Response of Dicamba-Resistant Soybean to Glyphosate/Dicamba Application Rate and Timing

Abstract

Three field experiments were conducted during 2016 and 2017 in southwestern Ontario, Canada to evaluate the effect of glyphosate/dicamba (2:1 ratio) applied at the V2/V3 or V4/V5 growth stage on dicamba-resistant (DR) soybean injury, growth response and yield. At 1 DAA, glyphosate/dicamba at 450, 900, 1350, 1800 and 3600 g·ae·ha -1 caused 0, 0.3, 1.8, 3.3 and 5.8% growth response (leaf droop) when applied at the V2/V3 growth stage and 0.3%, 1.3%, 1.5%, 2.3% and 4.5% growth response when applied at V4/V5 growth stage in DR soybean, respectively. The growth response was similar at 3 DAA. This response was transient with no growth response observed at 7 DAA, 2 WAB, 4 WAB, 8 WAB, and late Sept. (R8 growth stage). Glyphosate/dicamba at 450, 900, 1350, 1800 and 3600 g·ae·ha -1 caused 0.0, 1.4%, 3.6%, 5.7% and 10.7% injury (speckled chlorosis and necrosis and leaf distortion) when applied at the V2/V3 growth stage and 0.5%, 0.7%, 2.0%, 3.7% and 6.9% injury when applied at the V4/V5 growth stage in DR soybean, respectively. The injury observed was transient with no injury observed at 4 and 8 WAB and late Sept. (R8). There was no impact of glyphosate/dicamba at various rates evaluated on maturity and seed yield except at 1350 g·ae·ha -1 (V2/V3) and 3600 g·ae·ha -1 (V4/V5) which caused a 6% reduction in yield of DR soybean yield compared to the weed-free control. Based on these results, gly phosate/dicamba at the labelled rates can be safely applied at the V2/V3 and V4/V5 growth stage in DR soybean. However, care is needed to avoid spray overlaps as the 2 × rate can result in significant crop injury and yield reduction in DR soybean.