Winter cover crops can provide several environmental benefits, including reduced soil erosion and enhanced soil organic matter and nutrient retention. They can also provide ecologically-based weed suppression by producing a soil-covering mulch into which a cash crop can be planted. Such cover crops compete with weeds during cover crop growth, and the mulch they provide can inhibit germination and establishment of weeds during the early phase of growth of the cash crop, potentially reducing both the density and competitiveness of the weed population. The weed suppressiveness of such mulch may be enhanced by maximizing cover crop biomass through earlier cover crop planting or later cover crop termination. However, the level of cover crop biomass required to achieve adequate weed control may depend on the size of the weed seedbank in the field as well as the species composition. We tested the weed suppressiveness of varying levels of a cereal rye cover crop mulch in no-till soybean managed with conventional and organic control strategies. The soybeans either received supplemental weed control (post-emergence herbicide or high-residue cultivation) or received none. We measured rye biomass, weed biomass, and soybean yield. Weed biomass was also collected in sub-plots supplemented with summer annual weed seeds. 
Results/Conclusions

Earlier rye planting and later rye termination produced greater rye biomass. Greater rye biomass was negatively correlated with weed biomass, indicating that a more weed suppressive mulch layer was formed. When weed seedbanks were supplemented, greater cover crop biomass was required to adequately suppress weeds without supplemental control. This finding suggests that cover crop mulches alone may provide sufficient weed suppression when weed seedbanks are relatively low, and that supplemental weed control may be required when weed seedbanks are high. Additional research will focus on how species composition and emergence timing impacts weed suppression with surface mulches.