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

Abstract

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.

Effects of cover crops sown in autumn on N and P leaching

“…This confirms findings by Gieske et al. () and Munkholm and Hansen () that radish can substantially reduce the soil mineral N content in late autumn, compared with bare soil. In three of the 6 years in the present study, the significant difference derived from the subsoil samples (30–60 cm), not the topsoil (0–30 cm), while in the other 3 years both soil depth contributed equally (Figure b).…”

“…Gieske et al. () observed that radish cover crop increased the yield of oats grown in the following year, but concluded that this effect was not linked to increased N availability.…”

“…Baggs et al (2000) also found no effect of cover crops on subsequent main crop yield, while Thorup-Kristensen (1994) found that barley grown after cover crops had a higher N content than barley grown after bare soil. Gieske et al (2016) observed that radish cover crop increased the yield of oats grown in the following year, but concluded that this effect was not linked to increased N availability.…”

Chemical termination of cover crop rapeseed

“…Rapeseed, as a cover crop, has many potential benefits (Chen et al 2007;Chen and Weil 2010;Gieske et al 2016;Virginia NRCS 2015;Williams and Weil 2004). However, termination can be difficult, as demonstrated in these experiments and reported by growers.…”

“…The rapeseed taproot is cylindrical and fast-growing, allowing it to act as a "biodrill" that can reach one or more meters into the soil (Virginia NRCS 2015). Producers in the midwestern United States utilize brassica species to scavenge residual nitrogen left after cash crop harvest (Gieske et al 2016). Gieske et al (2016) found that brown mustard (Brassica juncea L.), rapeseed, radish (Raphanus sativus L.), and white mustard (Sinapis alba L.) all accumulate comparable amounts of nitrogen and biomass.…”