Abstract. Drift is one of the most hazardous consequences of an improper aerial application of glyphosate. Wind, droplet size, application height, and distance to sensitive areas are the most important factors for drift. Droplet size is affected by nozzle, operating pressure, flight speed, deflection angle, and physicochemical properties of the spray solution. The objective of this study was to evaluate the effect of flight speed and the use of adjuvants on droplet size spectra in aerial applications of glyphosate. The study was conducted in a high-speed wind tunnel at the Pesticide Application Technology Laboratory (University of Nebraska-Lincoln, West Central Research and Extension Center, North Platte, Neb.). Aerial applications were simulated with four different airspeeds (44.4, 52.8, 61.1, and 69.4 m/s) and glyphosate combined with adjuvants (high surfactant oil concentrate, microemulsion drift reduction agent, nonionic and acidifier surfactant, polyvinyl polymer, and glyphosate alone). Droplet size spectra were evaluated using a Sympatec Helos laser diffraction instrument measuring 90 cm from the nozzle tip (CP11-4015). The volumetric droplet size distribution parameters (VMD, DV0.1, and DV0.9) and the percentage of droplets smaller than 100 µm were reported. The relative span was calculated to indicate the droplet size homogeneity [(DV0.9 - DV0.1) / DV0.5]. Glyphosate solutions with adjuvants had a larger VMD than the glyphosate alone solution at 44.4 m/s wind speed. At 69.4 m/s only the glyphosate solution with polymer had a larger VMD. Conversely, the glyphosate with polymer had the smallest DV0.1, and the greatest relative span and percentage of droplets smaller than 100 µm. Generally, adjuvants influence on droplet size was diminished or muted as the airspeed was increased. The polymer tested in this study failed as a drift agent reduction agent, especially at higher airspeeds. While not all polymers were tested, cautions should be taken if using these types of adjuvants in aerial applications. The interaction of airspeed and adjuvants influencing droplet size distribution in aerial applications of glyphosate should be considered by applicators in order to mitigate glyphosate drift to the surrounding environment. Further studies are necessary to better understand the interaction between solution viscosity and air shear effect on the atomization process and droplet size distribution, as well as confirm that trends hold true for other adjuvants in the polymer class. Although applicators tend to operate aircrafts with increased flight speeds in order to optimize the application time efficiency, this practice can reduce or mute adjuvants effects, decrease the droplet size distribution, and increase drift potential in aerial applications of glyphosate. Keywords: Drift reduction technologies, Flight speed, High-speed wind tunnel, Laser diffraction.
