Brassica Cover Crop Effects on Nitrogen Availability and Oat and Corn Yield

Cover crops are used to scavenge residual soil nitrate, with the goal of reducing N losses from agricultural fields and increasing subsequent N availability. Two experiments were conducted to determine fall‐planted brassica cover crops’ effect on N availability for rotational crops. The first evaluated five species—brown mustard [Brassica juncea (L.) Czern], hybrid turnip (Brassica rapa L. × B. napus L.), radish (Raphanus sativus L.), rapeseed (Brassica napus L.), and white mustard (Sinapis alba L.)—in rotation with oat (Avena sativa L.). The second evaluated radish in rotation with corn (Zea mays L.). End of season cover crop biomass averaged 1160 to 6170 kg ha−1 across experiments, locations, and years. Biomass did not differ by species. Nitrogen accumulation was generally greater than 70 kg ha−1 (range 31–136 kg ha−1). In the subsequent spring and summer, brassica cover crops reduced soil nitrate N levels relative to a no‐cover control by 0 to 132 kg ha−1. In Exp. 1, all brassicas reduced oat N accumulation by ≥77 kg ha−1 and oat biomass by ≥1255 kg ha−1 in June 2012, relative to the control. Mustards reduced oat grain yield by 505 kg ha−1 in 2011, while radish increased oat yield by ≥578 kg ha−1 in each year. In Exp. 2, radish did not affect corn V8 biomass N concentration, grain yield, or response to N fertilizer. Nitrogen taken up by brassica cover crops often is not available when the subsequent crop needs it.