Cover crop biomass production is more important than diversity for weed suppression

Biotic resistance theory suggests that diverse cover crop mixes may be more effective at weed suppression than a cover crop monoculture. However, evidence for this has so far been inconsistent. To investigate, we designed a trial to explicitly test the role of cover crop diversity in weed suppression, through comparing eight cover crop mixes that varied in species diversity, functional diversity, and composition. Mixes contained either one, four or eight species, in equal proportions. Three mixes contained only cereal species, three contained only legumes, and two contained a mix of cereals, legumes and brassicas. Research was conducted on two farms in South Africa’s winter rainfall region, replicated over two years. Indicators of resource uptake by each mix in terms of light, soil nitrogen and water were measured at three time points throughout the season, approx. 50, 85 and 110 days after establishment (DAE). Aboveground biomass (dry weight) of cover crops and weeds within each mix was measured twice, at approximately 70 and 120 DAE. Regression analyses indicated that cover crop biomass was key to resource uptake and weed suppression, and that early-season nitrogen and later-season light availability had the strongest influence on weed biomass. Neither species diversity nor functional diversity affected resource uptake or weed suppression by cover crops. These results indicate that it is important to consider the competitiveness of individual species when designing cover crop mixes. Diverse mixes remain valuable to perform multiple functions, but may contribute to weed problems if composed of poorly competitive species.