Cover crop species and termination timing impact on soybean production in the Mid‐Atlantic United States

Abstract While cover crop adoption in the United States has increased, limited research exists on the impact of cover crop mixtures on soybean ( Glycine max (L.) Merr.) production. This study evaluated the effects of cereal rye ( Secale cereale L.), cereal rye + hairy vetch ( Vicia villosa ) mixture (rye + vetch), and cereal rye + crimson clover ( Trifolium incarnatum ) mixture (rye + clover), along with pre‐plant and at planting termination timings, on total aboveground biomass, nitrogen (N) accumulation, C:N ratio, soil inorganic N, and soybean yield. A 3‐year study at two Delaware locations revealed that at planting terminated rye + clover produced the highest biomass, reaching 17.56 Mg ha − 1 in 2020, exceeding regional averages. Grass + legume mixtures generally outperformed monoculture rye in biomass and N content, except in 1 year at Middletown. The C:N ratio of mixtures (9:1–43:1) was consistently lower than monoculture rye (17:1–68:1). Soybean yield was influenced by cover crops only in 2018 with rye + vetch increasing yield by 0.45 Mg ha − 1 at Georgetown, but no differences were observed between rye + clover, rye, and control. The results suggest that rye + legume mixture (with low rye seed rate) can enhance biomass production, reduce C:N ratios, and potentially supply N through more rapid decomposition to high‐yielding soybeans, particularly under irrigated conditions. This study highlights the benefits of grass + legume cover crop mixtures for improving biomass and N dynamics without compromising soybean yield.