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.

Amaranthus palmeri S. Watson has become a weed of economic importance throughout the South-eastern United States in the last 20 years (Webster and Nichols, 2012) being especially problematic in soyabean (Glycine max L. Merr.) and cotton (Gossypium hirsutum L.) fields (Bensch et al., 2003;Berger et al., 2015). Recently, multiple reports have indicated that this weed species is invading agricultural