The control of glyphosate-resistant (GR) horseweed (Conyza canadensis) in soybean has been variable with glyphosate plus saflufenacil. The objective of this research was to determine the biologically effective rate (BER) of saflufenacil, saflufenacil mixed with glyphosate, and metribuzin mixed with saflufenacil and glyphosate applied preplant (PP) for the control of GR horseweed in no-till soybean; a study was conducted to determine each of the three treatments. For each study, seven field sites infested with GR horseweed were used over a 2-yr period (2014, 2015). Saflufenacil alone at 25 and 36 g ai ha−1 provided 90 and 95% control of GR Horseweed 8 wk after application, while the BER to achieve 98% control was outside of the treatment range tested. The saflufenacil plus glyphosate (900 g ai ha−1) BER experiment found less saflufenacil was required as 25, 34, and 47 g ha−1 provided 90, 95, and 98% control of GR horseweed respectively. The metribuzin BER experiment found 61, 261, and 572 g ha−1 was required to provide 90, 95 and 98% control of GR horseweed, respectively, mixed with saflufenacil (25 g ha−1) and glyphosate (900 g ha−1). The addition of metribuzin with the recommended rate of saflufenacil (25 g ha−1) plus glyphosate improved control and a second effective herbicide mode of action for the control of GR horseweed. The use of a three-way herbicide mixture can be an effective weed management strategy to control GR horseweed in soybean.
Nomenclature: Glyphosate; metribuzin; saflufenacil; horseweed, Conyza canadensis (L.) Cronq.; soybean, Glycine max (L.) Merr.