John Wilhoit and Qingyue Ling

Calibration of spreader equipment used to apply poultry litter to land can be a complicated and time-consuming process. AAES researchers, however, have found a way to calibrate spreaders on paper, rather than having to operate the machines, which can save producers time and trouble.

Extensive testing has been conducted at Auburn University on the effects of machine and operational parameters on the performance of poultry litter spreaders. Results of those tests revealed a number of ways to help improve the precision with which poultry litter spreaders can be controlled and calibrated. In the last issue of Highlights (Vol. 46, Number 3), an article (“Precisely Right! Research Helping Achieve Precision Control and Calibration of Poultry Litter Spreaders”) reviewed ways to alter spreader design and operation to improve the uniformity of litter application. This article discusses ways to perform calculations using pen and paper without actually running the machines. This method, which not only determines application rate with reliable accuracy but also clearly indicates how to adjust the formula to achieve target rates, should be increasingly useful as concerns about the loading rates of nutrients, such as phosphorous, push poultry litter application rates down as low as one ton per acre.

Standard calibration methods involve operating the spreader and collecting and measuring litter from some known area on the ground (either a tarp or collection pans). These methods are accurate for determining the overall application rate. However, it is difficult to make adjustments to achieve a target rate using this method because the spreader has to be operated each time an adjustment is made to determine the effect on the application rate. For the AAES studies, researchers needed to calculate on paper the required values for machine parameters, such as ground speed, conveyor speed, and gate height, to achieve target application rates. They developed a method for performing the calculations based on the theoretical volumetric flow rate of material exiting the hopper and being spread by the spinners.

Test results showed that the volumetric efficiency of material metering, which is defined as the ratio of theoretical volumetric flow rate to actual volumetric flow rate, averaged approximately 90% for poultry litter. Taking the volumetric efficiency and the density of the poultry litter into account, they achieved excellent calibration accuracy in the spreading tests, consistently coming within about 10% of target application rates. Several measurements are required for the paper calibration method. These include ground and conveyor speeds, gate height and width, and density of the litter.

The ground speed can be determined by timing the spreader as it drives a certain distance. For a pull-type spreader with PTO-driven spreader hydraulics, the ground speed can be checked without actually operating the spreader, but it is important to check the speed with the PTO running, and to use the same tractor engine speed for the speed check and for spreading operations.

Conveyor speed can be more difficult to check. However, by determining the pitch size of the conveyor chain and the number of teeth on the conveyor sprocket, operators can calculate conveyor speed from the rotational speed of the shaft on which the sprocket is mounted. Pitch size is the distance from the roller center of one chain link to the roller center of the next. Using these measurements, the following formula will provide conveyor speed measurements:
      conveyor speed = pitch size × # sprocket teeth × rotational speed.

The density of the litter can be determined by loosely filling a container of known volume with litter and determining the weight of that litter. The density of poultry litter is fairly consistent (depending on the moisture content). If the actual density cannot be measured, an estimated value of 30 pounds per cubic foot can be used.

The relationships used for calibration calculations can be determined based on what is happening during the spreading operation. The conveyor is pulling a stream of material out of the hopper through the gate opening. That material falls onto the spinners and is broadcast over a certain area.

The theoretical volumetric flow rate of material coming out of the hopper is:
      flow rate = conveyor speed × gate opening area.

The mass flow rate is the volumetric flow rate × the density of the litter mass flow rate = conveyor speed × gate opening area × density of poultry litter.

The application rate is simply the mass flow rate divided by the area covered per unit time:
      application rate = mass flow rate/ground speed × swath width.

These equations are based on the theoretical volumetric flow rate. Taking into account the estimated volumetric efficiency of 90%, the actual flow rate would be lower by about 10%. To achieve a target application rate, a spreader parameter, such as gate height or conveyor speed, would be adjusted upward by dividing by 0.9. These equations can be used to calculate the exact value for a spreader parameter to achieve a target application rate, or to determine the specific effect of changing a parameter such as swath width or ground speed.

Based on the fact that the gate opening area is the product of the gate height times the gate width, and including a conversion factor to account for units, the equation for calculating the application rate can be formulated as follows:
AR = (CS × GH × GW × $)/(S × SW × 581)
where:
	AR = application rate (tons per acre)
	CS = conveyor speed (feet per minute)
	GH = gate height (inches)
	GW = gate width (inches)
	$ = density of poultry litter (pounds per cubic foot)
	S = ground speed (miles per hour)
	SW = swath width (feet)
This equation can be rearranged to solve for different parameters to achieve target application rates. For example:
GH = AR × S × SW × 581/CS × GW × $
or
CS = AR × S × SW × 581/GH × GW × $
In Metric units, the equation becomes:
AR = CS × GH × GW × $/S × SW × 1.67
where:
	AR = application rate (tons per hectar)
	CS = conveyor speed (miles per minute)
	GH = gate height (meters)
	GW = gate width (meters)
	$ = density (kilograms per meter squared)
	S = ground speed (kilometers per hour)
	SW = swath width (meters)