Miller, R. T., Soltani, N., Robinson, D. E., Kraus, T. E. and Sikkema, P. H. 2012. Biologically effective rate of saflufenacil/dimethenamid- in soybean (Glycine max). Can. J. Plant Sci. 92: 517–531. A total of five field studies were conducted over a 2-yr period (2009, 2010) at three Ontario locations to determine the biologically effective rate of saflufenacil/dimethenamid-p applied preemergence (PRE) for the control of annual weeds in soybean. The rate of saflufenacil/dimethenamid-p required for the control of annual weeds was influenced by environmental conditions. With adequate moisture and above-average temperatures in 2010, between 224 and 374 g a.i. ha^-1 of saflufenacil/dimethenamid-p was required for 80% control of common ragweed, common lamb's quarters, and green foxtail 4 wk after treatment (WAT). In contrast, below-average temperatures and excessive moisture in 2009 resulted in higher rates of saflufenacil/dimethenamid-p being necessary for the same level of weed control. Pigweed species were least affected by environmental conditions after application at 4 WAT with only 245 g a.i. ha^-1 required for 80% control in both years. By 11 WAT, 320 g a.i. ha^-1 or less of saflufenacil/dimethenamid-p was required to achieve 80% control of these species in 2010, while 845 g a.i. ha^-1 or more was needed in 2009 for equivalent control. The potential of saflufenacil/dimethenamid-p as a set-up treatment prior to a postemergence (POST) glyphosate application was also examined. Excellent full season control of all weed species was achieved with saflufenacil/dimethenamid-p applied PRE followed by glyphosate POST. However, there was no difference in yield when saflufenacil/dimethenamid-p was followed by glyphosate POST compared with a single glyphosate POST application.