Is “relatedness” a good predictor of suppression of weeds by cover crop mixtures?

Background/Question/Methods

Cover crops can be important components of diversified cropping systems; however, little is known about how particular cover crop species or mixtures of cover crops may affect weed population and community dynamics. Given that ecological theory suggests that diverse plant communities should be more resistant to weed invasion, and that competition between plant species should be strongest when species are closely related and/or have similar traits for resource acquisition and utilization, we assessed the ability of five spring-sown cover crops (grown in monoculture and mixture) to suppress weeds emerging from an ambient and “surrogate” weed seed bank. Specifically, we tested the following hypotheses: (1) cover crop mixtures are more weed suppressive than cover crop monocultures and (2) individual cover crops more strongly suppress weed species that are ecologically similar to the cover crop. The cover crop treatments included monocultures of buckwheat, mustard, cereal rye, sorghum-sudangrass, and hairy vetch, and two mixtures containing all five cover crop species sown at full rate (Mix1X) or at 20 percent of the full rate (Mix20). The “surrogate weeds” were crop species in the same (spring wheat, sorrel, canola, and field pea) or different (sunflower) family as the cover crops. 

Results/Conclusions

Cover crop treatments were established in early June 2011, surrogate weeds were sown three days later, and weed and cover crop biomass were sampled in late July. The Mix1X and buckwheat monoculture treatments both reduced surrogate and ambient weed biomass by more than 50 percent compared to the no cover crop control (fallow); however, weed biomass in the Mix20 treatment did not differ from the control. Surrogate weed species were not more strongly suppressed by cover crops that belonged to the same family as the “weed”. These data suggest that in regards to the five spring-sown cover crop species assessed in this study, (1) buckwheat is the most weed suppressive, (2) mixtures containing all five species are not more weed suppressive than the most suppressive monoculture (i.e., buckwheat), and (3) weed suppressive ability is not well linked to the relatedness of the “weed” to the cover crop, at the family level.