From a manure management standpoint, when and where manure is applied can have a large impact on the risk to surface water quality. In the Michigan State University Extension News article "What is your manure strategy?" three questions were answered: 1) what is winter manure spreading? 2) what are the risks associated with winter spreading? and 3) what are winter manure management strategies? Reducing the environmental risks associated with winter manure spreading requires access to information and planning tools. The following resources and tools are available to help livestock farmers evaluate, on a field-by-field basis, the risk of winter spreading.
Michigan Right to Farm Generally Accepted Agriculture Management Practices – Manure Nutrient Utilization
Regardless of the size of the livestock operation, producers in Michigan should understand and follow the guidelines provided by the Michigan Right to Farm Generally Accepted Agriculture Management Practices (GAAMPs) for Manure Management and Utilization. "Section V – Manure Application to Land", outlines specific practices for winter application of manure in Michigan. The GAAMPs state that application of manure on frozen or snow-covered soils should be avoided but when necessary solid manures should be applied to fields with slopes of 6 percent or less; and liquid manures should be applied to fields that have a slope of 3 percent or less. The GAAMPs emphasize that in fields where winter manure will be applied conservation practices (such as vegetative buffer strips) should be utilized to control runoff and erosion to surface waters. Livestock producers should pay particular attention to field slopes and manure application rates. The GAAMPs also recommend maintaining a 150 foot buffer from surface water inlets and areas of concentrated flow. Utilizing a field-by-field assessment such as the Manure Application Risk Index (MARI) to evaluate fields acceptable for winter spreading is also recommended.
Concentrated Animal Feeding Operation (CAFO) regulations
Large livestock farms that fall into the category of Concentrated Animal Feeding Operations (CAFOs) are required by the National Pollutant Discharge Elimination System (NPDES) general permit to follow specific guidelines in order to spread manure in the winter. CAFOs are required to have storage facilities large enough to hold six months of waste. For the most part, this gives CAFOs the opportunity to avoid winter manure spreading. In times when winter spreading is a necessity, farms must complete a field-by-field assessment. The purpose of the assessment is to identify fields, or portions of fields, that could be used for surface application of CAFO waste without incorporation to frozen or snow-covered ground demonstrating that the application will not result in runoff entering waters of the state. This has to be done in accordance to the "Department 2005 Technical Standard for the Surface Application of CAFO Waste on Frozen or Snow-Covered Ground without Incorporation or Injection". This standard is found on the last page of the permit. The permit also states that "CAFO waste surface applied to ground that is frozen or snow-covered shall be limited to no more than one crop year of phosphorous per winter season, including pastures, forages crops such as alfalfa, wheat stubble or where no-till practices are used."
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.
Source: Michigan State University Extension