Diverse Cover Crop Mixtures in Organic Grain Crops: Ecology and Management for Winter-Annual Weed Suppression.

Multi-species cover crop mixtures, or “cocktails,” are an increasingly popular agronomic management tool. Cover crop cocktails (CCCs) have the potential to suppress winter-annual weeds, but little is known about the temporal dynamics of CCC weed suppression and the role of cover crop diversity therein. We hypothesize that both species and functional diversity enhance weed suppression by increasing cover crop competition with weeds at critical developmental stages. We investigated CCCs planted after corn (Zea mays) and after wheat (Triticum aestivum) in a rotation with soybean (Glycine max). 12 cover crop treatments ranging from zero to seven species were planted in late August (following wheat) and mid-October (following corn) of 2012 and 2013. We took repeated measures of canopy light attenuation and percent ground cover of cover crops and weeds between fall planting and spring termination. Paired plots were destructively sampled and sorted to species before winter dormancy and spring termination to quantify weed pressure.  Preliminary results for 2012-2013 indicate that CCCs with vigorous early growth can effectively suppress winter annual weeds, regardless of the mixture’s diversity and winter survival. Mean spring weed biomass in the five multi-species mixtures was minimal: 18.0 kg/ha following wheat and 81.3 kg/ha following corn. Cover crop percent ground cover at 39-44 days after planting (DAP) explains half of the variation in spring weed biomass at termination. Moreover, winter-killed cover crops that established canopies quickly were equally as weed-suppressive as winter-hardy cover crops. Cover crop species richness explained only 12.6% of the variation in spring weed biomass in the after-wheat window and none of the variation in the after-corn window. Partial least squares path modeling confirms the importance of vigorous early growth.