To maximize crop productivity and minimize adverse environmental effects, it is critical that land applications of manures meet, but not exceed, crop nutrient requirements. To assure that the actual manure application rate matches the desired application rate, manure-spreading equipment must be calibrated. The goal of manure spreader calibration is to determine the amount of manure, on a weight per unit area basis, that is being applied to a field. This publication describes three methods for manure spreader calibration for spreaders handling solid and semi-solid manures.
The application rate is a function of gear and throttle settings, which determine PTO (power take off) speed and ground speed; spreader equipment settings; and the amount of overlap due to width between passes in the field. A separate but related issue is the uniformity of the manure application.
It is a good idea to calibrate your manure application equipment before spreading any large volume of manure. The required calibration schedule for confined animal feeding operations holding a Virginia Pollution Abatement (VPA) permit for poultry or livestock is specified within the operation's nutrient management plan. Typically, a nutrient management plan specifies calibration before major cleanouts and/or at least twice a year. For manure spreaders handling solid materials, you should do calibration whenever the consistency of the manure is obviously different from the last batch used for calibration. Possible causes for changes in manure consistency include changing the bedding material, changing feed ingredients, or any change in the production system that affects the moisture content of the manure.
Manure spreader calibration results should be a permanent part of every farm's records for crop nutrient application. Regulated producers are required by their permits to keep written records of their spreader calibrations for at least five years.
The tarp method has the advantage of providing information about the uniformity of the manure application and helps you determine the appropriate overlap between application passes. The load-area methods (methods 2 and 3) are easier and more accurate at determining application rates, but do not provide any information regarding the uniformity of spreading. Therefore, you should use the tarp method at least once to determine the correct overlap distance and the settings required for a uniform distribution. Once the overlap distance and settings for uniform distribution are determined, using either of the load-area methods, which involve a greater area of spread, will provide a more accurate determination of the application rate.
Both of the load-area methods require that you know the weight of manure in a spreader load. The user's manual for your manure spreader specifies the load capacity of the spreader on a volume basis. Unfortunately, it is difficult to accurately convert from manure volume to manure weight for solid and semi-solid manure, because the conversion depends upon how the manure is packed into the spreader. The denser the packing, the more weight the same volume will hold.
The most accurate way to determine the weight of a load of manure in a spreader is to weigh the spreader both empty and full and then subtract the empty weight from the full weight to arrive at the weight of the manure. The regional watershed offices of the Virginia Department of Conservation and Recreation (listed at the end of this publication) have portable scales that you can use for this task. Contact them to arrange to borrow the scales. While the offices are willing to loan out the scales, you must plan ahead. Do not wait until the day before you plan to spread manure to contact them to schedule the use of their scales.
If you have access to scales capable of weighting your spreader, the load-area methods are recommended for calibrating the application rate. Load-area Method One requires that you spread enough loads (a minimum of one) to cover approximately one acre and then measure the area covered. Load-area Method Two involves spreading a field of known area (it should be less than five acres), and tracking the total weight of manure applied.
If you do not have access to large scales, you can use the tarp method, which only requires small scales capable of weighting a bucket and a manure-laden tarp.
For all the calibration methods described, you should record the gear setting, throttle setting (or tachometer reading), and spreader equipment settings at which you apply the manure. The calibration procedure should be repeated at different spreader settings or tractor gear/tachometer settings until you achieve the desired application rate. By keeping records of the tractor and spreader settings for each calibration run, you will be able to reproduce the conditions and thus the application rates that you achieved during calibration.
This method requires a small scale capable of weighing a bucket containing manure and tarps that have received a manure application. The following steps are required to use this method:
To determine the uniformity of the manure application, compare the weight of manure collected on each of the five tarps. If the weights of manure on all five tarps are reasonably close, no adjustment is needed in uniformity and you should continue to the calibration calculations.
