Impact of application volume and spray droplet size on soybean harvest aid efficacy

Throughout the mid-southern United States, indeterminate maturity group (MG) IV soybean (Glycine max L. Merr.) have largely taken the place of determinate MG V and VI soybean in order maximize yield. However, this shift towards earlier maturity groups results in more green plant material at physiological maturity. Consequently, harvest aids are of interest to producers who seek to defoliate and desiccate soybean in a timely and uniform manner. Field experiments were conducted from 2019 to 2020 to evaluate the impact of harvest aid, application volume, and spray droplet volume mean diameter (VMD) on soybean harvest aid efficacy in Mississippi. Applications of paraquat, saflufenacil, and sodium chlorate were made when soybean averaged 65% brown pods, with application volumes of 5 and 20 gal. acre⁻¹, and VMDs of 200, 500, and 800 µm. Defoliation, desiccation, and green stems were evaluated at 3, 7, 10, and 14 days after treatment (DAT). Soybean grain yield was obtained at harvest and adjusted to 13% moisture. Harvest aid efficacy did not vary due to application volume. Spray droplet spectra with a VMD of 200 µm maximized both defoliation and desiccation efficacy. At 14 DAT, applications of paraquat and sodium chlorate resulted in 95% defoliation but were similar to saflufenacil at 89%. Applications of paraquat resulted in desiccation of 16 more percentage points than saflufenacil, and 8 more percentage points than sodium chlorate. These data indicate soybean harvest aid applications should contain paraquat. Although efficacy was maximized with spray droplet spectra with a VMD of 200 µm, the total variance in efficacy due to VMD of spray droplets was minimal.