Convoluted interactions occur between droplet size, carrier volume, and other application parameters. Recommendations for optimizing herbicide applications based on droplet size should be based on a site-specific management approach to better account for these interactions. © 2018 Society of Chemical Industry.

Chemical weed control remains a widely used component of integrated weed management strategies because of its cost-effectiveness and rapid removal of crop pests. Additionally, dicamba-plus-glyphosate mixtures are a commonly recommended herbicide combination to combat herbicide resistance, specifically in recently commercially released dicamba-tolerant soybean and cotton. However, increased spray drift concerns and antagonistic interactions require that the application process be optimized to maximize biological efficacy while minimizing environmental contamination potential. Field research was conducted in 2016, 2017, and 2018 across three locations (Mississippi, Nebraska, and North Dakota) for a total of six site-years. The objectives were to characterize the efficacy of a range of droplet sizes [150 µm (Fine) to 900 µm (Ultra Coarse)] using a dicamba-plus-glyphosate mixture and to create novel weed management recommendations utilizing pulse-width modulation (PWM) sprayer technology. Results across pooled site-years indicated that a droplet size of 395 µm (Coarse) maximized weed mortality from a dicamba-plus-glyphosate mixture at 94 L ha–1. However, droplet size could be increased to 620 µm (Extremely Coarse) to maintain 90% of the maximum weed mortality while further mitigating particle drift potential. Although generalized droplet size recommendations could be created across site-years, optimum droplet sizes within each site-year varied considerably and may be dependent on weed species, geographic location, weather conditions, and herbicide resistance(s) present in the field. The precise, site-specific application of a dicamba-plus-glyphosate mixture using the results of this research will allow applicators to more effectively utilize PWM sprayers, reduce particle drift potential, maintain biological efficacy, and reduce the selection pressure for the evolution of herbicide-resistant weeds.

Core Ideas Model fit increased by predicting optimum droplet sizes for site‐specific scenarios. Generally, an Extremely Coarse spray would be recommended for a 2,4‐D choline plus glyphosate application. Site‐specific weed management using PWM sprayers was both manageable and effective. Weed control reductions were observed as droplet size increased at several site‐years. Alternative drift reduction efforts must be identified to avoid weed control losses. ABSTRACT The delivery of an optimum herbicide droplet size using pulse‐width modulation (PWM) sprayers can reduce potential environmental contamination, maintain efficacy, and provide more flexible options for pesticide applicators. Field research was conducted in 2016, 2017, and 2018 across three locations (Mississippi, Nebraska, and North Dakota) for a total of 6 site‐years. The objectives were to evaluate the efficacy of a range of droplet sizes (150 µm [Fine] to 900 µm [Ultra Coarse]) using a 2,4‐D choline plus glyphosate pre‐mixture and to create novel weed management recommendations using PWM sprayer technology. A pooled site‐year generalized additive model explained less than 5% of the model deviance, so a site‐specific analysis was conducted. Across the Mississippi and North Dakota sites, a 900‐µm (Ultra Coarse) droplet size maintained 90% of the maximum weed control. In contrast, at the Nebraska sites, droplet sizes between 565 and 690 µm (Extremely Coarse) were almost exclusively required to maintain 90% of the maximum weed control, likely due to weed leaf architecture. Severe reductions in weed control were observed as droplet size increased at several site‐years. Alternative drift reduction practices must be identified; otherwise, weed control reductions will be observed. This research illustrated that PWM sprayers paired with appropriate nozzle–pressure combinations for 2,4‐D choline plus glyphosate pre‐mixture could be effectively implemented into precision agricultural practices by generating optimum herbicide droplet sizes for site‐specific management plans. To fully optimize spray applications using PWM technology, future research must holistically investigate the influence of application parameters and conditions.

The adoption of auxin-tolerant crops has increased awareness regarding herbicide off-target movement. Deposition aids are promoted as a possible solution to off-target movement, although their effect on spray canopy deposition are not well understood. Studies were conducted to determine the impact of deposition aids tank-mixed with herbicides on spray droplet size and canopy deposition. Commonly used herbicides were applied on soybean and cotton in combination with deposition aids (oil, polymer, and guargum). Interactions between herbicide solution and deposition aid influenced droplet size parameters for both cotton and soybean herbicides tested herein (p ≤ 0.0001). Generally, the addition of polymer and guargum deposition aids increased spray droplet size, whereas the addition of oil deposition aid decreased droplet size for some treatments. When herbicides were combined, the inclusion of deposition aids did not influence overall spray deposition on cotton (p = 0.82) and soybean (p = 0.72). When herbicide solutions were evaluated individually, the advent of deposition aids had inconsistent results with cotton and soybean spray deposition being unaffected, increased, or even decreased depending on the herbicide solution tested. For example, the polymer-based deposition aid increased spray deposition on cotton for applications of glyphosate + dicamba + S-metolachlor resulting in 1640.6 RFU (relative fluorescence units). However, the same deposition aid decreased spray deposition on cotton for applications of glyphosate + dicamba + acetochlor (1179.3 RFU). Although deposition aids influenced spray deposition on cotton and soybean for some herbicide combinations, their use should be determined on a case-by-case scenario.

