More growers across the U.S. Midwest are considering interseeding or overseeding cover crops into corn for soil health purposes. One challenge of this practice is the potential injury from soil residual herbicides applied preemergence (PRE) for weed control in corn to the interseeded and overseeded cover crop species. Field-treated soil was collected in 2021 and 2022 at Janesville, WI, and Lancaster, WI, to investigate the impact of PRE residual herbicides on establishment of interseeded and overseeded cover crops via greenhouse bioassay. Soil samples (0 to 5 cm depth) were collected from field experiments at 0, 10, 20, 30, 40, 50, 60, and 70 days after treatment (DAT). Treatments consisted of 14 single and multiple sites of action (SOAs) PRE herbicides plus a nontreated check (NTC). Four bioindicator cover crop species were used in the greenhouse bioassay: annual ryegrass, cereal rye, radish, and red clover. Cover crop biomass was collected 28 d after bioassay seeding. Cover crop species responded differently across herbicide treatments. Annual ryegrass and cereal rye were sensitive to treatments containing herbicide Group 15, whereas Groups 2, 4, 5, 14, and 27 had minimal impact on their establishment when field soil was collected at 30 DAT (interseeding scenario) and 70 DAT (overseeding scenario) compared to the NTC. Radish and red clover were sensitive to herbicide Groups 2, 4, and 27, whereas Groups 5, 14, and 15 had minimal impact on their establishment. Annual ryegrass, radish, and red clover were more sensitive to PRE herbicides containing two and three SOAs than to herbicides with a single SOA. On the basis of these greenhouse bioassay results, cover crop species should be carefully selected depending on the soil residual herbicide when interseeded and overseeded into corn. Field studies will be conducted to validate these results and support recommendations to growers interested in this system.

Nomenclature: Annual ryegrass, Lolium multiflorum Lam.; cereal rye, Secale cereale L.; radish, Raphanus sativus L.; red clover, Trifolium pratense L.; corn, Zea mays L.