Soil‐residual herbicides: Is there a trade‐off between waterhemp control and soybean injury?

Abstract

AbstractThe perceived risk of pre‐emergence (PRE) herbicide injury negatively affecting soybean [Glycine max (L.) Merr.] grain yield may cause growers to select herbicides and/or rates that, consequently, reduce waterhemp (Amaranthus tuberculatus (Moq.) J.D. Sauer] control. Research was conducted over 3 years with flumioxazin and sulfentrazone to evaluate the influence of herbicide rate and mixture on waterhemp control and soybean injury, density, development, and grain yield. In a year with limited rainfall, waterhemp control was >90% up to 9 weeks after planting (WAP) and soybean injury never exceeded 7% from all treatments. In years with more adequate moisture, reduced rates of either flumioxazin or sulfentrazone applied with any mixture herbicide provided similar waterhemp control (>95%) to a full‐labeled rate of flumioxazin or sulfentrazone applied alone at 3 WAP. A full‐labeled rate of flumioxazin or sulfentrazone in mixture with fomesafen + S‐metolachlor provided the greatest and most uniform waterhemp control (>90%) at 9 WAP. When applied at full‐labeled (1x) rates, soybean injury at 3 WAP from flumioxazin‐ and sulfentrazone‐based treatments ranged from 8 to 18% and 2 to 6%, respectively. Similar trends among treatments were observed for soybean density. Despite early‐season injury, treatments applied at a 1x rate did not reduce grain yield. However, applicators should be cognizant of spray boom overlap as 2x rates increased the extent of yield loss by up to 10 bu acre–1. Soybean injury at 9 WAP was a more accurate predictor of grain yield loss (R2 = .51) than injury at 3 WAP (R2 = .14).