Michigan is the number one producer of organic dry beans in the United States. Commercial pesticides and fertilizers cannot be used in organic systems, therefore it is essential to maximize the potential benefits of cover crops for increasing nutrient availability, weed control, and crop yields. Field experiments were conducted from 2011 to 2013 on Michigan State University research farms (main sites) and at organic grower cooperator farms (satellite sites) to determine the impact of cover crop species on dry bean production, including weed management and nitrogen (N) availability. Cover crops included: medium red clover, oilseed radish, and cereal rye; a no cover control was also included. Within each cover crop treatment there were four bean varieties: `Zorro' and `Black Velvet' black beans and `Vista' and `R-99' (non-nodulating) navy beans. Overall, the commercial dry bean varieties responded to the cover crops similarly and showed few differences among varieties with regard to nodulation, percent N derived from the atmosphere in the grain, grain total N, and yield. The non-nodulating variety `R-99' had less total grain N and yield, showing the benefit of N fixation. Cover crops altered some soil, dry bean, and weed characteristics. Soil inorganic N increased following red clover by as much 34 kg N ha-1 at planting and 55 kg N ha-1 at V2 compared with the no cover crop control; yield was not increased but grain N increased by up to 32% in some site-years. Cereal rye reduced soil inorganic N in some instances and caused early maturity of the beans in two of six site-years. At maximum biomass production (12.8 Mg ha-1), cereal rye reduced dry bean yield, however grain N was not affected. Oilseed radish occasionally increased inorganic N availability at the V2 dry bean stage, but had no impact on bean maturity, yield, or grain N. Oilseed radish and cereal rye did not impact weed populations or biomass during the dry bean growing season. However, when clover biomass exceeded 5 Mg ha-1, soil inorganic nitrogen was often higher compared with the no cover crop control, which occasionally resulted in increased weed populations and biomass. Weed responses appeared to also be dependent upon fall seed inputs while the cover crop was growing, precipitation during cover crop establishment and the dry bean growing season, and weed seed bank composition. High concentrations of cereal rye residues, with a high C:N ratio, increased giant foxtail and velvetleaf seed persistence by 12 and 6%, respectively, compared with the no cover crop control in one of two years. Red clover, with a low C:N ratio, decreased common lambsquarters seed persistence by 25% in one of two years. Understanding how the C:N ratio and N content of cover crops change during development and impact N release and N cycling after cover crop termination will improve the synchronization of N availability with cash crop needs, improve weed management, and potentially increase crop yield and quality. Enhancing the predictability of cover crop performance and nutrient availability through management recommendations and breeding will encourage cover crop use among growers.
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