Manure Application Guided By Grid Soil Sampling: Minnesota Case Studies.

Case studies evaluating the economic and environmental effects of using grid soil sampling compared to field-average sampling to guide manure application were prepared for eight farms in southern Minnesota.  The cases include liquid dairy and swine manure, and solid dairy, beef, and poultry manure. The grid phosphorus maps were used to delineate manure application and no-application zones within fields, with supplemental fertilizer applied where necessary. Nitrogen and phosphorus-based manure application rates were compared, following University of Minnesota (UM) nutrient rate guidelines. The economic comparisons were based on the fertilizer substitution value of manure nutrients, and were made using the UM spreadsheet “What’s Manure Worth?” MANURWKST.XLS, available at http://z.umn.edu/manureworth. For case studies where soil erosion was a likely risk, the Minnesota Phosphorus Index, http://www.mnpi.umn.edu/, with its included RUSLE2 erosion calculator was used to estimate potential loss of sediment and P in comparing manure application and tillage options. Results indicated that the fertilizer savings value obtained by targeting manure application with grid soil sampling more than paid back in the first year the increased cost of sampling on these farms that had a history of non-uniform manure distribution. That comparison did not include the expected yield gain from targeting manure to areas of the field that would have received less than optimal manure nutrients if using field-average soil test values. The case studies, an introduction, and associated video presentations are available at http://manure.umn.edu.  The case studies were presented in twelve interactive workshops with livestock producers, agricultural professionals, and agency staff across Minnesota in 2013.