Evaluating the Influence of Various Seeding Rates on Weed Removal Timing in Glyphosate-Resistant Soybean

Abstract

Reduced seeding rate in glyphosate-resistant soybean [Glycine max (L) Merr.] may influence the critical time of herbicide application. Field studies were conducted across four environments in Kentucky to determine the effect of seeding rate and glyphosate application timing on soybean seed yield, weed control, light interception at growth stage R1 and canopy closure at R5. Soybean was seeded at rates of 75,000, 125,000, and 175,000 seeds/acre, while glyphosate was applied at 3 weeks after planting (WAP), 5 WAP, 7 WAP, or 3 & 7 WAP. Seed yield at 125,000 seeds/acre was greater than at 75,000 seeds/acre rate in two of four environments. There were no yield differences between 125,000 and 175,000 seeds/acre. Sequential applications of glyphosate at 3 and 7 WAP as well as a single application at 5 WAP always resulted in seed yield equal to a weed free herbicide program. Yields from the 3 WAP treatment was lower than weed free in three of four environments. Seeding rate did not influence the optimum timing for a spray application for any of the factors measured, indicating that glyphosate herbicide programs do not need to be adjusted when seeding rate is reduced.