Walsh, K. D., Soltani, N., Hooker, D. C., Nurse, R. E. and Sikkema, P. H. 2015. Biologically effective rate of sulfentrazone applied pre-emergence in soybean. Can. J. Plant Sci. 95: 339-344. Sulfentrazone is a protoporphyrinogen (PPO)-inhibiting herbicide under evaluation for use in soybean in Ontario, Canada. The primary objective of this study was to determine the dose of sulfentrazone applied pre-emergence (PRE) needed to provide 50 and 90% control of redroot pigweed, common ragweed, common lambsquarters and green foxtail. Seven field trials were conducted over a 3-yr period (2007, 2008 and 2009) in southwestern Ontario to evaluate the efficacy of sulfentrazone applied PRE at doses ranging from 26 to 1120 g a.i. ha-1. The doses of sulfentrazone applied PRE to reduce redroot pigweed, common ragweed, common lambsquarters and green foxtail dry weight by 50% were 104, 139, 15 and 65 g a.i. ha-1; doses of 241, 514, 133 and 721 g a.i. ha-1 of sulfentrazone were required for 90% reduction in above-ground biom...