Fall 2001
Calibrating manure spreaders
by Jeff Lorimor, Department of Agricultural and Biosystems Engineering
As manure application issues become more critical, it is important to know exactly how much manure you are applying, and that you are applying the manure uniformly. If you do not know how much manure you are spreading, you are probably not using the manure effectively. Research in northeastern Iowa with farmer producers showed that they often thought they were applying significantly less than they actually were.Calibrating your spreader is a simple, effective way to know that you are doing what you think you are when you pull onto a field. Whether you spread solid or liquid manure, you should calibrate your spreader.
One way to calibrate a solid manure spreader (20 percent or more solids) is the tarp method. All you need is a bucket, a 56-inch by 56-inch tarp, and a set of small scales. First, weigh the bucket and tarp to get a tare weight. Then spread the tarp on the ground and drive over, and next to, the tarp while spreading manure. Make three passes; the first centered exactly over the tarp then one on each side. Be sure the tarp is always within the spread pattern. Weigh the bucket, tarp, and manure, and subtract the tare weight. Because the 56-inch by 56-inch tarp is 1/2000th of an acre and a ton is 2000 pounds, the weight you read from the scale equals tons per acre. Repeat the procedure at least three times. You can change application rates by changing speeds and swath widths, but be careful about changing swath widths because that also changes the uniformity across the field. As manure gets wetter it gets heavier, so your application rates change with different manures.
Another way to calibrate either liquid or solid spreaders is to weigh and apply an entire load, or loads, either with portable scales (if you can find some) or by pulling across a commercial scale such as those at grain elevators. Measure the area covered, length ´ width (in feet), and multiply the two dimensions to get square feet. Divide by 43,560 to get acres. Divide the manure weight by the acres to get pounds per acre. To get gallons per acre for liquid manure, divide your pounds per acre by 8.34. To get tons per acre for either liquid or solid, divide your pounds per acre by 2000.
The other calibration concern is uniform distribution across the width of the spreader pattern. The distribution should be as uniform as possible, taking into account the overlaps of adjacent passes. To determine uniformity and optimum overlap of broadcast systems, place a line of small, equally spaced pans or trays across the spreader path. They should be fairly large and shallow. Cookie or cake pans, or cafeteria trays work well. The number needed depends on the spread width but plan on a minimum of five pans. Figure 1 shows results of placing 13 pans across the 60-foot spread width of a dry poultry manure spreader.
Injectors that place the manure below the ground surface can be calibrated for overall application by weighing as described above. Determining distribution is more difficult because the trays cannot be used very well. The best way to determine distribution with injectors is to place a bucket under each injector and run the applicator in one spot for several seconds. See whether every bucket has the same amount of liquid in it. This type of uniformity check is especially important when low application rates are being used. If your injector has a rotating valve or other mechanism in the distribution manifold to ensure uniform distribution, check it anyway. It should be OK, but calibrating is your insurance policy for getting a good application.
Calibrating always seems like a nuisance for applicators, but it can yield big benefits. It will either confirm that you are doing a good job, or you might discover a problem that you can fix to improve the job you are doing. Either way, you win by calibrating.
