Saflufenacil/dimethenamid-P is a relatively new prepackaged herbicide mixture that has the potential to provide enhanced weed control in soybean when tank-mixed with reduced doses of imazethapyr. Six field experiments were conducted over a 3-yr period (2011, 2012, and 2013) near Ridgetown and Exeter, Ontario, Canada, to determine the dose of imazethapyr, applied PRE, that must be added to saflufenacil/dimethenamid-P (245 g ai ha−1) to provide effective weed control in soybean. The predicted dose of imazethapyr PRE for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf 8 wk after soybean emergence (WAE) was 66, 180, 137, and 48 g ai ha−1, respectively. In contrast, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1), the dose of imazethapyr PRE needed for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was reduced to 11, 80, 48, and 18 g ha−1, respectively. The control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was improved by 21, 23, 34, and 27%, respectively when saflufenacil/dimethenamid-P (245 g ha−1) was added to imazethapyr PRE. Imazethapyr at 104 g ha−1 resulted in soybean yield that was 95% of the weed-free control; however, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1) only 54 g ha−1 of imazethapyr was required for the same yield level. Based on this study, PRE application of saflufenacil/dimethenamid-P with reduced doses of imazethapyr has the potential to improve soybean yield and provide acceptable weed control (≥ 80%); however, the extent that imazethapyr dose can be reduced is dependent upon weed community composition.

Field experiments were conducted from 2011 to 2013 near Ridgetown and Exeter, Ontario, Canada to evaluate the tolerance of seven identity-preserved (IP) soybean cultivars to protoporphyrinogen oxidase (PPO)-inhibiting herbicides flumioxazin, saflufenacil/dimethenamid-P, and sulfentrazone applied PRE; fomesafen applied POST; as well as PRE followed by (fb) POST application. Ridgetown sites demonstrated excellent tolerance (< 10% injury) to PRE treatments, whereas PRE sulfentrazone caused up to 36% injury at 1 and 2 wk after application (WAA) at Exeter. Of the PRE fb POST treatments evaluated, those containing saflufenacil/dimethenamid-P and sulfentrazone were most injurious to soybean, with cultivar being a further determinant of injury. At Exeter 1 WAA, cultivars ‘S03W4’ and ‘S23T5’ showed 23 to 27 and 45 to 46% injury for saflufenacil/dimethenamid-P and sulfentrazone when followed by POST fomesafen, respectively. Sulfentrazone application (PRE alone or fb fomesafen) consequently reduced S03W4 yield up to 38% and S23T5 up to 25%, whereas saflufenacil/dimethenamid-P fb fomesafen reduced S03W4 by 18%. In general, PRE fb POST caused more injury than PRE treatments; however, injury diminished over time and no significant reduction to soybean yield could be attributed to adding POST fomesafen. On the basis of this study, the injury from PPO herbicides applied PRE is active ingredient, cultivar, and environment specific. Although PPO herbicides have the potential to cause unacceptable crop injury in some IP soybean cultivars, selecting a tolerant cultivar will minimize yield losses.