For Spreader Trucks: If tarps 1, 3, and 5 are heavier than tarps 2 and 4, the manure may not be contacting the spinners at the correct location resulting in too MUCH material landing directly behind the spreader. The chute may need to be moved away from the truck (See figure 2) or the gate setting/drag chain speed may be feeding too much material for the spinners to handle.
If tarps 2 and 4 are heavier than tarps 1, 3, and 5, the manure may not be contacting the spinners at the correct location resulting in too little material landing directly behind the spreader. The chute may need to be moved toward the truck (See Figure 2) or the width between passes may need to be increased (too much overlap).
If tarps 2 and 4 are not reasonably close in weight, the chute and divider are not supplying litter to the spinners at equal rates and the chute needs adjustment.
For Box Spreaders: If tarps 2 and 4 are heavier than tarps 1, 3, and 5, then there is too much overlap and the width between passes should be increased. If traps 2 and 4 are lighter than tarps 1, 3, and 5, then there is not enough overlap and the width between passes should be decreased.
The load-area method should be repeated until a reasonably uniform application is achieved. After uniformity is achieved, you can use the data collected to determine the application rate.
The application rate is the total weight of manure collected on the tarps divided by the collection area and multiplied by a constant to convert lb/ft2 to ton/acre (see Application rate below).
While it is possible to determine the application rate from the tarp method, it is less accurate than either of the load-area methods. Once uniformity of application has been established, it is best to determine the application rate using one of the two load-area methods.
This method of spreader calibration, based upon spreading a field of known size, requires the least effort. However, it has the distinct disadvantage of determining the application rate AFTER THE FIELD IS SPREAD. For a small field, this is not really a problem, but you should not use this method on a field larger than five acres. Another disadvantage of this method is that repeated tests to determine application rates at different tractor and spreader settings require additional fields of known size.
The authors would like to express their appreciation for the review and comments made by Bobby Clark, Virginia Cooperative Extension agent, Warren County; and Susan Gay, assistant professor and Extension engineer, and Bobby Grisso, professor and Extension engineer, both of the Department of Biological Systems Engineering, Virginia Tech.
If you have questions regarding manure spreader calibration, contact your local Virginia Cooperative Extension agent (listed in the phone book under County Government) or your local Department of Conservation and Recreation representative, as shown below:
Abingdon (Tennessee-Big Sandy Watersheds Office): 252 W. Main Street Suite 3, Abingdon, VA 24210; phone: (276) 676-5528, fax: (276) 676-5527
Chase City (Roanoke Watershed Office): 411 Boyd Street, Chase City, VA 23924; phone: (434) 372-2191, fax: (434) 372-4962
Dublin (New River Watershed Office): PO Box 1506, Dublin, VA 24084; phone: (540) 643-2590, fax: (540) 643-2597
Fredericksburg (Rappahannock Watershed Office): 2601 Princess Anne St., Suite 101, Fredericksburg, VA 22401; phone (540) 899-4463; fax: (540) 899-4389
Henrico (James River Watershed Office): 3800 Stillman Pkwy., Suite 102, Richmond, VA 23233; phone (804) 527-4484, fax: (804) 527-4483
Staunton (Shenandoah Watershed Office): 44 Sanger Lane, Suite 102, Staunton, VA 24401; phone: (540) 332-9991, fax: (540) 332-8956
Suffolk (Chowan-Albemarle Coastal Watersheds Office): 1548 Holland Road, Suffolk, VA 23434; phone: (757) 925-2468, fax: (757) 925-2388
Tappahannock (York Watershed Office): PO Box 1425, Tappahannock, VA 22560; phone: (804) 443-6752, fax: (804) 443-4534
Warrenton (Potomac Watershed Office): 98 Alexandria Pike Suite 33, Warrenton, VA 20186-2849; phone: (540) 347-6420, fax: (540) 347-6423
Reviewed by Jactone Arogo, Extension Specialist, Biological Systems Engineering
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Interim Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 1, 2009