Using a spatially explicit approach to assess the contribution of livestock manure to Minnesota’s agricultural nitrogen budget

Authors: PORTER, SARAH - Environmental Working Group; James, David

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2020
Publication Date: 4/1/2020
Publication URL: https://handle.nal.usda.gov/10113/6934610
Citation: Porter, S.A., James, D.E. 2020. Using a spatially explicit approach to assess the contribution of livestock manure to Minnesota’s agricultural nitrogen budget. Agronomy. 10(4):480. https://doi.org/10.3390/agronomy10040480.
DOI: https://doi.org/10.3390/agronomy10040480

Interpretive Summary: The size and density of agricultural livestock operations has grown significantly over the past twenty-five years, raising concern over the environmental problems associated with the byproducts of animal agriculture, such as contamination of lake and streams. A computer mapping process was developed to estimate where and how much livestock manure would likely be applied to nearby agricultural fields. After accounting for losses, animal manure retains a significant amount of nitrogen that could be used to replace commercial fertilizer on farms, resulting in cost reductions for the farmer. We examined crop rotations by field and estimated the required nitrogen fertilizer based on commonly used fertilizer guidelines for the various crops. Because nitrogen fertilizer rates may vary by farmers, we chose to use three different fertilizer rates, ranging from economically optimized rates on the low end to yield goal driven rates on the high end. Trucking manure is expensive, so manure haul distance was capped at distances based on livestock and manure type. By capping the haul distance, there were places where manure had to be over-applied. Lastly, commercial nitrogen fertilizer sales were combined with estimated manure nitrogen to examine statewide nitrogen fertilizer used for agriculture. Regardless of which the three application rate guidelines were used, total nitrogen applied exceeded statewide crop nitrogen requirements. This suggests that farmers are not replacing commercial fertilizer with manure but applying more nitrogen than needed by crops. Over application of nitrogen fertilizer may lead to unnecessary producer fertilizer costs, and potentially, widespread environmental contamination of water resources. The approach introduced in this paper can provide the geographic analysis necessary for understanding the dynamics of using manure as a fertilizer sourced at a scale appropriate to influence future regulatory and permitting guidelines. The research informs farmers, advisors and regulatory agencies concerned with effective manure and nutrient management.

Technical Abstract: The size and density of concentrated animal feeding operations has 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 gage the sensitivity of the landscape to varying nitrogen application rates. Manure haul distance was capped at distances unique to each livestock type, forcing manure to be over-applied in areas where manure remained after traveling the maximum distance allotted. Lastly, commercial nitrogen fertilizer sales were combined with manure sourced nitrogen to estimate statewide nitrogen application. Regardless of which application rate guidelines were modeled, total nitrogen applied exceeded statewide crop requirements, suggesting that recommended manure nitrogen credits are not being included in manure management plans and widespread environmental contamination of water resources from nitrogen over-application is likely occurring.