Glyphosate‐Resistant Soybean Response to Micro‐Rates of Three Dicamba‐Based Herbicides

Abstract

Core Ideas The impact of simulated dicamba drift on growth and yield of glyphosate‐resistant soybean was similar among dicamba formulations. The impact of dicamba drift on soybean could be influenced by moisture condition of the environmental field. Late vegetative or early flowering stage of soybean was the most sensitive growth stage to dicamba drift. New dicamba‐based herbicides such as Engenia (N,N‐bis‐(3‐aminopropyl) methylamine salt) and XtendiMax (diglycolamine salt) with VaporGrip technology were developed to reduce dicamba volatility and drift; however, there are claims that these products can still volatilize or drift. Field studies were conducted to evaluate glyphosate (N‐(phosphonomethyl)glycine))–resistant (GR) soybean response to micro‐rates (0, 1/1000, 1/500, 1/100, 1/50, and 1/10 of the label rate, 560 g a.e. ha−1) of the two new dicamba products compared with Clarity (diglycolamine salt) applied at three growth stages. The GR soybean [Glycine max (L.) Merr.] was equally impacted by the micro‐rates of all three products as measured by visual injury, height reduction, delayed physiological maturity, and yield reduction. The greatest visual injury (80%), plant height reduction (65%), maturity delay (22 d), and soybean yield loss (96%) was caused by 1/10 of the dicamba label rate when applied at V7/R1 soybean growth stage. In addition, estimation of effective dose for 5, 10, or 20% yield reduction suggested that V7/R1 was the most sensitive soybean growth stage to the three dicamba products. For example, 10% yield reduction occurred when 1.83 to 1.85 g a.e. ha−1 (∼1/300 of the label rate) of Engenia was applied at V2 or R2, whereas, a lower dose of 0.32 g a.e. ha−1 (1/1750 of the label rate) of Engenia caused the same level of yield reduction when applied at V7/R1. Similar doses were estimated for Clarity and XtendiMax; therefore dicamba drift should be avoided at all costs, because GR soybean was equally sensitive to low rates of all three tested products with different formulations or technologies.