Invariably when I talk about manure spreader calibration there are a few chuckles. The image of a manure spreader doesn’t call to mind a piece of equipment that needs calibration; it is the equivalent of a hammer in a carpenter’s toolbox. No calibration or explanation needed — you just use it. However, as nutrient management and its corresponding linkage to water quality continue to grow in importance, all livestock owners and anyone who hauls and applies manure needs to become more aware of managing manure as a source of nutrients. When nutrients are purchased as commercial or synthetic fertilizer, we talk about an application rate in pounds of a particular nutrient that should be applied per acre. The desired application rate is achieved by using a calibrated fertilizer spreader. We can do the same with our manure spreaders.
The application rate for manure spreaders is generally expressed as tons per acre or gallons per acre. Two common calibration methods to determine manure application rate are the swath or load-area method and the tarp or weight-area method. The swath method involves measuring the amount of manure in a typical spreader load and then measuring the land area covered by applying this load. This method is often used to calibrate liquid manure spreaders. The tarp method involves laying out several tarps, running a manure spreader over them and then calculating the amount of manure applied per acre. This method works well for solid manure. Let’s now examine each of these calibration methods in a little more detail.
To use the swath or load-area method, for liquid manure spreader calibration, fill the manure spreader to a typical load level. The biggest question that must be answered is: how many gallons of manure are in the spreader? The manufacturer’s capacity rating can be used, but to what fill level does that capacity refer? Often the spreader may be filled to a different level. In other cases the manufacturer’s capacity rating may not be known. If there is doubt about the spreader capacity it can be calculated by some simple math. The volume for a round tank spreader is determined by the following formula: tank length x tank diameter x tank diameter x 0.8. For a noncircular tank spreader the volume formula is: tank length x width x depth x 0.8. Using these formulas, the volume will be in cubic feet.
To convert to gallons multiply the cubic feet figure by 7.48. After the spreader is filled apply the load to a field using a typical tractor and spreader settings. If the area to be covered is not long enough for a single pass make sure to apply with typical overlaps. Next, determine the area covered in square feet by measuring the length and width of the application and multiplying those figures. The square feet covered divided by 43,560 will give you the acres or fraction of an acre covered by the spreader load. The application rate is then the spreader capacity divided by the acres covered, resulting in gallons per acre.
To use the tarp or weight-area method, get three to four tarps or pieces of plastic of equal size. Plastic that was used to cover the bunker silo can work well for this. I like to use plastic cut to six feet by six feet, but almost any size can work. Weigh the plastic or tarps and get an average weight. Lay out the plastic or tarps in the field and stake them down so the wind will not blow them around. Load the manure spreader with a typical load of manure and then drive over the plastic at the tractor and spreader settings that are typically used. Gather the tarps and weigh them with the manure. Subtract the empty tarp weight from this value to get the weight of the manure. Divide the manure weight by the tarp area in square feet. Multiply that value by 21.8 to get a tons per acre figure. The 21.8 figure is the conversion of pounds per square foot to tons per acre derived from 43,560 square feet per acre divided by 2,000 pounds per ton.
For example, let’s say I used six-foot-by-six-foot pieces of plastic that averaged three pounds. The square foot area of each piece is 36. After laying out the plastic in the field and running the spreader over it, I get an average weight of 28 pounds. Subtracting the empty plastic weight gives me a figure of 25 pounds of manure over 36 square feet or 0.69 pounds per square foot. Multiplying 0.69 x 21.8 gives a result of 15 tons of manure per acre as my application rate.
Spreader calibration is an important piece of managing manure as a source of nutrients. It provides the operator with an application rate. The next step is to determine if that spreader application rate is too high, too low or just right. To answer that question depends upon a manure nutrient analysis, current soil test levels and crop nutrient removal rates, each a topic for another column.