Environmental conditions outweigh seeding rates for cover crop mixture performance across the Northeast US

Authors: BONIFACE, HELEN - University Of Maryland; Mirsky, Steven; RYAN, MATTHEW - Cornell University; SMITH, RICHARD - University Of New Hampshire; ACKROYD, VICTORIA - University Of Maryland; Bybee-Finley, Kristine; DARBY, HEATHER - University Of Vermont; DUIKER, SJOERD - Pennsylvania State University; HASHEMI, MASOUD - University Of Massachusetts, Amherst; HIRSH, SARAH - University Of Maryland

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2024
Publication Date: 9/7/2024
Citation: Boniface, H., Mirsky, S.B., Ryan, M.R., Smith, R.G., Ackroyd, V.J., Bybee-Finley, K.A., Darby, H.M., Duiker, S.W., Hashemi, M., Hirsh, S.M. 2024. Environmental conditions outweigh seeding rates for cover crop mixture performance across the Northeast US. Ecological Applications. Article 109564. https://doi.org/10.1016/j.fcr.2024.109564.
DOI: https://doi.org/10.1016/j.fcr.2024.109564

Interpretive Summary: Cover crops in a mixture can provide complementary benefits, as in a leguminous vetch contributing nitrogen to a system while its rye mixture counterpart protects against fall soil erosion. However, there is a need to fine-tune seeding rate recommendations for species in a mix to account for interactions among climate, the environment, and management and thus maximize the benefits a mix provides and encourage cover crop use. This study characterized how site conditions affected the performance of a grass/legume/brassica (rye/vetch/rapeseed) cover crop mix in the Northeast US using metrics including total biomass, species evenness, and seed cost. 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 rate proportions between the constituent species. Growers had more flexibility in species selection and could use lower seeding rates in temperate than colder climates to produce high yielding, multi-functional mixtures. Cereal rye dominated at sites with colder winter seasons while forage rape biomass was highly dependent on climate and performed poorly at colder sites. Evenness among mixtures that included hairy vetch was primarily influenced by baseline soil nitrogen levels. This study benefits farmers by providing concrete guidelines for cover crop mixture species and seeding rate selections, which in turn supports the sustainability of US agriculture.

Technical Abstract: Cover crop mixtures that include complementary species can increase resource use efficiency, total cover crop biomass, and agroecosystem benefits. However, current seeding rate recommendations may limit the extent of potential benefits due to the influence of climatic, environmental, and management conditions. The development of site-specific seeding rates may be necessary to optimize cover crop mixture services and increase farmer adoption. The aim of this study was to characterize how site conditions influence mixture performance across the northeastern US, with total biomass, species evenness (yield distribution of constituent species), and seed cost used as metrics of performance. A field experiment was implemented at seven research farms across a latitudinal gradient in the northeast US, spanning from Maryland to Maine. Monocultures and 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. 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 rate proportions between the constituent species. Further, growers had more flexibility in species selection and could use lower seeding rates in temperate than colder climates to produce high yielding, multi-functional mixtures. Cereal rye dominated at sites with colder winter seasons due to its winter hardiness compared to the other species. Evenness among mixtures that included hairy vetch was primarily influenced by baseline soil nitrogen levels, and slightly less influenced by spring growing degree days. As expected, forage rape biomass was highly dependent on climate and performed poorly at colder sites; this study demonstrates the feasible geographical range of this species as a winter cover crop. Results suggest that anticipated growing degree days in the cover crop season and baseline soil fertility should be considered before selecting species and seeding rate proportions for multi-functional mixtures.