Is Retention the Old-New Problem in a Drift-Control Era?

Spray drift has been a problem for years, and a joint effort to reduce drift has been made by all those involved with agrochemical application, including pesticide and adjuvant industries, regulatory bodies, universities, applicators, equipment and nozzle manufacturers, and farmers. Because the risk of spray drift is primarily related to spray droplet size, the main focus in addressing drift is the reduction of driftable fines. However, a corresponding increase in the number of large droplets may reduce the biological efficacy of an agrochemical due to a lower degree of coverage and retention. The objective of this research was to evaluate drift potential versus the retention of glyphosate and dicamba applied with flat fan and air-induction nozzles. The herbicides were evaluated with and without tank-mix adjuvants used for drift control. It was therefore possible to determine both the effect of droplet size and the different adjuvant technologies (such as emulsified oil and guar gum polymer) had on herbicide retention. The main methodologies used in this study were the measurement of droplet size (ASTM E2798, Standard Test Method for Characterization of Performance of Pesticide Spray Drift Reduction Adjuvants for Ground Application) and spray retention, determined as micrograms of dye/cm2 foliage of Chenopodium album. Although air-induction nozzles reduced the drift potential of the herbicides, they also reduced spray retention. The emulsion-based adjuvant was more effective in decreasing driftable fines through flat-fan nozzles, whereas the polymer was more effective through air-induction nozzles. It was interesting to notice that the adjuvants designed for drift control also influenced retention. The emulsion based adjuvant resulted in a 2- to 2.6-fold increase in retention compared with the herbicide without adjuvant (applied through a TTI11004 nozzle). Because retention is an essential feature for ensuring biological efficacy, this paper focused on the balance between drift and biological efficacy.