. Effect of carrier water pH (4, 6.5, or 9) and coapplied Zn or Mn foliar fertilizer was evaluated on glufosinate efficacy for horseweed and Palmer amaranth control in the field. Greenhouse studies were conducted to evaluate the effect of: (1) carrier water pH, foliar fertilizer (Zn, Mn, or without fertilizer), and ammonium sulfate (AMS) (at 0 or 2.5% v/v); and (2) carrier water hardness (0 to 1,000 mg L−1) and AMS (at 0 or 2.5% v/v) on glufosinate efficacy for giant ragweed, horseweed, and Palmer amaranth control. In a 2014 field study, control, plant density reduction, and biomass reduction were at least 8% greater for horseweed and at least 14% greater for Palmer amaranth when glufosinate was applied at carrier water pH 4 compared with pH 9. Glufosinate efficacy was at least 10 and 17% greater for giant ragweed and Palmer amaranth control, respectively, with carrier water pH 4 compared with pH 9 in the greenhouse. In the greenhouse studies, coapplied Zn or Mn fertilizer had no effect on glufosinate efficacy. Increased carrier water hardness from 0 to 1,000 mg L−1 negatively influenced glufosinate efficacy and resulted in 20 and 17% lesser control and biomass reduction, respectively, on giant ragweed or Palmer amaranth. Use of AMS enhanced glufosinate efficacy on giant ragweed control in both greenhouse studies, whereas only the Palmer amaranth control was enhanced in the water hardness study. Horseweed control with glufosinate as affected by carrier water pH, hardness, or AMS remained unaffected in both greenhouse studies. Carrier water at alkaline pH or hardness > 200 mg L−1 has potential to reduce glufosinate efficacy. Therefore, carrier water free of hardness cations and at acidic condition (pH = 4 to 6.5) should be considered for optimum glufosinate efficacy.

Pigweeds are among the most abundant and troublesome weed species across Midwest and mid-South soybean production systems because of their prolific growth characteristics and ability to rapidly evolve resistance to several herbicide sites of action. This has renewed interest in diversifying weed management strategies by implementing integrated weed management (IWM) programs to efficiently manage weeds, increase soybean light interception, and increase grain yield. Field studies were conducted across 16 site-years to determine the effectiveness of soybean row width, seeding rate, and herbicide strategy as components of IWM in glufosinate-resistant soybean. Sites were grouped according to optimum adaptation zones for soybean maturity groups (MGs). Across all MG regions, pigweed density and height at the POST herbicide timing, and end-of-season pigweed density, height, and fecundity were reduced in IWM programs using a PRE followed by (fb) POST herbicide strategy. Furthermore, a PRE fb POST herbicide strategy treatment increased soybean cumulative intercepted photosynthetically active radiation (CIPAR) and subsequently, soybean grain yield across all MG regions. Soybean row width and seeding rate manipulation effects were highly variable. Narrow row width (≤ 38 cm) and a high seeding rate (470,000 seeds ha−1) reduced end-of-season height and fecundity variably across MG regions compared with wide row width (≥ 76 cm) and moderate to low (322,000 to 173,000 seeds ha−1) seeding rates. However, narrow row widths and high seeding rates did not reduce pigweed density at the POST herbicide application timing or at soybean harvest. Across all MG regions, soybean CIPAR increased as soybean row width decreased and seeding rate increased; however, row width and seeding rate had variable effects on soybean yield. Furthermore, soybean CIPAR was not associated with end-of-season pigweed growth and fecundity. A PRE fb POST herbicide strategy was a necessary component for an IWM program as it simultaneously managed pigweeds, increased soybean CIPAR, and increased grain yield.

Production of maize in western Kenya is severely constrained by the parasitic weed striga. Although productivity of maize can be improved through adoption of improved varieties, adoption of such varieties remains low in the region, as the majority of smallholder farmers still grow unimproved open-pollinated varieties (landraces). The performance of two improved hybrid varieties was evaluated against six landraces in striga-infested soils in western Kenya. The varieties were planted in plots under natural striga infestation and were supplemented with pot experiments under artificial infestation. Striga emergence was lower in landraces than in the hybrid varieties in both field and pot experiments. Similarly, height of maize plants at harvest and grain yields were higher in the landraces than in the hybrids. After three continuous cropping seasons, in all treatments, striga seedbank density increased two to seven times. Seedbank increase was higher with hybrids and two of the landraces, ‘Rachar' and ‘Endere'. These results provide an insight into the potential role landraces could play in efforts toward an integrated management approach for striga in smallholder cropping systems. They also highlight the need to develop hybrid maize lines with local adaptation to biotic constraints, specifically striga.

