Environmental Conditions Outweigh Seeding Rates for Cover Crop Mixture Performance Across the Northeast US

Context or problem: Cover crop mixtures that include complementary species can increase resource use efficiency, total cover crop biomass, and agroecosystem benefits. In the northeastern US, farmers need information on how climatic, environmental and management factors influence the performance of various cover crop mixtures. The development of site-specific seeding rates may be necessary to optimize cover crop mixture services and increase farmer adoption. Objective or research question: The aim of this study was to characterize how site conditions influence mixture performance across the northeastern US, with total shoot biomass, species evenness (yield distribution of constituent species), and seed cost used as metrics of performance. Methods: A field experiment was implemented at seven research farms across a latitudinal gradient in the northeast US, spanning from Maryland to Maine. Monocultures and 12 bicultures were established at 0 %, 25 %, 50 %, 100 %, and 150 % of the recommended rate of that species in monoculture. Winter cover crops from three plant families were planted: cereal rye (grass; Secale cereale L.), hairy vetch (legume; Vicia villosa Roth), and forage rape (brassica; Brassica napus L.), which were selected for their differing functional traits and popularity among northeastern growers. Results: Classification and regression tree analysis showed that climate variables (spring growing degree days, hardiness zone) and soil conditions (soil nitrogen, pH, organic matter) were more influential on cover crop mixture performance than seeding rates of the constituent species. As soil inorganic nitrogen stocks increased, hairy vetch competitiveness and overall shoot biomass decreased compared to cereal rye or forage rape. Cereal rye dominated at sites with colder winters due to its winter hardiness compared to the other species. Forage rape shoot biomass was highly dependent on climate and performed poorly at colder sites. Conclusions: In order to maximize mixture performance, it is important to understand initial soil nitrogen levels if including legumes. Sites with milder winters had more flexibility in species selection and could use lower seeding rates compared to colder sites to produce high yielding, multi-functional mixtures at lower overall seed costs. In colder climates, it is important to include cereal rye to ensure productive mixtures that establish early and are winter hardy. Implications or significance: Understanding anticipated growing degree days in the cover crop season and baseline soil fertility is key when selecting species and seeding rates to ensure high performance of multi-functional mixtures.