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Calculating Manure Spreader Application RatesManure as a Resource SeriesManure Spread Application RatesOne of the most critical components of a comprehensive nutrient management plan that includes animal manure is the application rate. Calibration of a manure spreader helps livestock producers use the nutrients contained in manure more efficiently. Calibrating a spreader only takes a short time but can save hundreds of dollars in fertilizer costs. This publication describes two calibration methods that effectively estimate the amount of nutrients applied to a field. Applying the nutrients in manure according to crop needs reduces production costs and protects water resources. Manure Application PlanThe key to successful nutrient management is a sound manure application plan. This includes:
Calibrating Manure Spreader Application RatesThere are two basic approaches for calibrating a manure spreader. They are the load area and the weight area methods. The load area method is the most accurate and can be used for both liquid and solid manure. The weight area method works only with solid or semi-solid manure. Load Area MethodThe load area method is a 3-step process. First, determine the amount of manure in the spreader. The most accurate way to determine the amount of manure in a spreader is weighting the spreader when it is empty and again when it is full. For a reliable estimate of spreader capacity, weigh several representative loads (at least five) to determine the average gross weight. Subtract the empty spreader weight. Then, calculate the average net loaded weight. Next, determine the area of spread using the method at the right. Width measurements near the beginning and end of the spread pattern should be avoided because the spreader may not be operating at full capacity. Finally, calculate the application rate. The application rate is calculated using the formula for either liquid or solid manure. Formula for Solid Manure Equals Tons/Acre (Average Loaded Weight (lbs) x 21.81) divided by (Distance Traveled (ft) x Width of Spread (ft)) Formula for Liquid Manure Equals Gal/Acre (Tank Volume (gal) x 43,5602) divided by (Distance Traveled (ft) x Width of Spread (ft)) Weight Area MethodWhen a scale is not available, the application rate of a box spreader can be determined by collecting manure on a tarp or piece of heavy material. The weight area method is less accurate than the load area method. This method consists of eight steps:
1. Factor for converting pounds to tons and square feet to acres (21.8 =
43,560 sq ft per acre divided by 2,000 lbs/ton). Determining Area of SpreadThe “area of spread” is the length and width of the ground covered with one load of manure. The area of spread is affected by speed and equipment settings. Spreaders discharge manure at varying rates depending on travel and PTO speed, gear box settings, and discharge openings. It is important to adjust the spreader so the pattern is as uniform as possible. Accurately measuring the length and width of this area is essential. To determine width, measure two adjacent spreads and divide by two to find the “effective” spread width. This accounts for overlap, which is often needed for a more uniform application. The length of spread is determined using the following three values:
From these values, calculate the distance or lengths of spread using these formulas. Formula: Solid Manure Equals Feet/Load (Average Load Weight (lbs) x 21.81) divided by (Spread Width (ft) x App. Rate (tons/ac)) Formula: Liquid Manure Equals Feet/Load (Tank Volume (gal) x 43,5602) divided by (Spread Width (ft) x Desired App. Rate (gal/ac)) Spread a load. If the distance traveled does not equal the calculated distance, adjust the speed or equipment settings. For More InformationFor information or assistance with your management options, contact your local Natural Resources Conservation Service (NRCS) or conservation district office. Natural Resources Conservation Service Montana Assoc. of Conservation Districts For information or assistance with regulatory requirements of manure management, contact the Montana Department of Environmental Quality. Department of Environmental Quality Manure as a Resource SeriesThree additional brochures are in this series. To receive a copy of any of the following brochures, visit your local NRCS office.
If you encounter any problems with the file provided on this page, please contact Technical Resources at 406-587-6822. This document requires
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