Six field trials were conducted over a two-year period (2014, 2015) to determine the level and consistency of glyphosate-resistant (GR) Canada fleabane control with glyphosate plus saflufenacil plus a third tankmix partner. GR Canada fleabane interference reduced soybean yield 73% compared with the weed free control. At 4 and 8 weeks after application (WAA), glyphosate plus saflufenacil provided 99% and 88% control of GR Canada fleabane respectively, and at 8 WAA, reduced GR Canada fleabane density by 96% and biomass by 89%. Glyphosate plus saflufenacil plus dicamba improved the control of GR Canada fleabane to 100% and 97% at 4 and 8 WAA, respectively. At 8 WAA, glyphosate plus saflufenacil plus amitrole reduced GR Canada fleabane density and biomass 99% and 97%, respectively. At 8 WAA, glyphosate plus saflufenacil plus dicamba at 300 or 600 g a.i. ha −1 reduced GR Canada fleabane biomass 97% and 98%, respectively. Tank-mixing dicamba with glyphosate plus saflufenacil applied pre-plant improved control of GR Canada fleabane; however, this caused 14% and 46% crop injury at 2 and 4 WAA, respectively. Soybean yield for saflufenacil alone and saflufenacil tankmix treatments were similar to the weed free control, with the exception of dicamba (600 g a.i. ha −1).

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Control of glyphosate-resistant (GR) horseweed in soybean with glyphosate (900 g ai ha−1) plus saflufenacil (25 gaiha−1) has been variable. The objective of this research was to determine the effect of GR horseweed height, density, and time of day (TOD) at application on saflufenacil plus glyphosate efficacy in soybean. All experiments were completed six times during a 2 yr period (2014, 2015) in fields previously confirmed with GR horseweed. Applications from 0900 to 2100 hours provided optimal control of GR horseweed 8 WAA. Soybean yield paralleled GR horseweed control with the highest yield of 3000kgha−1at 1500 hours, and the lowest yield of 2400kgha−1at 0600 hours. The height and density of GR horseweed at application had minimal effect on saflufenacil efficacy. Saflufenacil provided>99% control of GR horseweed when applied to small plants and low densities; however, control decreased to 95% when>25 cm tall, and to 96% in densities>800 plants m−2at 6 WAA due to some plant regrowth. TOD of application had a greater influence on GR horseweed control with saflufenacil than height or density. To optimize control of GR horseweed, saflufenacil should be applied during daytime hours to small plants at low densities. Optimizing GR horseweed control minimizes weed seed return and weed interference.

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 threeway herbicide mixture can be an effective weed management strategy to control GR horseweed in soybean.