Cover Crop Mixture Diversity and Weed Suppression.

The diversity-invasibility hypothesis proposes that increasing plant diversity increases invasion resistance. The competition relatedness hypothesis proposes that like species compete with each other more than unlike species. The goal of this study was to test both of these hypotheses in the context of cover crops and weed suppression. Specifically, the objectives were (1) to evaluate the effect of cover crop mixture diversity—as measured by species richness—on weed suppression and (2) to evaluate the effect of cover crop type on weed suppression by weed type—specifically, to determine whether grass cover crops are better at suppressing grass weeds than broadleaf weeds and vice versa. Twenty to forty treatments were planted at three sites in southeastern Nebraska reflecting varying levels of species richness. Six grass species—barley, oats, wheat, proso millet, sorghum sudangrass, and teff—and twelve broadleaf species—Austrian winter pea, red clover, yellow blossom sweetclover, radish, rapeseed, turnip, chickpea, cowpea, sunn hemp, buckwheat, safflower, and sunflower—were used in this study. Cover crop planting dates ranged from July 20 to August 31. Cover crop and weed species specific biomass measurements were taken approximately two months after planting. While weed suppression was correlated to species richness, this was a result of cover crop species richness covarying with cover crop biomass productivity. Once the positive effect of cover crop biomass on weed suppression was controlled for, there was no observable effect of cover crop species richness on weed suppression. Similarly, there was no observable effect of cover crop type on weed suppression by weed type. The effect of a cover crop on weed suppression was overwhelmingly a function of cover crop biomass rather than of cover crop diversity or type.