Distribution and Control of Glyphosate-resistant Common Ragweed (Ambrosia artemisiifolia L.) in Ontario Soybeans (Glycine max (L.) Merr.)
Glyphosate resistant (GR) weeds have started to be selected for in Ontario over the last decade following the repeated use of glyphosate in GR crops. An Ontario population of common ragweed was confirmed to be resistant to glyphosate in 2011. In 2012 and 2013, field surveys were conducted to determine the spread of glyphosate resistance in Ontario, where four addition sites with resistance were found in Essex county. Alternative control methods in Roundup Ready (glyphosate resistant) and Roundup Ready 2 Extend (glyphosate plus dicamba resistant) soybean (Glycine max L. Merr.) were evaluated with twenty-eight field experiments. The three objectives of the research were to determine 1) the biologically effective rate of glyphosate on a resistant and susceptible biotype, 2) the level of control of glyphosate tankmixes applied prior to and following crop emergence, 3) the efficacy of glyphosate and dicamba tankmixes in Roundup Ready 2 Xtend soybean applied at various timings. Linuron and metribuzin applied prior to planting, provided >80% control of GR common ragweed 4 and 8 weeks after application. None of the postemergence herbicides provided commercially acceptable control. Sequential applications of dicamba provided 64 to 100% control throughout the season.