Calibrate manure spreaders to determine nutrient credits

Kevin Shelley, UW NPM Program

As much of Wisconsin continues to either thaw out, or dry out, now might be a good time for manure spreader weighing and calibration on dairy and livestock farms.  Determining the weight of an average load of manure is an important step in the process of determining valuable nutrient credits from a farm’s solid or semi-solid manure applications.  Accurate nutrient crediting depends on knowing both the application rate and the nutrient content of the manure.  Knowing how much manure is being applied from each load helps determine the application rate part of the equation.

Farmers can get a load weight by weighing a “representative” load of the solid or semi-solid manure they have on the farm.  This can always be done by driving a load to the nearest scale, such as at a grain elevator, feed mill or gravel pit.  If there are no stationery scales nearby, portable weigh pad scales can be brought to the farm.  Many county land and water conservation agencies or extension offices have scales, or have access to them through the University of Wisconsin Nutrient and Pest Management Program (NPM).  Farmers or farm consultants can check with their County Cooperative Extension agricultural agent to see who in the county has scales.

Once the average load weight is known, manure application rates in tons per-acre can be calculated by keeping track of the number of loads spread on a field or area of known acreage.  Knowing the application rate, together with the nutrient content of the manure, will allow calculation of the amount of N, P and K that can be credited toward crop needs as recommended by a soil test.

The Nutrient and Pest Management Program has produced an instructional video on determining manure application rates by weighing a spreader with portable scales.  The step by step process of weighing the tractor and spreader, both full and empty, is shown.  Manure spreader calibration is also discussed.  In the video, calibration is shown as measuring an area to which two or more loads have been spread, calculating the application rate in tons per-acre, and then making ground speed or discharge rate adjustments to achieve a specific application rate goal.

The NPM video, Calibrating a Manure Spreader, can be viewed on YouTube.


Educational publications and worksheets useful for spreader calibration can be accessed on the IPCM website at  Click on “publications” and then “nutrient management” to find publications such as:

Know How Much You haul

Manure Load Worksheet

Nutrient Management Fast Facts provides guidelines for estimating manure nutrient content