Calibrating a Manure Spreader

Manure spreaders have been used for many years to apply manure to agricultural fields. With the increased emphasis water quality and nutrients in runoff from non-point sources it is important to know how much manure a spreader is applying over a given area. There is a good discussion about calibrating manure spreaders in the Natural Resources Conservation Service (NRCS) Agricultural Waste Management Handbook, Chapter 13, and appendix 13a. The appendix explains both the "load-area" and the "weight-area" calibration techniques. It also has worksheets for figuring Spreader Capacity, Load-Area Calibration, Weight-Area Calibration, and Uniformity Testing.

Remember that calibrating a manure spreader is only part of the process. The largest variable in waste management application may be the nutrient content of the manure. It is difficult to obtain a true representative sample for nutrient analysis from a large pile of manure. The assumption that the nutrient content is uniform throughout the manure pile is probably not valid anyway. With a large potential variable in nutrient content, the calibration of the manure spreader does not have to be very precise. A rough estimate of tons per acre applied for a given setting and speed is usually all you need in practice.

A "quick and dirty" way to calibrate a manure spreader is as follows: First, determine the spreader capacity in cubic feet. If manure is commonly heaped in the spreader, then take this into consideration in measuring or estimating the capacity. Second, convert the cubic feet per loaded spreader into tons per loaded spreader. Moist manure with bedding will weigh about 55 to 60 pounds (lbs) per cubic foot. When the manure is wet enough to be a semi-solid or thick slurry then use 62 lbs. per cubic foot. Remember that it is easy to weigh a few buckets of manure to get a better estimate. Third, empty the spreader at a given speed and setting and measure the area covered by manure. This will give you a weight factor (pounds) over an area factor (square feet) which can easily be converted to tons per acre.

While calibrating the spreader, consider also the uniformity of application. Using the "weight-area method" (describes in appendix 13e in the NRCS Agricultural Waste Management Handbook) with a light colored cloth (like a common canvas drop cloth used by house painters) is a good way to visually check uniformity. It is possible to take pictures of the cloth after an application of the manure and process the image through a computer to get a measure of the uniformity, but a simple visual check will serve most practical purposes. If possible, adjust the manure spreader to apply manure uniformity across the application swath. If this can not be done, the operator can compensate by making sure the spreader is not always going down the same paths in the field, or by crossing the fields in different directions.

Demonstrating the calibration of a manure spreader is a good thing to do at farm shows and field days. It gets attention, and you can take advantage of this to talk about waste and nutrient management in general.

For more information contact:
David Moffitt at (817) 509-3315 david.moffitt@ftw.usda.gov

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