Efficacy of Diflufenican Plus Metribuzin Applied Preemergence in Soybean

Diflufenican is a selective phenyl ether herbicide belonging to WSSA Group 12 Limited information is available regarding its efficacy, crop safety, and interaction with metribuzin when applied preemergence (PRE) in soybean under Ontario growing conditions. Field experiments were conducted in 2018 and 2019 across three environments in southwestern Ontario, Canada, to evaluate soybean tolerance and weed control with preemergence (PRE) applications of diflufenican, metribuzin, and their combinations. Treatments included metribuzin (150, 200, and 300 g ai ha-1), diflufenican (60, 75, 90, 105, 120, 150, and 180 g ai ha-1), and diflufenican plus metribuzin (1-2 ratio) mixtures (225, 300, and 450 g ai ha-1) applied PRE. Soybean injury was ≤ 8% at 1 and 2 weeks after emergence (WAE) for all treatments. Soybean injury was not observed beyond 2 WAE. Metribuzin applied PRE controlled Powell amaranth (13-97%), redroot pigweed (30-38%), common ragweed (0-38%), common lambsquarters (11-35%), flower-of-an-hour (0-33%), ladysthumb (0-87%), common purslane (5-87%), and wild mustard (25-33%) at 8 WAA and controlled barnyardgrass (14-66%) and green foxtail (14-65%) at 2 WAA. Diflufenican applied PRE controlled Powell amaranth (52-100%), redroot pigweed (35-100%) common ragweed (0-11%), common lambsquarters (0-13%), flower-of-an-hour (0-85%), ladysthumb (≤0-13%), common purslane (10-87%), and wild mustard (47-99%) at 8 WAA and barnyardgrass (0-20%) and green foxtail (0-20%) at 2 WAA. Diflufenican plus metribuzin mixtures applied PRE provided broader and more consistent control, including Powell amaranth (72-89%), redroot pigweed (71-100%), common ragweed (8-43%), common lambsquarters (26-54%), flower-of-an-hour (31-97%), ladysthumb (16-76%), common purslane (46-94%), and wild mustard (56-99%) at 8 WAA and barnyardgrass (10-69%) and green foxtail (11-68%) at 2 WAA. Observed responses were generally additive, with minor antagonistic or synergistic interactions for specific species at specific rating timings. Results indicate that diflufenican has activity on Powell amaranth, redroot pigweed and wild mustard. Diflufenican combined with metribuzin improved the spectrum and level of weeds controlled. Diflufenican co-applied with metribuzin is a safe and effective PRE option for control of some weed species in soybean.