Calibrating Poultry Litter Spreader Trucks

Traditionally, poultry litter application rates based on truckloads per field have been considered acceptable. Today, there are increasing environmental concerns regarding soil test phosphorus levels and the potential impact on water quality. As a result of these concerns, it is generally recommended, and in some cases a permit requirement, that each poultry farm develop a comprehensive nutrient management plan. These plans document the volume of litter produced and managed on each farm. The documentation includes estimates of the nitrogen, phosphorus and potassium content of the litter, and calculations of the appropriate application rates to meet the farm’s pasture and hay fertility needs while at the same time, protecting water quality. In these nutrient management plans, the application rates are expressed as tons of litter per acre. For more information on soil test phosphorus concerns and comprehensive nutrient management plans, contact your local county Extension office.

To identify how many tons of litter are being applied, litter application equipment should be calibrated for specific equipment settings and operating conditions. With proper calibration, controlled applications can be made to meet crop needs, protect the environment, reduce litter wastage and possibly provide cash revenue from the sale of unused litter.

Several factors need to be considered when applying litter. The primary factors are the average application rate and the application uniformity. If the application rate is too low, the crop’s nutrient needs are not met. If they are too high, not only are nutrients wasted that may have been marketed, there also is an increased risk for the excess nutrients to enter streams with rainfall runoff. If the application is not uniform, some areas are over-fertilized and other areas under-fertilized, resulting in streaking and a reduction in forage production. Spreader swath width is a key factor in controlling application uniformity.

The spreader swath width, litter application rate and distribution uniformity can be evaluated fairly quickly. Methods explained in the following sections emphasize simple approaches that can be used to modify spreading techniques to remedy any weaknesses. A typical analysis first samples the distribution of poultry litter. If this distribution is uniform, a good estimate (±20 percent) of the tonnage applied per acre can be determined. Obtaining this litter application rate, in tons per acre, is the best way to estimate the nutrients applied.

The University of Arkansas Agricultural Diagnostic Laboratory will analyze litter for nutrients. Well- blended samples of litter from several loads or from multiple sites in the poultry house should be combined to obtain realistic average values. Interested persons should contact their county Extension agent for details on how to benefit from a laboratory litter analysis and how to obtain a representative sample.

To maximize the value of a litter calibration procedure, choose a level area in the field and sample when the wind is not gusty. Samples obtained when the wind velocity is below 10 miles per hour display the characteristics of the litter and the spreader. A smooth wheel track reduces spreader bouncing and tilting. Any distortions of the spread pattern caused by wind or spreader bounce will only confuse the analysis rather than simulating typical pasture operating conditions. The discharge rate, gear settings and PTO speed should all be at the settings normally used. The spreader should be engaged and distributing litter at a uniform rate well before it crosses the sampling surface.

STEP 1. Obtain a roll of paper 24 to 36 inches wide that contrasts with the general color of the poultry litter to be spread. (Grocers, butchers and office supply stores may carry large rolls of white paper.) Locate a smooth area where poultry litter is to be spread. Unroll 50 to 75 feet of paper perpendicular to the intended path of the litter spreader. Place weights (rocks, sticks, bricks, etc.) along the length of the paper as you unroll it. This will prevent the wind from lifting it or causing portions of it to flutter. A litter sample typical of the spreader is needed from the entire width where litter is thrown.

STEP 2. Locate the center of the unrolled paper and position two or three small flags in a line perpendicular to the paper at the center. Flags should be chosen that can be straddled with equipment without concern for damage. Figure 1 shows the flags located about 100 yards apart to guide the operator.

STEP 3. With spreader settings and speed typical of normal operation, make one pass directly across the center of the paper using the flags for guidance. If another person is available to operate the spreader, it is helpful to observe the paper while sampling is underway. One possible cause of sampling error is the wind turning the paper over, etc., while the spreader passes. The observer can position himself behind and off to the side of the spreader to monitor the discharge as the vehicle crosses the sample paper to assure a normal feed rate. Since rocks and chunks of litter can be thrown from the spreader, observers need to be alert and far enough away for personal safety.

STEP 4. Evaluate the litter deposition along the entire length of the sample paper. The maximum deposition should be at the center, and the rate gradually decreases to the limits of the spreader “throw.” The “throw” distance and amount of litter spread on both sides of the spreader should also be about the same. If these conditions are met, good litter uniformity is possible. If the “throw” and litter distribution are significantly different from one side to the other, then the cause should be determined. If necessary, faulty equipment should be repaired before proceeding. Examples of mechanical items to look for are damaged vanes, and rotors that spin at different speeds.

