Radish and other brassica cover crop effects on nitrogen availability and weed management

Abstract

Use of radish (<italic>Raphanus sativus</italic> L.) and other brassica species as cover crops has increased in the US Midwest in recent years, as farmers seek new ways to reduce nitrogen losses and manage weeds. Brassicas can take up large amounts of nitrogen quickly, but their effects on nitrogen available to subsequent crop are mixed. Fall-seeded radish cover crops suppress fall weed growth. Little is known about the ability of spring-seeded radish to suppress weeds. The first objective of this research was to determine the effects of radish seeding date (mid-August [Date 1], late August [Date 2], mid-September [Date 3], late September [Date 4]), accession, and seeding rate (4.1 to 22.4 kg ha-1) on canopy cover, biomass, and nitrogen accumulation of a fall-seeded radish cover crop. To accomplish this objective, two-year field experiments were established at St. Paul and Lamberton, MN in 2010 and 2011. The effect of seeding date was larger and more consistent than the effects of accession and seeding rate. At St. Paul, radish cover was &#8805; 79% for all rates and accessions at seeding dates 1-3, but &#8804; 59% at Date 4. Delaying seeding decreased radish biomass by 143 kg ha-1 day-1. Total nitrogen accumulation averaged 96-225 kg ha-1 at Date 1 and 57-132 kg ha-1 at Date 3. At Lamberton, severe drought resulted in poor radish growth at Dates 2-4. In Minnesota, radish cover crops should be planted by mid-September at 5 kg ha-1.The second objective of this research was to determine the effect of fall-seeded brassica cover crops on nitrogen availability in the subsequent growing season. Two-year field experiments were established at St. Paul, Rosemount, and Lamberton, MN in 2010 and 2011. Brassica cover crops accumulated large amounts of nitrogen, up to 151 kg ha-1. In 2010-2011, a wet year, they had little effect on nitrogen availability in the subsequent growing season, but in 2011-2012, a dry year, they reduced nitrogen availability. Brassica cover crops have the potential to reduce nitrogen leaching, but their failure to increase later nitrogen availability suggests that nitrogen taken up by the cover crops is not available when the subsequent crop needs it.The third objective of this research was to determine the effect of fall- and spring-seeded radish cover crops on the density, cover, and biomass of weeds in organically-managed corn. Again, two-year field experiments were established at Rosemount and Lamberton, MN. Spring-seeded radish does not appear feasible as a cover crop in Minnesota. Shoot biomass of spring-seeded radish averaged 385 kg ha-1, compared to 3057 kg ha-1 for fall-seeded radish in the same fields. Effects of radish cover crops on weed density, cover, and biomass were inconsistent. The effect of fall-seeded radish ranged from a 44% decrease in August weed cover to an 88% increase in June weed density in the reduced tillage treatment relative to plots without fall-seeded radish. The effect of spring-seeded radish ranged from a 46% decrease in June weed density to a 94% increase in August non-crop biomass. Radish cover crops are unlikely to improve management of summer annual weeds in organic systems over the short term.