Benefit of tank mixing dicamba with glyphosate applied after emergence for weed control in dicamba- and glyphosate-resistant soybean

Soybean resistant to both glyphosate and dicamba (Roundup Ready 2 Xtend T) has been developed by Monsanto Inc. and was commercially available for the first time in Canada in 2017. Six field trials were conducted over a 2-yr period (2014 - 2015) at three locations in southwestern Ontario to determine whether there is a benefit of including dicamba in a postemergence application of glyphosate at two application timings for the control of non-glyphosate-resistant weeds in Roundup Ready 2 Xtend T soybean. Adding dicamba to glyphosate did not increase control of grass weed species. The tank mix of glyphosate and dicamba increased the control of redroot pigweed, common ragweed, common lambsquarters, and lady's thumb by as much as 14%, 3%, 7%, and 5%, respectively, at 8 weeks after the late-postemergence application. In general, broadleaf weed density and biomass collected 6 weeks after the late-postemergence application was reduced more with dicamba applied alone or together with glyphosate than when glyphosate was applied alone early postemergence. Due to the absence of a grass herbicide, weed interference with dicamba applied alone resulted in a yield loss of 30%-33% while treatments containing glyphosate resulted in a yield loss of only 3%-7%. The tank mix of glyphosate and dicamba improved broadleaf weed control, but it should not be applied alone due to poor control of grass weeds.