Non-mechanical factors that can result in a poor application pattern are wind, ground slope and overloading the rotors. Wind and excessive ground slope can skew the pattern to one side. Overloading the rotors occurs when more litter is discharged onto the rotors than they can “throw” to the side of the spreader, resulting in large amounts of the litter falling off of the back edge of the rotors and directly to the ground. When this occurs, there is a heavy application directly behind the spreader and possibly a significant reduction in swath width. This is corrected by slowing the litter delivery rate (truck bed drive) to the rotors.

STEP 5. By examining litter deposition on the paper, select the distance on each side of the spreader where litter deposition appears to be half the maximum. The distance between these points is an estimate of the ideal swath width. The best distribution uniformity is possible if the distance from the spreader to the “half-maximum-deposition” point is equal on both sides. An average of three estimates is a reliable way to estimate the swath width for the combination of equipment settings and litter characteristics in the current calibration trial.

After determining the swath width, the distribution uniformity is easily confirmed using a similar approach.

STEP 1. Unroll a length of paper equal to two swath widths perpendicular to the intended path of the litter spreader. Place weights (rocks, sticks, bricks, etc.) along the length of the paper as you unroll it. This will prevent the wind from lifting it or causing portions of it to flutter. A litter distribution sample typical of the field can be obtained across portions of three passes.

STEP 2. The ends of the paper should be unrolled to the center of alternate passes. Again, small flags can be used to guide the spreader operator to approach perpendicular to the paper for all three passes. Locate flags at both ends of the paper and in the center to guide the operator on a serpentine path shown in Figure 2. Flags should be chosen that can be straddled with equipment without concern for damage. These flags can be spaced along the serpentine path about 100 yards apart to maintain the swath width determined by the procedure in the previous section.

STEP 3. With spreader settings and speed typical of normal operation, distribute litter on the paper following the serpentine pattern. If another person is available to operate the spreader, it is helpful to observe the paper while sampling is underway. One possible cause of sampling error is the wind turning the paper over, etc., while the spreader passes. The observer can position himself off to the side of the spreader to observe the discharge when the vehicle crosses the sample tarp to assure a normal feed rate. Since rocks and chunks of litter can be thrown from the spreader, observers need to be alert and far enough away for personal safety.

STEP 4. Evaluate the litter deposition along the entire length of the sample paper. The litter should be uniformly distributed throughout the length of the sample paper. If this is not the case, the swath width can be adjusted and a second test performed. If the litter applied in the center point between adjacent passes is too light, then decrease the swath width. If it is too heavy in the center, try increasing the swath width.

Using these techniques to obtain swath width and application uniformity increases confidence in the distribution uniformity of the litter and prepares you for an application rate measurement. The two basic approaches to determining application rate are the loads per field and weight per area methods.

With the loads per field method, an estimate of the litter weight per load times the number of loads per field divided by the number of acres in the field determines the rate at which the litter was applied. This approach is primarily a method of keeping records, not a planning tool, because the application is completed before the rate is determined. Reducing the rate is not possible after the litter has been applied to the entire field.

To use the loads per field method, an estimate of the litter density is required. One method of obtaining an estimate is to weigh a truck load of litter. The litter weight per load is calculated as the full weight minus the empty truck weight. Another approach is to calculate the volume of the litter in a load and multiply that times the density of the litter. Both of these approaches assume that all the loads applied to a given field are the same weight. The litter volume and density worksheets at the end of this publication provide formulas to help estimate the litter densities and truck volumes. Table 1 summarizes the typical range of weight, volume and density of loads of broiler litter measured in calibration workshops in Arkansas.

Table 1. Findings From Poultry Litter Spreader Truck Calibration Workshops in Arkansas*
	Weight	Volume	Density**
Minimum	2.9 tons	150.1 ft³	27.9 lb/ft³
Average	5.1 tons	317.2 ft³	32.7 lb/ft³
Maximum	6.1 tons	356.7 ft³	38.2 lb/ft³
* Information is from the calibration of 10 trucks. ** Minimum, Average, Maximum values of the broiler litter densities calculated for the individual truck loads.

With the weight per area method, tarps are used to catch litter applied under normal application conditions. The application rate is then calculated by dividing the weight of the litter caught on the surface of the tarp and converting the results to tons/acre. This estimated application rate is much more reliable after the correct swath width is chosen, and the litter distribution is uniform.

The weight per area method has the advantage of determining the application rate for a given set of equipment settings prior to completing the application of the litter. This allows the equipment settings to be adjusted to increase or decrease the rate for the field as needed.

