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 yearʹs 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 latevs. 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.