Two studies consisting of six field experiments each were conducted at three locations in southwestern Ontario, Canada, in 2014 and 2015 to evaluate the possible antagonism when dicamba was added to quizalofop-p-ethyl or clethodim for the control of volunteer glyphosate-resistant (GR) corn. At 4 wk after application (WAA), quizalofop-p-ethyl at 24, 30, or 36 g ai ha−1provided 88, 94, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ae ha−1to quizalofop-p-ethyl (24 g ha−1) reduced the activity of quizalofop-p-ethyl on volunteer GR corn by 12 and 20%. At 4 WAA, clethodim at 30, 37.5, and 45 g ai ha−1provided 85, 91, and 95% control of volunteer GR corn, respectively. The addition of dicamba at 300 or 600 g ha−1to clethodim (30 g ha−1) resulted in antagonism, causing a reduction in volunteer GR corn by 12 and 11%, respectively. In general, there was greater antagonism when the high rate of dicamba was tank-mixed with the lower rate of the graminicide. There was no antagonistic effect on soybean yield by tank-mixing dicamba with either graminicide at all rates evaluated. Based on these results, volunteer GR corn can be controlled effectively by increasing the rate of the graminicide when tankmixed with dicamba.

Soybean resistant to both glyphosate and dicamba (Roundup Ready 2 Xtend™) 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™ 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.

Six field trials were conducted over a two-year period (2014, 2015) at two locations in southwestern Ontario to compare the level of weed control provided by dicamba applied alone and in combination with dimethenamid-p applied preemergence (PRE) in glyphosate/dicambaresistant soybean to current industry standards when used in a two-pass weed management program. Crop injury, weed control, soybean seed yield, environmental impact (EI), and profitability were evaluated in this study. No significant injury was documented. Several PRE herbicides provided excellent early season grass and broadleaved weed control, though early season weed control of those weed species was not acceptable with glyphosate applied alone or in combination with dicamba, dicamba plus dimethenamid-p, 2,4-D, or saflufenacil applied PRE.

Traditional two-pass weed management strategies need to be compared with new strategies in glyphosate/dicamba-resistant soybean. Weed control, soybean yield, partial profitability and environmental impact (EI) were evaluated in glyphosate/dicamba-resistant soybean using dicamba applied alone or in a tank-mix with dimethenamid-P applied preemergence (PRE). Trials were conducted at three locations during 2014 and 2015. Several PRE herbicides provided excellent control of broadleaf and grass weeds. Dicamba provided > 91% control of broadleaf weeds, and the addition of dimethenamid-P improved grass control. All weed species at the trial locations were controlled > 94% following a postemergence (POST) application of glyphosate. Weed interference reduced soybean seed yield 33% where no herbicide was applied. A single POST application of glyphosate had the lowest EI. Several treatments improved early-season weed control and reduced early-season weed density and biomass compared to glyphosate and had similar EI values. In this study, there was no benefit to yield or partial profit by including a PRE herbicide for weed management; however, the inclusion of multiple modes-of-action in a herbicide program may reduce the selection for herbicide-resistant weeds.