Maximizing cereal rye biomass has been recommended for weed suppression in cover crop—based organic no-till planted soybean; however, achieving high biomass can be challenging, and thick mulch can interfere with soybean seed placement. An experiment was conducted from 2012 to 2014 in New York to test whether mixing barley and cereal rye would (1) increase weed suppression via enhanced shading prior to termination and (2) provide acceptable weed suppression at lower cover crop biomass levels compared with cereal rye alone. This experiment was also designed to assess high-residue cultivation as a supplemental weed management tool. Barley and cereal rye were seeded in a replacement series, and a split-block design with four replications was used with management treatments as main plots and cover crop seeding ratio treatments (barley:cereal rye, 0:100, 50:50, and 100:0) as subplots. Management treatments included high-residue cultivation and standard no-till management without high-residue cultivation. Despite wider leaves in barley, mixing the species did not increase shading, and cereal rye dominated cover crop biomass in the 50:50 mixtures in 2013 and 2014, representing 82 and 93% of the biomass, respectively. Across all treatments, average weed biomass (primarily common ragweed, giant foxtail, and yellow foxtail) in late summer ranged from 0.5 to 1.1Mg ha^-1 in 2013 and 0.6 to 1.3Mg ha^-1 in 2014, and weed biomass tended to decrease as the proportion of cereal rye, and thus total cover crop biomass, increased. However, soybean population also decreased by 29,100 plants ha^-1 for every 1Mg ha^-1 increase in cover crop biomass in 2013 (P = 0.05). There was no relationship between cover crop biomass and soybean population in 2014 (P = 0.35). Soybean yield under no-till management averaged 2.9Mg ha^-1 in 2013 and 2.6 Mg ha^-1 in 2014 and was not affected by cover crop ratio or management treatment. Partial correlation analyses demonstrated that shading from cover crops prior to termination explained more variation in weed biomass than cover crop biomass. Our results indicate that cover crop management practices that enhance shading at slightly lower cover crop biomass levels might reduce the challenges associated with excessive biomass production without sacrificing weed suppression in organic no-till planted soybean.

Nomenclature: common ragweed, Ambrosia artemisiifolia L.; giant foxtail, Setaria faberi Herrm.; yellow foxtail, Setaria pumila (Poir.) Roemer & J. A. Schultes; barley, Hordeum vulgare L.; cereal rye, Secale cereale L.; soybean, Glycine max (L.) Merr.