The importance of species selection in cover crop mixture design.

Cover crops are increasingly being included in crop rotations as a mechanism to promote diversity and provide agroecosystem services including weed suppression. Recently, cover crop mixtures have increased in popularity in an attempt to provide a greater diversity in ecological services as compared to monocultures. Several recent studies, however, have failed to detect a positive effect of cover crop diversity on biomass production or weed suppression. Here we assessed biomass productivity and weed suppression in 19 cover crops seeded as monocultures and 19 mixtures of varying species composition and functional richness (2- and 3-species mixtures) of full-season cover crops in Atlantic Canada. Cover crop biomass production and weed suppression varied by species identity, functional diversity, and species richness. As cover crop biomass increased regardless of diversity, weed biomass declined. Highly productive forbs and grasses provided the greatest weed suppression in monoculture. In line with previous observations, mixtures were on average not more productive nor weed suppressive than the most productive monocultures. We observed that the inclusion of the highly productive species buckwheat and sorghum-sudangrass in a mixture increased stand evenness, productivity, weed suppression and spatiotemporal stability. Taken together our results suggest that effects of diversity on mixture productivity and weed suppression are species specific. This further demonstrates the importance of species selection in cover crop mixture design.