Effects of a Grass‐Selective Herbicide in a Vetch–Rye Cover Crop System on Nitrogen Management

Cover crop kill date affects N fixation by hairy vetch (Vicia villosa Roth), N uptake by cereal rye (Secale cereale L.), residue C/N ratio, and subsequent N availability. Data are needed on spring management of vetch–rye cover crop mixtures, compared to pure stands, to estimate fertilizer nitrogen (FN) equivalents. A 2‐yr study evaluated spring management of hairy vetch (HV), pure rye, a vetch–rye mixture, and a no‐cover check on N accumulation and subsequent no‐till corn N uptake following corn FN rates of 0, 45, 90, 180, and 270 kg ha−1. A grass‐selective herbicide (GSH) was applied in late March to the pure rye and the vetch–rye mixture, leaving HV to accumulate N until early May. These treatments were compared to the same covers killed in early May. Cover crop N uptake was lowest for rye, intermediate for the mixtures, and highest for HV. The N content in the pure rye and vetch–rye mixture was significantly increased if the previous year's corn had received excess FN. The cover crop mixture produced greater rye growth if fall soil nitrate N was high, while low soil nitrate N resulted in greater yield of HV in the mixture. There was no difference in corn N uptake for the late‐ vs. early kill pure rye, or of the rye component in the vetch–rye mixture. A vetch–rye mixture functioned like a “dual purpose” cover by conserving fall residual N, producing a lower C/N ratio residue than pure rye, and supplying more N to the succeeding corn than pure rye, although the N supplied was still less than pure vetch.