Weed Community Dynamics in Cover Crop-Based, Organic Rotational No-till Soybean.

Cover crops can be used to overcome tradeoffs between soil health and weed management goals in organic cropping systems. Cover crops can facilitate reductions in tillage, and recent research has shown that no-till planting of organic soybean into rolled-crimped cereal rye can result in weed suppression and soybean yields that are equivalent to tillage-based management. As in conventional no-till systems, reductions in tillage in organic systems can result in weed community shifts toward perennial species. However, anticipating and managing for such community shifts is even more critical in organic systems because of the reduced options for weed control. In this research, we created a range of cover crop biomass and soybean densities to assess their weed suppressive ability and effect on weed community composition and structure. The study was conducted in 2008 and 2009 in Maryland and Pennsylvania using five levels of cereal rye residue representing 0, 0.5, 1, 1.5, and 2 times the ambient level and five soybean densities ranging from 0 to 74 seeds m^-2. Weed biomass ranged from 0 to 967 g m^-2 across all site-years. The highest levels of weed biomass were observed in MD 2008 (mean = 246 g m^-2), whereas the lowest levels were observed in PA 2009 (mean = 45 g m^-2). Weed biomass decreased with increasing rye residue and decreased with increasing soybean density. In rank order, over 75% of weed biomass was comprised of giant foxtail, large crabgrass, smooth pigweed, and yellow nutsedge in MD 2008; giant foxtail and smooth pigweed in MD 2009; giant foxtail, common ragweed, and redroot pigweed in PA 2008; and common ragweed, eastern black nightshade, cereal rye, velvetleaf, yellow foxtail, redroot pigweed, and giant foxtail in PA 2009. Preliminary results indicate that cover crop-based weed suppression selects for perennial and early emerging annual weed species.