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Title: Droplet size impact on efficacy of a dicamba-plus-glyphosate mixture

item BUTTS, THOMAS - University Of Nebraska
item SAMPLES, CHASE - Mississippi State University
item FRANCA, LUCAS - Mississippi State University
item DODDS, DARRIN - Mississippi State University
item REYNOLDS, DANIEL - Mississippi State University
item ADAMS, JASON - North Dakota State University
item ZOLLINGER, RICHARD - North Dakota State University
item HOWATT, KIRK - North Dakota State University
item Fritz, Bradley - Brad
item HOFFMANN, WESLEY - Former ARS Employee
item LUCK, JOE - University Of Nebraska
item KRUGER, GREG - University Of Nebraska

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2018
Publication Date: 3/14/2019
Citation: Butts, T., Samples, C., Franca, L., Dodds, D., Reynolds, D., Adams, J., Zollinger, R., Howatt, K., Fritz, B.K., Hoffmann, W.C., Luck, J., Kruger, G. 2019. Droplet size impact on efficacy of a dicamba-plus-glyphosate mixture. Weed Technology. 33(1):66-74.

Interpretive Summary: Chemical weed control is dependent on precise application of herbicide products and requires proper selection of the applied spray droplet size as a function of the target species, environment conditions and chemical type to ensure effective control with minimal off-target impacts. Proper selection and setup of spray delivery systems ensures that optimum droplet sizes are delivered to provide uniform and sufficient deposition of crop protection products to target species required for efficacy. Field trials across three geographically diverse sites spanning three years were conducted to determine optimum spray rates and droplet sizes to effectively control multiple weed species using a dicamba and glyphosate spray solution. Overall results demonstrated that a coarse spray delivered at 10 gallons per acre resulted in maximum weed mortality. However, droplet size could be doubled and still maintain more than 90 percent control while reducing the potential for off-target movement. Optimizing the spray droplet size and rate to maximize weed mortality, while minimizing spray drift, provides for site-specific, precise delivery of herbicides preventing non-target damage and herbicide resistance selection pressure.

Technical Abstract: Chemical weed control remains a widely-used component of integrated weed management strategies due to its cost effectiveness and rapid removal of crop pests. Additionally, dicamba plus glyphosate tank-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 the application process to 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 tank-mixture and create novel weed management recommendations utilizing pulse-width modulation (PWM) sprayer technology. Results across pooled site-years indicated a droplet size of 395 µm (Coarse) maximized weed mortality from a dicamba plus glyphosate tank-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 tank-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.