Mulching Cover Crop Mixtures to Increase Weed Suppression, Soil Moisture and Grain Yield.

The objective of this study is to determine the effects of cover crop diversity and termination method on weed populations, soil microbial community structure, soil nutrient availability, soil water content and grain yield in a certified organic cropping system. A field experiment was conducted in 2009 and 2010 near Mead, NE. Spring-sown mixtures of 2, 4, 6 and 8 cover crop species were included in a sunflower – soybean – corn crop rotation. Cover crops were terminated in late-May using a field disk or sweep plow undercutter and summer annual crops were planted within one week. Cover crop and/or weed biomass, soil N and soil microbial communities were sampled three times throughout the season and soil water content was measured weekly to a depth of 8 cm. In 2009, cover crop biomass was greatest in the 6 cover crop mixture (318 g m^-2) and lowest in the 2 cover crop mixture (114 g m^-2).  Weed suppression and soil moisture were greater in the undercut compared to the disk termination treatment. Reduced levels of weed biomass in the undercut treatment may be due to physical interference with weed germination or an increased competitive advantage of the crop due to greater soil moisture. The lack of weed suppression in the disked treatment indicates a lack of phytotoxic activity in the soil. Soil N levels did not differ among treatments and the soil microbial community is currently being analyzed using the fatty acid methyl ester method. Late in the growing season broadleaf weed cover was greatest in the weed/cover crop free control treatment (28.7%) and lowest in the 8 cover crop mixture (20.0%), indicating increased weed suppression as diversity of the cover crop mixture increased. Crop yield was reduced in the disked treatment due to reduced levels of early-season soil moisture and weed suppression.