Agronomy and Horticulture Department



Samuel E. Wortman

Date of this Version

Spring 4-6-2012


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Agronomy, Under the Supervision of Professor John L. Lindquist. Lincoln, Nebraska: May, 2012

Copyright (c) 2012 Sam E. Wortman


Organic grain cropping systems typically depend on intensive mechanical cultivation for weed control and manure or compost applications to meet plant nutrient demands. However, cover crops may contribute to weed suppression and soil fertility, potentially increasing crop yield and sustainability of the system. The utility of individual cover crop species have been well documented, but the agronomic benefits of diverse cover crop mixtures have received less attention. Cover crop mixtures are an appealing option for farmers, as increasing species diversity has been shown to increase resource-use efficiency, stability, resiliency, and productivity of plant communities. Despite the growing interest in cover crop mixtures, little is known about the effect of increasing cover crop diversity on cropping system performance. Moreover, organic farmers have questions about the most effective method for cover crop mixture termination.

In an effort to increase knowledge about cover crop mixtures and management for the western Corn Belt, an organic cropping systems trial was initiated in 2009 at the UNL ARDC near Mead, NE. Spring-sown mixtures of cover crops, ranging from two to eight species, were included in a sunflower – soybean – corn crop rotation. Cover crops were planted in late-March and terminated mechanically with either a field disk or sweep plow undercutter in late-May. Changes in cover crop mixture influenced cover crop productivity and early-season weed biomass, while termination method drove differences in weed community composition, soil microbial community structure, soil moisture and nitrogen, and crop yield. Interestingly, the management of ambient weed communities as a cover crop led to unique shifts in soil microbial community structure, but did not alter soil nitrogen or crop yield when compared to cover crop mixtures. When considering cropping system performance in combination with potential environmental benefits, diverse cover crop mixtures paired with a sweep plow undercutter for termination seems to be a profitable and sustainable management option for organic grain farmers in the western Corn Belt.