Using a Spatially Explicit Approach to Assess the Contribution of Livestock Manure to Minnesota’s Agricultural Nitrogen Budget

Abstract

The size and density of concentrated animal feeding operations have grown significantly over the past twenty-five years, raising concern over the ability of the surrounding landscape to sustainably handle the byproducts of animal agriculture. A novel geographic information system program was developed to spatially model the application of manure nutrients to proximal agricultural fields. Nutrient losses during storage and field application were accounted for to determine the amount of manure sourced nitrogen available annually for land application. By-field nitrogen requirements were estimated using six-year crop rotations and commonly used guidelines on fertilizer recommendations for agronomic crops. Three different nitrogen fertilizer recommendation approaches, ranging from economically optimized rates on the low end to yield goal-driven rates on the high end, were modeled to gauge the sensitivity of the analysis approach to varying nitrogen application rates. For each fertilizer N rate, three manure haul distance scenarios were modeled, allowing for manure travel distance to be capped at distances unique to each livestock type. Lastly, commercial nitrogen fertilizer sales data were combined with manure sourced nitrogen estimates to assess statewide agricultural nitrogen application. Results indicated minimal (<5%) over-application from manure alone when applied at recommended rates and using the haul distances specified. However, regardless of which application rate guidelines were used, combined manure and commercial fertilizer nitrogen exceeded statewide crop requirements (110%–155%). This suggests that significant application of nitrogen above recommended rates is likely occurring. Information on commercial fertilizer application at the field level is sparse, precluding greater understanding of the relative contribution of manure and commercial sources. Despite this knowledge gap, additional focus should be placed on cumulative nitrogen application in areas with dense animal concentrations. Adequate crediting of all nitrogen sources, including the recognition of manure as a valuable fertilizer resource, presents the opportunity for substantial producer cost savings and potential widespread reduction in the contamination of water resources.