The distribution and control of glyphosate-resistant waterhemp (Amaranthus tuberculatus var. rudis) in soybean (Glycine max) in Ontario
Field surveys were conducted in 2014 and 2015 to gain an understanding of the distribution of glyphosate-resistant waterhemp in Ontario. Following the first confirmed case on Walpole Island, Lambton County in 2014, 39 additional glyphosate-resistant populations were discovered in 2015. In addition to glyphosate, waterhemp populations were found to be resistant to group 2 (imazethapyr) and group 5 (atrazine) herbicides in greenhouse experiments at an incidence of 100% and 75%, respectively. Of the 49 collected waterhemp samples, 61% were resistant to all three herbicide groups (2, 5, and 9). In addition, six greenhouse and thirty-six field experiments were conducted over a two-year period (2015 and 2016) at four locations to determine the biologically effective rate of glyphosate, and at two locations to evaluate the efficacy of pre-emergence and post-emergence herbicides for the control of glyphosate- resistant waterhemp in soybean. Determination of the biologically effective rate of glyphosate revealed a resistance factor in the greenhouse and field of 4.8 and 28, respectively. Among the 14 pre-emergence herbicides available for use in soybean that were tested, pyroxasulfone/flumioxazin, pyroxasulfone/sulfentrazone, and s-metolachlor/metribuzin all provided greater than 90% control. A two- pass weed control strategy was found to be the most efficacious, with a residual pre-emergence herbicide followed by an alternative to glyphosate applied postemergence for waterhemp escapes providing excellent control in all control systems tested.