Calibration of manure application equipment is a critical part of routine farm management as manure management is being examined closely across the country. Knowing and managing how much manure along with its nutrients is being applied to a particular field is paramount for several reasons:

1. Manure spreader calibration and the application records that depend on it are required of all dairy and livestock farms.

2. The inadvertent or advertent over-application of manure is considered a primary factor contributing to the risk of discharge of manure associated with field application into surface waters via surface runoff or field tile drainage.

3. Manure is a valuable source of nutrients for crop production; however, just like commercial fertilizer, manure needs to be applied and credited accurately and uniformly based on crop needs.

Although all three reasons for calibrating manure application equipment potentially can affect a farm’s profitability, the ability to confidently credit the fertilizer value of manure is the one reason that is absolutely certain to have a financial impact year after year.

Once a field has been identified as a potential recipient of manure, a desired application rate needs to be calculated based on a recent soil test (within the last three years), realistic yield goals and the nutrient content of the manure as determined by laboratory analysis.

Many crop consultants are proficient at recommending manure application rates, but accurately applying manure at the desired rates has, at times, been a guessing game. The resulting inaccurate application rates can result in inadequate nutrient levels available for maximum crop yields. More commonly, however, manure is applied in excess to insure adequate nutrients for crops, but the risk of build-up of soil phosphorus levels, direct discharge to surface water and contamination of ground water are increased under these conditions.

Not only are there potential environmental costs associated with inaccurate application of manure, but the over- or under-application of manure results in inefficient use of nutrients. Under-application results in compromised crop yields, whereas over-application has an opportunity cost because the excess nutrients could have reduced purchased fertilizer costs in the cropping system.

Simple calibration method
Several manure application equipment calibration methods are accurate when practiced correctly. Unfortunately, many of the methods are inconvenient because of the physical and labor resources required. Therefore, few farmers actually conduct routine calibrations.

Although a farmer should utilize the most comfortable calibration method(s) with which he or she is most comfortable, the following is a method requiring only a matter of minutes using equipment available on every farm. No method provides perfect accuracy, but calibration can reduce application rate errors from as high as 100 to 200 percent down to 10 to 20 percent.

The ground speed of the application equipment is often the ultimate determinate of application rate. In fact, the transmission ratio of the tractor is commonly the limiting factor in accurately matching desired application rates. Utilizing a ground speed application rate chart based on actual available ground speeds for a particular tractor or truck is often the most practical option for managing application rates.

The following simple formula can be used to easily develop a user-friendly chart:

Rate per acre=C ÷ W ÷ T ÷ S x 29,700

C = capacity of spreader (tons or gallons minus unused volume)
W = width of a pass with applicator (feet, considering overlap)
T = time required to empty spreader at the selected PTO RPM (seconds)
S = ground speed of equipment in a particular gear and the selected PTO RPM (miles per hour)

A completed chart can easily be developed and used by the equipment operator to achieve a desired application rate with reasonable accuracy.

Verify actual application rate
Routinely measuring actual application rates to verify the accuracy of the predicted rate is equally important. Variables such as manure consistency, wind and operator differences make this essential. The following formula can be used to quickly calculate the actual rate in field:

Rate per acre=C ÷ W ÷ L x 43,560

C = capacity of spreader (tons or gallons minus unused volume)
W = width of a pass with applicator (feet, considering overlap)
L = length of pass to empty spreader (feet)

Alternatively, the number of gallons applied to a known number of acres also would serve as a check of calibration accuracy.

Determining spreader capacity
Knowing the actual capacity of manure application equipment in gallons or tons is critical to accurately calibrate and record field application rates. The following are some relatively simple methods for determining spreader capacity for calibration purposes:

Liquid tankers
The gallon capacity of liquid manure tankers is generally known; however, it is important to recognize that there will be unused capacity. Filling a tanker all the way to the very top greatly increases the risk of manure splashing out during transport and spilling onto roads and driveways.

Open liquid spreaders
The gallon capacity of open spreaders such as box or V-bottom spreaders may be listed in the operator’s manual; however, most must be calculated. Capacity in gallons can be determined with a simple tape measure and the conversion factor of 7.48 gallons per cubic foot.

Solid manure
For solid manure, capacity is expressed in tons. Ideally, a scale, fixed or portable, can be used to measure the actual weight of the spreader with and without its load for each specific type of manure. The difference would be the capacity. If a scale is not conveniently available, an alternative method, based on the volume of the spreader and the manure’s density can be used.

The density (pounds per gallon) of the manure being measured can be used to convert gallons into tons. A small scale such as a spring scale can be used to measure the net weight in pounds of a 5-gallon bucket of manure. When weighing a bucket of manure, attempt to duplicate the density (packing) of the manure in the actual spreader. Divide net weight of the bucket of manure by the volume (i.e., 5 gallons) to get density (pounds per gallon). The solid capacity of the spreader in tons can then be calculated.

Summary
Manure application equipment calibration has become so important to properly manage manure nutrients that it should be routine for every dairy and livestock farm. The small investment in developing simple tools such as ground speed application rate charts will be recovered readily in improved accuracy of nutrient crediting in the cropping system and reduced environmental risk. As with any tool, calibration is only valuable when put into practice. ANM

References omitted but are available upon request at editor@progressivedairy.com

—Excerpts from Michigan Dairy Review