It is often convenient to combine the application uniformity test described above with the application rate procedures described below. If this approach is taken, two tarps should be placed so that each receives litter from two passes as shown in Figure 2. Sampling this way provides two measured application rates, while the visual inspection verifies uniformity.

Normal spreader operational settings should be used during calibration. Ideally, to increase the reliability of the results, the application rate should be determined as the average of three calibration repetitions. The recommended procedures are given below. An example data sheet with formulas is included at the end of this publication.

STEP 1. Select a 6 mil polyethylene sheet or plastic tarp and a scale that will accurately read 1/2 pound increments or less. Locate an area where poultry litter is to be spread that is reasonably smooth, allowing the tarp to lay reasonably flat on the test area surface. Weigh the tarp before sampling to obtain a tare value. It is important to take this tare weight each time a sample is taken if the tarp is reused and wet litter remains on the surface. Turning the tarp over and shaking it will minimize material clinging to it from previous samples.

STEP 2. Locate the tarp securely in the path of the spreader. Two tarps used simultaneously on both sides of the spreader path provide helpful data. Place weights (rocks, sticks, bricks, etc.) to keep the tarps down and somewhat flat over the area. Locate an area that minimizes any driving obstacles and obstructions in the path of the litter to obtain a representative sample from the spreader. Avoid situations that increase the probability of litter rebounding off the “catch” surface.

STEP 3. With spreader settings and speed typical of normal operation, make the required number of passes past the tarps. Each tarp should catch litter from two adjacent passes of the spreader truck. If another person is available to operate the spreader, it is helpful to observe the tarp while sampling is underway. One should be alert for gusts turning the sample tarp over while the spreader passes, etc., causing sample error. The observer can position himself behind and off to the side of the spreader to observe the discharge as the vehicle passes the tarps to assure a normal feed rate. As soon as the litter has been spread, immediately weigh the tarp to minimize the opportunity for the wind or evaporation to reduce the sample weight, thus reducing its accuracy.

STEP 4. Calculate the litter weight for each tarp. To find the weight of the litter:

STEP 5. Compute the application rate for each tarp. To find the weight of litter spread in tons per acre:

STEP 6. Adjust the spreader gate opening height, swath width or bed chain speed to modify the application rate as needed to match the desired application rate. Any one of these adjustments directly affects the number of tons per acre applied. You need to repeat these six steps to determine the application rate for each set of equipment settings. Do not attempt swath widths that are too wide because this introduces considerable non-uniformity in the distribution. Segments of the swath behind the spreader will be high; and halfway to the next pass (wheel tracks), the application rate will be low.

These techniques can be hampered by wind, especially with dry litter. The amount retained on the tarp may not represent the actual application rate if dusty litter moves with the wind.

Producers desiring to better utilize their poultry litter can benefit from the proper calibration of litter spreading equipment. Modifying spreading procedures will improve soil fertility and reduce potential water pollution from litter.

Tire		Full	Empty
RF	=			lb
LF	=			lb
RR	=			lb
LR	=			lb
Total	=

Truck: ______________________________________________________ Date: ________/________/________

To reduce math errors, fill in the blanks and do the math in order from left to right.

Tarp Area (ft2) = Length (in) x Width (in) ÷ 144 (in2/ft2)

Tarp A Area = ________in x ________in ÷ 144 (in2/ft²) = ________ft²

Tarp B Area = ________in x ________in ÷ 144 (in2/ft²) = ________ft²

Litter Weight (lb) = Full Tarp Weight (lb) – Empty Tarp Weight (lb)

Tarp A Litter Weight = ________lb – ________lb = _________lb

Tarp B Litter Weight = ________lb – ________lb = _________lb

Rate (ton/acre) = Litter Weight (lb) x 43,560 (ft²/acre) ÷ Tarp Area (ft²) ÷ 2,000 (lb/ton)
Rate (ton/acre) = Litter Weight (lb) x 21.78 ÷ Tarp Area (ft²)

Tarp A Rate = ________(lb) x 21.78 ÷ ________ (ft2) = ________ ton/acre

Tarp B Rate = ________(lb) x 21.78 ÷ ________ (ft2) = ________ ton/acre

Average Rate = ________ ton/acre

Layout Diagram and Comments

Target swath width = ______ ft

Karl VanDevender - Ph.d., P.E., Extension Agricultural Engineer
Gary Huitink - P.E., Extension Agricultural Engineer

DR. KARL VANDEVENDER, and GARY HUITINK are Extension engineers with the University of Arkansas Cooperative Extension Service in Little Rock.

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Cooperative Extension Service	Agricultural Experiment Station
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Calibrating Poultry Litter Spreader Trucks