Manure is a valuable resource, and farmers incorporate it as part of their farm’s nutrient management plan. Manure spreader calibration is an essential and valuable nutrient management tool that helps maximize the efficient use of available manure nutrients.
One method of determining how many tons per acre are being applied is the tarp method. The tarp method involves several tarps of equal, predetermined size; over which manure will be spread. By averaging the manure weights collected from each tarp, you can calculate the average tons per acre spread.
The tarp method requires the following tools: three or more tarps of equal size, a scale, bucket, nails and a measuring tape.
First, measure the tarps to determine the square footage of each tarp. Tarps that are cut to even measurements make calculations easier.
Although the calibration can be done with one tarp, a far more accurate calibration can be achieved by using at least three tarps.
Weigh each tarp and bucket prior to placing the tarps in the field. Nails or tent pins can be used to anchor the tarps to the ground.
Because the area of the tarps is known, the weight of the manure spread per unit area can be calculated.
Position the tarp in a field where manure can be spread. Place the tarp or tarps far enough into the field to allow ample distance for the tractor to be moving at the desired speed and the spreader in the desired gear. Avoid placing the tarp where the beginning or the end of the load will fall — the goal is to get a good representative sample.
Prior to spreading manure for calibration, prepare to make a note of tractor gear, throttle setting, tractor speed, PTO speed and any settings on the spreader so that you can make any necessary adjustments.
Operate the spreader at the speed that would normally be driven for manure application. After the tractor and spreader make a pass, weigh each of the tarps and their manure content. If the manure is dry, you can sweep the manure toward the center and dump it into a bucket for weighing. If the manure is somewhat wet, collect the entire tarp and its manure contents for weighing. Weigh the bucket, and subtract the weight of the bucket or the tarp and bucket combination to determine the weight of manure collected on each tarp.
For example, if the tarps measure 8’ x 8’, each tarp is 64 square feet in size. After making a pass with the spreader and collecting the manure, Tarp A weighs 10 pounds, Tarp B weighs 14 pounds and Tarp C weighs 12 pounds.
After determining manure weights on each tarp, calculate pounds of manure per square foot by dividing the weight of the manure on the tarp by the square footage of the tarp. In this example, Tarp A has 0.16 pounds per square foot, Tarp B has 0.22 and Tarp C has 0.19. Determine the average weight per square foot of the three tarps. The average weight is .19 pounds of manure spread per square foot of tarp.
Once the average amount of manure being applied is determined, calculate the manure application rate. The number 22 is a constant that relates acres to tons. Multiply the average weight of 0.19 by 22 to determine the application rate expressed in tons per acre. In this example, the application rate is 4.2 tons per acre.
Additional factors such as spread pattern also play a role in manure distribution. After spreading, manure should be incorporated to minimize nutrient loss. Because the characteristics of manure vary throughout the year, it’s important to calibrate the spreader to account for these changes.
Contact your local county extension office or nutrient management office for worksheets that help with spreader calibration.