Fertilizer Value and Weed Seed Destruction Potential of Digested Manure

The number of anaerobic digester (AD) systems on dairy and swine farms in the U.S. has approximately doubled in the last 5 years, nearly all at larger feedlot operations. Although the odor reductions due to AD can be fairly obvious to the nose, there is a critical need to better understand some of the more subtle environmental and economic impacts of AD systems. A multi-year collaborative effort was begun three years ago to answer some of these questions, and will wrap up next year. The NRCS-funded project is led by the non-profit organization The Minnesota Project and utilizes the research and outreach expertise of the University of Minnesota. The project is composed of three components, focusing on dairy AD systems. The first, augmented with field trial research, compares soil quality and yield response for commercial fertilizer, stored digested manure, and un-digested manure. The second considers the destruction of weed seeds, a widely-claimed, but little studied, benefit of AD. The project tests viability of weed seeds suspended in an anaerobic digester. The final portion of the project considers the economic performance of AD systems, which will be considered in another paper. Results to date indicate the following key preliminary conclusions: (1) stored, digested manure can result in crop yields equivalent to undigested manure or fertilizer when applied at similar nitrogen rates, while simultaneously allowing the capture of bioenergy; and (2) the digestion process does not appear to significantly destroy weed seed viability, although germination times may be impacted.