A study was established to evaluate interactions between imazamox at 0 and 44 g ai ha−1 mixed with propanil at 0, 1,120, 2,240, 3,360, and 4,480 g ai ha−1 for the control of red rice and barnyardgrass. Blouin's Modified Colby's procedure was used to test for interactions. At 7 d after treatment (DAT), a synergistic response occurred for red rice treated with imazamox at 44 g ha−1 mixed with propanil at 3,360 and 4,480 g ha−1 by increasing expected control of 62% to an observed control of 67 and 75%, respectively, and the synergistic response continued across all evaluations through 49 DAT. No antagonism occurred for any imazamox plus propanil mixture for red rice control. An antagonistic response was shown for barnyardgrass control with imazamox at 44 g ha−1 mixed with any rate of propanil, at 7 DAT. However, imazamox plus propanil at 4,480 g ha−1 resulted in a neutral response at 14 through 49 DAT. Rice treated with imazamox plus propanil at 4,480 g ha−1 plus imazamox resulted in a yield of 6,640 kg ha−1. The synergistic response observed for red rice control with a mixture of imazamox plus propanil can benefit producers by increasing control of red rice, and this mixture contains two different modes of action that can be part of an overall resistance management strategy.

Two separate field experiments were conducted over a 2-yr period in Fayetteville, AR, during 2012 and 2013 to (1) evaluate POST herbicide programs utilizing a premixture of dimethylamine (DMA) salt of glyphosate + choline salt of 2,4-D in a soybean line resistant to 2,4-D, glyphosate, and glufosinate and (2) determine efficacy of herbicide programs that begin with PRE residual herbicides followed by POST applications of 2,4-D choline + glyphosate DMA on glyphosate-resistant Palmer amaranth. In the first experiment, POST applications alone that incorporated the use of residual herbicides with the glyphosate + 2,4-D premixture provided 93 to 99% control of Palmer amaranth at the end of the season. In the second experiment, the use of flumioxazin, flumioxazin + chlorimuron methyl, S-metolachlor + fomesafen, or sulfentrazone + chloransulam applied PRE provided 94 to 98% early-season Palmer amaranth control. Early-season control helped maintain a high level of Palmer amaranth control throughout the growing season, in turn resulting in fewer reproductive Palmer amaranth plants present at soybean harvest compared to most other treatments. Although no differences in soybean yield were observed among treated plots, it was evident that herbicide programs should begin with PRE residual herbicides followed by POST applications of glyphosate + 2,4-D mixed with residual herbicides to minimize late-season escapes and reduce the likelihood of contributions to the soil seedbank. Dependent upon management decisions, the best stewardship of this technology will likely rely on the use multiple effective mechanisms of action incorporated into a fully integrated weed management system.

Desiccants are currently used to improve lentil dry-down prior to harvest. Applying desiccants at growth stages prior to maturity may result in reduced crop yield and quality, and leave unacceptable herbicide residues in seeds. There is little information on whether various herbicides applied alone or as a tank-mix with glyphosate have an effect on glyphosate residues in harvested seed. Field trials were conducted at Saskatoon and Scott, Saskatchewan, Canada, from 2012 to 2014 to determine whether additional desiccants applied alone or tank mixed with glyphosate improve crop desiccation and reduce the potential for unacceptable glyphosate residue in seed. Glufosinate and diquat tank mixed with glyphosate were the most consistent desiccants, providing optimal crop dry-down and a general reduction in glyphosate seed residues without adverse effects on seed yield and weight. Saflufenacil provided good crop desiccation without yield loss, but failed to reduce glyphosate seed residues consistently. Pyraflufen-ethyl and flumioxazin applied alone or tank mixed with glyphosate were found to be inferior options for growers as they exhibited slow and incomplete crop desiccation, and did not decrease glyphosate seed residues. Based on results from this study, growers should apply glufosinate or diquat with preharvest glyphosate to maximize crop and weed desiccation, and minimize glyphosate seed residues.

Feral populations of cultivated crops have the potential to function as bridges and reservoirs that contribute to the unwanted movement of novel genetically engineered (GE) traits. Recognizing that feral alfalfa has the potential to lower genetic purity in alfalfa seed production fields when it is growing in the vicinity of foraging pollinators in alfalfa seed fields, industry has established production standards to control feral plants. However, with the commercialization of GE glyphosate-resistant (GR) alfalfa and the need to support the coexistence of both GE and conventional production, effective methods to control transgenic feral alfalfa need to be developed. Therefore, a study was conducted in 2012, 2013, and 2014 to determine the effect of several synthetic auxin herbicides on seed development in GR alfalfa. GR alfalfa, var. Genuity (R44BD16), was treated with dicamba, 2,4-D, triclopyr, and aminopyralid when alfalfa plants contained green seed pods. Two weeks after herbicide application, plants were harvested, air dried, and seed yield, seed germination, and seedling emergence from the soil were determined. In 2013, dicamba, triclopyr, and 2,4-D decreased alfalfa seed yield per plant compared wih nontreated plants, whereas in 2014, all four herbicides decreased alfalfa seed yield per plant 24 to 49% (by weight) compared with nontreated plants. The same trend was evident in 2012, but seed yield was variable and was not significantly different among treatments. Seed germination averaged 43, 50, and 72% in 2012, 2013, and 2014, respectively, and was not affected by the four herbicides applied at early pod-fill stage. However, seeds harvested from plants treated with dicamba, 2,4-D, and triclopyr often produced deformed and abnormal seedlings, and when planted in soil, frequently failed to emerge. The combined effects of dicamba, 2,4-D, and triclopyr in reducing seed yield, seedling emergence, and seedling growth could contribute to managing feral alfalfa populations.

