Glyphosate-resistant (GR) horseweed management continues to be a challenge in no-till cotton systems in Tennessee and Mississippi. Field studies were conducted in 2009 and 2010 to evaluate saflufenacil in tank mixtures with glyphosate, glufosinate, or paraquat on GR horseweed prior to planting cotton. Saflufenacil and saflufenacil tank mixtures were applied 7 d before planting (DBP). Three broad spectrum herbicides were tank-mixed with saflufenacil at rates of 0, 6.3, 12.5, 25, and 50 g ai ha⁻¹. Saflufenacil at 25 and 50 g ai ha⁻¹ in tank mixture with all three broad-spectrum herbicides provided similar GR horseweed control when compared to the current standard of glyphosate dicamba. Across all saflufenacil rates, lint cotton yield among the glyphosate, glufosinate, and paraquat tank mixture treatments did not differ from each other. Control of horseweed with 25 or 50 g ha⁻¹ of saflufenacil across all tank mixtures also was not different from the standard of glyphosate dicamba. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more cotton injury than glyphosate applied 7 d or more before planting. Saflufenacil applied alone at 25 g ha⁻¹ provided lower control of GR horseweed than the standard, which translated to lower lint yield compared to the glyphosate dicamba treatment or saflufenacil with each tank mixture partner. The 12.5 g ha⁻¹ rate of saflufenacil tank mixed with either paraquat or glufosinate provided less horseweed control (< 85%) than if higher rates of saflufenacil were used (> 95%). However, lint cotton yield was not different between these treatments. This research suggests that saflufenacil at 25 g ha⁻¹ is the most optimal rate for tank mixtures with glyphosate, glufosinate, or paraquat. It also reaffirms earlier research that the 25 g ha⁻¹ saflufenacil rate safely can be applied inside the currently labeled 42-d waiting period between a saflufenacil application and cotton planting.

Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; cotton, Gossypium hirsutum L. ‘Phytogen 375 WRF’.