John Wilhoit and Qingyue Ling

Poultry litter is a valuable resource for farmers who use this poultry industry by-product to fertilize farm land. However, in order for the litter to be an asset and not a liability, it must be applied at a controlled and uniform rate, which can be difficult because of the design limitations of standard poultry litter spreaders. AAES research has shown that a few minor adjustments to the machinery can greatly increase the precision of land application of poultry litter.

Poultry litter spreaders have two horizontal spinners that distribute the litter across some fixed width behind the spreader. The application rate at any distance from the center of travel of the spreader varies, with the highest application rate usually in the middle and decreasing amounts farther from the center. The average overall application rate and the uniformity of application depends on the overlap of litter at any point, produced by adjacent passes of the spreader (see Figure 1). The amount of overlap depends on how far the spinners throw the material (distribution pattern) and on the swath width, which is the distance from the center of travel of one pass of the spreader to the center of travel of the adjacent pass.

Figure 1. Single and overlapped spreader distribution patterns.

This overlap affects the two main objectives for poultry litter spreading operations—applying the litter at the desired application rate and as uniformly as possible. Precision application is likely to become more important as concerns about nutrient loading rates, especially phosphorous, push poultry litter application rates down to as low as one ton per acre.

AAES research has investigated the effects of machine and operational parameters on the performance of a poultry litter spreader. Through those studies, several methods have been devised to improve the precision of control and calibration of poultry litter spreaders. A portion of the research investigated certain machine and operational parameters, and results are leading to new information about spreader design and operation that have the potential to significantly improve uniformity.

Calibration checks of commercial poultry litter spreaders often show significant skewing of the distribution pattern, indicating that more litter is falling on one spinner than the other. With such skewing, it is difficult to adjust the swath width to get a more even distribution. Commercial spreaders usually have a horizontal flow deflector located just above the spinners that divides the flow only after the material falls off the conveyor. These flow dividers can only be adjusted front to back, in an attempt to affect the location that the material drops on the spinners (drop point). For the AAES tests, it was critical that researchers obtained an equal flow of material to each of the two spinners. To accomplish this, a vertical flow divider was added at the gate opening located at the center line of the chain conveyor (Figure 2). The front edge of the divider is sharp and inclined downward to help pull the material out of the gate and cut any large lumps pulled underneath the divider. Flow deflectors are located at the bottom of the divider. Test results showed that this flow divider was very effective. In hundreds of distribution pattern tests conducted with the spreader, there was rarely any skewing of the pattern. The flow dividers are a simple addition to spreaders that could be made by the manufacturer or by the spreader owner.

Figure 2. Vertical flow divider at the spreader gate opening: A. horizontal support bar, B. divider, C. deflector.

The adjustment provided by standard deflectors or flow dividers that come with commercial spreaders is very coarse at best. And, even with what little adjustment is possible, it is difficult to tell what effect the adjustment will have on the actual location of the point where the material is dropped onto the spinners. To address this problem, AAES researchers added a box-type flow chute to the spreader to carefully control the flow of material to four different drop locations on the spinners. Each drop point location was produced by an arc-shaped opening in the bottom of the box that the material falls through. The openings transcribed a 45-degree angle, starting at the designated angle (45, 90, 135, and 180 degrees) measured from the front end of the spinners.

Extensive testing was conducted to evaluate the effect of drop point location on uniformity and effective swath width over a range of spinner speeds and material flow rates. The flow rates ranged from 16 to 63 cubic feet per minute, corresponding to application rates of approximately one to four tons per acre for the spreader travel speed, swath width, and litter density used in the tests. Statistical analysis of the test results showed that the most effective spread patterns, based on both uniformity and effective swath width, were achieved at the 90- and 135-degree drop point locations for all of the spinner speeds and flow rates used in the tests.

Further analysis indicated that the 90-degree drop point location tended to produce patterns that had slightly better uniformity, while the 135-degree drop point location produced patterns with slightly higher effective swath widths (indicating that the material was thrown farther). From a practical standpoint, these results indicate that flow dividers or deflectors should be adjusted to drop the material as close as possible to the center of the spinners (as measured from the front to the rear of the spreader) to produce the best spread patterns.

The spinner speeds used in the tests were 420, 625, and 850 rpms. The middle speed is the approximate spinner speed that is standard with most poultry litter spreaders. The lower and higher values were used in the tests so researchers could look at the effect that spinner speed had on spreader performance. The test results showed that for all of the flow rates and drop point locations, the higher the spinner speed, the better the uniformity and the greater the swath width. For high application rates, these results are what would be expected, because the higher speed would help keep the spinners from becoming overloaded with material at the higher material flow rate.

These results could also be useful for low application rates. If a higher spinner speed can be used to throw the material farther, then wider swath widths can be used. This will produce lower application rates for a given flow rate of material onto the spinners. Most spreaders have spinners that are hydraulically-powered, which is important for being able to adjust the spinner speed. These results indicate that the spinners should be adjusted to as high a speed as possible. Spreader manufacturers should probably be encouraged to build spreaders with increased spinner speed capacity.

The flow rate of material coming out of the hopper and falling on to the spinners is the product of the area of the gate opening times the conveyor speed. The area of the gate opening, in turn, is the product of the gate width times the gate height. It follows, then, that to achieve a certain flow rate, different combinations of gate height and conveyor speed can be used. Research showed that there often is a cyclic variation in material falling onto the spinners, possibly from the material falling off the conveyor in large chunks rather than in a continuous, uniform stream. This might be more likely to happen when the height of the layer of material on the conveyor was greater (i.e. when the gate height was greater). To investigate the effects of gate height and conveyor speed on material metering, researchers conducted tests at theoretical flow rates ranging from 16 to 63 cubic feet per minute, with four combinations of gate height and conveyor speed at each flow rate. The uniformity of material metering was evaluated by measuring the pressure drop across the hydraulic motor powering one of the spinners. Variation in pressure drop should indicate changes in load on a spinner due to the amount of material falling on it.

Results showed that the variation in pressure drop, indicated by the coefficient of variation, increased with increasing gate height at all of the flow rates. The increase in variation was greatest at the lowest flow rate, and the variation decreased as the conveyor speed increased. Based on these test results, the variation in flow onto the spinners for a given flow rate would be minimized by having a combination of low gate height and high conveyor speed. Assuming that less variation of flow onto the spinners will mean better spread uniformity, then the gate height should be kept as low as possible to get the best uniformity. If the conveyor is driven hydraulically, the conveyor can be easily adjusted to the appropriate speed (dependent on the gate height) to achieve a target flow rate.

This is the first of a two part series on poultry litter spreading. Part two will appear in the next issue of Highlights and will present a calibration method that accurately determines application rate, and clearly indicates how to adjust machinery to achieve target rates, without actually running the spreader.