XtendFlex® technology from Bayer allows growers to apply glyphosate, glufosinate, and dicamba POST to cotton. Since the evolution and spread of glyphosate-resistant weed species, early POST applications with several modes of action have become common. However, crop injury potential from these applications warrants further examination. Field studies were conducted from 2015 to 2017 at two locations in Mississippi to evaluate XtendFlex® cotton injury from herbicide application. Herbicide applications were made to XtendFlex® cotton at the 3 to 6 leaf stage with herbicide combinations comprised of two, three, and four-way combinations of glyphosate, glufosinate, S-metolachlor, and three formulations of dicamba. Data collection included visual estimations of injury, stand counts, cotton height, total mainstem nodes, and nodes above whiteflower at first bloom. Data collection at the end of the season included cotton height, total mainstem nodes, and nodes above cracked boll. Visual estimations of injury from herbicide applications were highest at 3 days following applications containing glufosinate + S-metolachlor (36 to 41% injury) and glufosinate + S-metolachlor in combination with dicamba + glyphosate (39 to 41% injury), regardless of the dicamba formulation. Crop injury decreased at each rating interval and dissipated by 28 days following applications (p = 0.3748). Height reductions were present at first bloom and at the end of the season (p < 0.0001), although cotton yield was unaffected (p = 0.2089) even when injury at 3 days after treatment (DAA) was greater than 30%. Results indicate that growers may apply a variety of herbicide tank-mixtures to XtendFlex® cotton and expect no yield penalty. Furthermore, if growers are concerned with cotton injury after herbicide applications, the use of glufosinate in combination with S-metolachlor should be approached with caution in XtendFlex® cotton.

The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is one of the most economically important pests of cotton, Gossypium hirsutum L., in the mid-southern U.S. Experiments were conducted during 2012 and 2013 to evaluate the effect of nitrogen fertilizer application rate on tarnished plant bug populations and management as well as cotton growth, development, and yield. Fertilizer (N) was applied as a 32% urea ammonium nitrate (UAN) solution at pinhead square at five different application rates: 0, 45, 90, 134, and 179 kg N ha-1. Plots were managed for tarnished plant bug with insecticides using treatment thresholds recommended by the Mississippi State University Extension Service. A corresponding set of plots for each N fertilizer application rate were not treated with insecticides fto determine tarnished plant bug infestation level and subsequent damage. The interaction of N fertilizer application rate and tarnished plant bug management level (treated or not treated) was significant for total number of plant bugs observed during the growing season. Fertilizer N application rate and tarnished plant bug management each had a significant impact on the mean number of plant bugs observed on a weekly basis and cotton lint yield. Fertilizer N application rate had a significant impact on the number of applications required to manage tarnished plant bug populations. This research demonstrated that there was an optimal level of N availability to balance yield and insecticide applications for tarnished plant bug, thus maximizing profits.

Growers desiring to maximize productivity of farm land have driven interest in double-cropping cotton (Gossypium hirsutum L.) following wheat (Triticum aestivum L.) production. However, the optimum approach for wheat stubble management and cotton seeding rates to achieve optimum cotton yield following wheat production yields is not completely defined. The objective of this study was to evaluate wheat stubble management practices and cotton seeding rates following wheat production. Field research was conducted in 2012 and 2013 at the R.R. Foil Plant Science Research Center in Starkville, MS and at the Black Belt Branch Experiment Station near Brooksville, MS. Wheat stubble management techniques included: no-till planting of cotton seed into undisturbed wheat stubble (None); double-disking wheat stubble followed by re-forming beds with a one-pass bedding implement (Re-bed); and burning wheat stubble and planting cotton seed without additional tillage (Burn). Delta and Pineland 0912 B2RF cotton seed was seeded at the following rates (planted seeds ha-1): 49,000; 86,500; 123,500; and 160,500. Generally, as cotton seeding rates increased, percent cotton emergence decreased. Burning wheat stubble prior to planting cotton seed resulted in greater cotton emergence when compared to other wheat stubble management techniques. Cotton height at the end of the season was unaffected by wheat stubble management technique or cotton seeding rate. Cotton yields were highest when wheat stubble was burned and cotton was seeded at 160,500 seeds ha-1. These data suggest that increasing cotton seeding rate and planting cotton seed into burned wheat stubble could increase the success rate of double cropping cotton following wheat.

Enlist® cotton with tolerance to 2,4-D choline, glyphosate, and glufosinate became publicly available in 2016 to aid growers in controlling glyphosate-resistant weed species. Little data exist regarding the tolerance of Enlist cotton to herbicide tank mixtures containing glyphosate, glufosinate, 2,4-D choline, and S-metolachlor. The objective of this study was to evaluate the tolerance of Enlist cotton to herbicide tank mixtures including these herbicides. Field studies were conducted in 2016 and 2017 where cotton was sprayed with herbicide combinations containing glyphosate, glufosinate, S-metolachlor, 2,4-D choline, and a premix formulation of glyphosate and S-metolachlor. Crop injury consisted of necrosis, chlorosis, visual stunting, injury on new growth, and total injury at 7, 14, and 28 days after application (DAA). Cotton lint yield was recorded at the conclusion of each growing season. The greatest levels of necrosis and total injury at 7 DAA were observed following applications of glufosinate + S-metolachlor, alone or in combination with glyphosate or glyphosate + 2,4-D choline. The least amount of necrosis and total injury at 7 DAA was observed following applications of glyphosate, glufosinate, S-metolachlor, glyphosate + glufosinate, or glyphosate + S-metolachlor, which produced less than 13% injury. Visual injury at 14 DAA ranged from 8 to 16% across herbicides applied. At 28 DAA, no differences in visual injury were reported. Lint yield was unaffected by herbicide application. Although transient visual injury is expected, Enlist cotton withstood herbicide applications with up to four modes of action in tankmixture without suffering yield reduction.