Using cover crops in our never ending battle against weeds

Cover crops provide a variety of agroecosystem services such as improvements to soil health, water holding capacity and reduced soil erosion. Recently, cover crops are being promoted for their use in weed management through suppression of weed abundance, biomass and seed shed. An ongoing debate continues, on the relative importance of cover crop biomass versus diversity for weed suppression. The majority of studies, have found little evidence to support the notion increased diversity provides increased weed suppression. To investigate this hypothesis, we evaluated 20 cover crop species grown as monocultures and 19 cover crop mixtures for their suitability under Atlantic Canadian conditions and their weed suppression potential. Overall, we found that grass, brassica and forb cover crop species grown in monoculture produced greater biomass than legumes. While cover crop biomass was on average greater in mixtures than monocultures, several species performed significantly better as monocultures. Buckwheat consistently produced greater biomass alone when compared to mixtures containing buckwheat. In contrast, brown mustard was more productive in functionally diverse mixtures than as a monoculture or a brassica-only mixture. Across years, however, mixtures had greater spatiotemporal stability in biomass production than monocultures. In terms of weed suppression, increasing cover crop biomass resulted in declines in weed biomass up to > 800 g m-2 at which point weed biomass tended to increase. Despite increasing cover crop richness resulting in greater weed suppression, greater cover crop diversity had the opposing effect. Of all cover crops and mixtures evaluated, buckwheat and sorghum-sudangrass consistently suppressed greater weed biomass in monoculture than in mixtures containing either species. The addition of buckwheat, however, to a cover crop mixture provided an additive effect on weed suppression regardless of mixture identity. Therefore, while cover crop mixtures had greater spatiotemporal stability in biomass production, we did not find evidence for enhanced weed suppression. If weed management is the primary goal, a monoculture may provide more consistent weed suppression. Our results suggest, the addition of buckwheat to a mixture will increase weed suppression and may buffer against reduced weed suppression observed in mixtures.