Crop safety is one of the many considerations when deciding which POST herbicide to use. This research examined relative corn injury as a result of POST herbicides and the effect of including the safener isoxadifen, the choice of a sensitive or tolerant hybrid, or both. The herbicides included commercial combinations of dicamba, diflufenzopyr, nicosulfuron, rimsulfuron, and thifensulfuron, all at twice the labeled rate. Isoxadifen reduced twisting from dicamba plus diflufenzopyr but not with dicamba plus rimsulfuron. Isoxadifen had negligible effect on chlorosis. In general, rimsulfuron plus thifensulfuron caused the most corn stunting, whereas including isoxadifen or using a tolerant hybrid often reduced corn injury. In two of the four years, treatments with rimsulfuron plus thifensulfuron resulted in yield reductions. Although using products with isoxadifen or selecting tolerant hybrids may influence injury, herbicide selection will have the greatest effect on corn injury.

Topramezone controls weeds in tolerant, cool-season turfgrasses, but injury potential during establishment has received limited investigation. The objectives of this research were to evaluate the tolerance of ‘Penn A-4’ creeping bentgrass, ‘Manhattan V' perennial ryegrass, and ‘Titan' tall fescue to topramezone at 18.5, 37, or 74 g ae ha−1 during establishment. Grasses were seeded in strips in October, and treatments were applied at 0, 2, 4, or 6 wk after seeding (WAS). Perennial ryegrass and tall fescue had minimal (≤ 8%) injury from all treatments, and ground cover was greater or equal to the nontreated at all application timings. Topramezone applied 4 WAS at 37 and 74 g ha−1 injured creeping bentgrass 16 and 23% at 2 wk after treatment, respectively. However, all other topramezone rates and timings caused < 10% injury. Mesotrione at 175 g ai ha−1 injured creeping bentgrass 14 to 43% at all timings and was more injurious than topramezone. Mesotrione applied at 2, 4, or 6 WAS controlled lesser swinecress ≥ 99% at 20 WAS, whereas applications on the day of seeding provided 71% control. All topramezone treatments provided poor control (< 70%) of lesser swinecress at 20 WAS. Overall, perennial ryegrass and tall fescue are tolerant to topramezone during establishment at the rates tested. Seedling creeping bentgrass has better tolerance to topramezone from 18.5 to 74 g ha−1, than to mesotrione at 175 g ha−1 and may provide end-users an HPPD inhibitor for use during establishment.

Two studies consisting of six field experiments each were conducted at three locations in southwestern Ontario, Canada, in 2014 and 2015 to evaluate the possible antagonism when dicamba was added to quizalofop-p-ethyl or clethodim for the control of volunteer glyphosate-resistant (GR) corn. At 4 wk after application (WAA), quizalofop-p-ethyl at 24, 30, or 36 g ai ha−1 provided 88, 94, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ae ha−1 to quizalofop-p-ethyl (24 g ha−1) reduced the activity of quizalofop-p-ethyl on volunteer GR corn by 12 and 20%. At 4 WAA, clethodim at 30, 37.5, and 45 g ai ha−1 provided 85, 91, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ha−1 to clethodim (30 g ha−1) resulted in antagonism, causing a reduction in volunteer GR corn by 12 and 11%, respectively. In general, there was greater antagonism when the high rate of dicamba was tank-mixed with the lower rate of the graminicide. There was no antagonistic effect on soybean yield by tank-mixing dicamba with either graminicide at all rates evaluated. Based on these results, volunteer GR corn can be controlled effectively by increasing the rate of the graminicide when tankmixed with dicamba.

Sprayer applicator-controlled variables, such as nozzle selection and spray volume, will become increasingly important for making labeled POST applications of dicamba in next-generation cropping systems. A field experiment was conducted in 2013 and 2014 at the Northeast Research and Extension Center in Keiser, AR. Tank mixtures of Engenia (a new form of dicamba), glyphosate, glufosinate, and S-metolachlor were applied with TeeJet AIXR, AITTJ60, and TTI nozzles. Two nozzle sizes, 11003 and 11006, were used to vary spray volume from 94 L ha−1 to 187 L ha−1, respectively. For barnyardgrass, a significant decrease in control was observed when spray volume was reduced for glyphosate + dicamba in 2013. In 2014, an overall decrease in control was observed for the TTI nozzle when spray volume was reduced to 94 L ha−1, averaged across all herbicide treatments. The addition of the product S-metolachlor to glyphosate + glufosinate + dicamba significantly reduced the droplet spectra for all nozzle types. For example, adding S-metolachlor into the tank-mix decreased the volume median diameter (Dv50) for the TTI nozzle at 187 L ha−1 spray volume from 789 μm to 570 μm. The results from this research demonstrate that using low spray volume and coarser nozzles could reduce efficacy of the herbicides on the weed species evaluated. Nozzle selection and spray volume have key roles in maximizing efficacy of POST applications in dicamba-resistant crops. Additionally, evaluating droplet spectra of potential dicamba-containing tank-mixtures is critical for producing the desired droplet size to minimize off-target movement.