Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.

Weed suppression is one possible benefit of including cover crops in croprotations. The late spring planting date of dry beans allows for more growthof cover crops in the spring. We assessed the influence of cover crops onweed dynamics in organic dry beans and weed seed persistence. Medium redclover, oilseed radish, and cereal rye were planted the year before drybeans; a no-cover-crop control was also included. After cover-cropincorporation, common lambsquarters, giant foxtail, and velvetleaf seedswere buried in the red clover, cereal rye, and no-cover control treatmentsand then retrieved 0, 1, 2, 4, 6, and 12 mo after cover-crop incorporation.Dry beans were planted in June and weed emergence and biomass measured.Eleven or more site-years of data were collected for each cover-croptreatment between 2011 and 2013, allowing for structural equation modeling(SEM), in addition to traditional analyses. Cereal rye residue increasedgiant foxtail and velvetleaf seed persistence by up to 12%; red cloverdecreased common lambsquarters seed persistence by 22% in 1 of 2 yr relativeto the no-cover-crop control. Oilseed radish and incorporated cereal ryerarely reduced weed densities. When red clover biomass exceeded 5 Mg ha−1, soil inorganic N was often higher (5 of 6 site-years),as were weed density and biomass (5 and 4 of 12 main site sample times,respectively). Using SEM, we identified one causal relationship betweencover-crop N content and weed biomass at the first flower stage (R1), asmediated through soil N at the time of dry bean planting and at the stagewith two fully expanded trifoliates. Increasing cover-crop C : N ratiosdirectly reduced weed biomass at R1, not mediated through changes in soil N.Cover crops that make a significant contribution to soil N may alsostimulate weed emergence and growth.