Control of glyphosate-resistant waterhemp with preemergence herbicides in glyphosate- and dicamba-resistant soybean

Glyphosate-resistant (GR) waterhemp was first discovered in Ontario, Canada, in 2014. In Ontario, GR waterhemp interference in previous studies reduced soybean yield up to 73%. Tank-mixes of herbicides with multiple modes-of-action are important for delaying the evolution of herbicide resistance. The objective of this study was to evaluate the efficacy of pyroxasulfone (150 g a.i. ha(-1)), S-metolachlor/metribuzin (1943 g a.i. ha(-1)), pyroxasulfone/sulfentrazone (300 g a.i. ha(-1)), and pyroxasulfone/flumioxazin (240 g a.i. ha(-1)) applied preemergence (PRE) with and without the addition of glyphosate/dicamba (1800 g a.e. ha(-1)) for the control of GR waterhemp in soybean. At 8 wk after treatment application (WAA), glyphosate/dicamba applied PRE controlled GR waterhemp 45%. Pyroxasulfone, S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, and pyroxasulfone/flumioxazin applied PRE controlled GR waterhemp 79%, 87%, 91%, and 95%, respectively. At 2, 4, 8, and 12 WAA, the addition of glyphosate/dicamba to the aforementioned PRE herbicides did not improve GR waterhemp control. There was no increase in GR waterhemp control with the addition glyphosate/dicamba; however, multiple herbicide modes-of-action should be utilized to reduce the selection intensity for herbicide-resistant weeds.