Control of glyphosate-resistant Canada fleabane [Conyza canadensis(L.) Cronq.] with preplant herbicide tankmixes in soybean [Glycine max. (L). Merr.]

Byker, H. P., Soltani, N., Robinson, D. E., Tardif, F. J., Lawton, M. B. and Sikkema, P. H. 2013. Control of glyphosate-resistant Canada fleabane [ Conyza canadensis (L.) Cronq.] with preplant herbicide tankmixes in soybean [ Glycine max . (L). Merr.]. Can. J. Plant Sci. 93: 659–667. Glyphosate previously provided excellent control of Canada fleabane; however, with the evolution of glyphosate-resistant (GR) Canada fleabane in Ontario, alternative herbicides must be identified for control of this weed in soybean. The objective of this study was to identify preplant herbicide tankmixes that provide effective control of GR Canada fleabane. A total of 12 field trials were completed over a 2-yr period (2011, 2012) in fields previously confirmed with GR Canada fleabane. Preplant tankmixes of glyphosate (900 g a.e. ha −1 ) plus saflufenacil (25 g a.i. ha −1 ) or saflufenacil/dimethenamid-p (245 g a.i. ha −1 ) provided greater than 87% control 4 wk after application (WAA) across all sites. Glyphosate (900 g a.e. ha −1 ) tankmixed with metribuzin (1120 g a.i. ha −1 ), cloransulam-methyl (35 g a.i. ha −1 ) or flumetsulam (70 g a.i. ha −1 ) provided 78 to 99% control 8 WAA. Control of GR Canada fleabane prior to soybean emergence is essential as currently there are no herbicides that provide acceptable control in-crop. Because of the lack of in-crop options, spring residual herbicides may be required for season long control in regions where Canada fleabane emerges throughout the year to reduce Canada fleabane seed production and populations in subsequent years.