Cover crops have the potential to increase the sustainability of agronomic cropping systems. Farmers are increasingly interested in using oilseed radish (Raphanus sativus L. var. oleiformis Pers.), both alone and in mixtures, to suppress weeds, reduce fertilizer inputs, and improve crop yields. However, there is limited information to guide cover crop species selection. To evaluate differences between and within species, we evaluated biomass accumulation of six oilseed radishes, two brown mustards (Brassica juncea [L.] Czern.), two white mustards (Sinapis alba L.), one rapeseed (Brassica napus L.), and one hybrid turnip (Brassica rapa L. x B. napus L.) in field trials. Cover crop biomass accumulation within and between species was similar. The accessions provided rapid ground cover and accumulated biomass at similar rates. In Minnesota, dry aboveground biomass ranged between 3410-5542 kg ha-1, while in Michigan biomass ranged between 2545-3572 kg ha-1. There were no differences in N uptake for any of the accessions in either trial. Brassicaceae cover crops accumulated 100-131 kg N ha-1 and 81-109 kg N ha-1 in aboveground tissues in Minnesota and Michigan, respectively. Experiments were then conducted to investigate the growth and weed suppression of oilseed radish, annual ryegrass (Lolium multiflorum Lam.), cereal rye (Secale cereale L.), oats (Avena sativa L.), crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), and winter pea [Pisum sativum var. arvense (L.) Poir.] both in monocultures and in biculture mixtures of oilseed radish plus each species. Cover crop and weed biomass varied across years. Oilseed radish comprised the majority of biculture fall biomass, and was more competitive in biculture with legumes than grasses. Grass monoculture and grass biculture treatments were more effective at weed suppression in fall 2012, fall 2013, and spring 2014 than legume monoculture treatments. Crimson clover failed to establish in two out of the three years, and winter pea failed to survive the winter in two out of three years. This study also evaluated the impact of the cover crop monocultures and bicultures on a following corn crop in the absence of applied fertilizer. Overall, the cover crops did not reduce corn grain yield, with the exception of annual ryegrass and cereal rye treatments each in one of three years. Annual ryegrass and cereal rye reduced corn yield by 51% and 24%, respectively, compared with the weedy control. An additional experiment was conducted in Lansing and Hickory Corners, MI to determine the impact of fall-planted oilseed radish, annual ryegrass, and radish + ryegrass cover crops on nitrous oxide (N2O) emissions. There were no differences between the cover crop treatments and the bare ground control for fall and spring-summer cumulative N2O –N emissions. It appears nitrous oxide emissions did not represent a major pathway for N loss in this study. This work adds to the cover crop body of knowledge and provides information which will be of use when making recommendations to farmers.
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