Influence of Plant Height and Glyphosate on Saflufenacil Efficacy on Glyphosate-Resistant Horseweed (Conyza canadensis)

A field study was conducted in 2007 and 2008 near Murphysboro, IL to determine the effect of plant height and addition of glyphosate on control of glyphosate-resistant horseweed with saflufenacil. Saflufenacil was applied at rates ranging from 25 to 125 g ai ha(-1) alone and in combination with glyphosate at 840 g ae ha(-1), and the efficacy compared to paraquat at 840 g ai ha(-1). Control of horseweed with glyphosate applied alone was less than 30%, confirming the presence of glyphosate-resistant plants. At 14 d after application, all treatments with saflufenacil or paraquat provided at least 90% control. Saflufenacil applied alone at the lowest rate of 25 g ha(-1) provided less control (92%) than all other treatments that included saflufenacil, and efficacy was reduced as horseweed height at application increased. Horseweed control from saflufenacil at 50 g ha(-1) was reduced as plant height increased in 2007 but not in 2008. However, saflufenacil applied at 50 g ha(-1) or greater resulted in at least 98 % control, regardless of horseweed height at application or tank mixture with glyphosate. Combining glyphosate with saflufenacil at 25 g ha(-1) increased horseweed control compared with saflufenacil applied alone and resulted in control similar to saflufenacil applied at 50 g ha(-1). Control of horseweed from paraquat declined over time as the growth continued from the apical meristem. The extent of horseweed regrowth from applications of saflufenacil alone was less than that observed from paraquat. The addition of glyphosate to saflufenacil further reduced the frequency of horseweed regrowth compared with saflufenacil applied alone.