The technical standard in the permit emphasizes that a field-by-field assessment must be completed. The following requirements must be met and documented:
- The Natural Resources Conservation Service’s Manure Application Risk Index (MARI) has been completed to identify fields, or portions of fields, that scored 37 or lower on MARI (explained below).
- An on-site field inspection of the entire field (or portion) that scored 37 or lower under the MARI has been completed. Slope and location of surface waters, tile line risers and other conduits to surface water will be taken into consideration.
- Based on field inspection, the Comprehensive Nutrient Management Plan (CNMP) will include documentation on topographic maps, the fields (or portions) where the runoff will not flow to surface waters, and designate these areas as the only areas authorized for winter surface application.
- The findings of the inspection and documentation in the CNMP will be approved by a certified CNMP provider.
Manure Application Risk Index (MARI)
“A Procedure for Determining the Land Available for Winter Spreading of Manure in Michigan” is a great resource for detailed information on using the Manure Application Risk Index (MARI). MARI is a tool that evaluates fields (or portions of fields) and identifies areas of concern when winter spreading may cause increased risk of runoff. MARI uses twelve specific field features to reach an overall “rating” for each site. The features included in the MARI evaluation are: soil hydrologic and management groups, percent slope, soil test phosphorous value, concentrated water flow, nitrogen leaching index, percent residue or cover crops, surface water setback, vegetative buffer width, manure P and N application rates, and application method. Fields that are rated as “Very Low (MARI score of <19)” and “Low (MARI score of 19-37)” have a reasonably good potential for winter spreading. Michigan farmers looking for assistance in identifying fields for winter spreading can contact their local Michigan Agriculture Environmental Assurance Program technician or NRCS conservation technician.