Manure Nutrient Utilization
Carrington Research Extension Center
North Dakota State University
Efforts to prevent pollution of water from livestock manure began over thirty years ago. Subsequently, systems were designed and built to collect and store manure and to contain runoff from open feedlots. It has been most economical to dispose of manure and feedlot runoff on fields close to the livestock feeding facility. Often, manure nutrient applications exceed crop removal and thus accumulate in the soil. When this happens, nutrients are easily leached or eroded from the manure treated soil. The pollution potential is simply moved from the animal feeding operation to the manure disposal area.
The two manure nutrients that are of most concern are nitrogen(N) and phosphorus(P), two elements important in crop production and that are typically part of most commercial fertilizer applications. When in excess amounts and in the wrong place, however, they result in problems such as high nitrates in groundwater and phosphorus fertilization of surface waters leading to eutrophication.
The most practical way to manage livestock manure is almost always applying it to cropland to utilize the nutrients. The objective should be to utilize all the N and P for crop production, thus keeping those nutrients from becoming pollutants. This will result in some economic benefit to the producer to offset the costs of manure management while preventing harm to the environment and keeping the livestock producer in compliance with regulations.
Manure Application Planning
Common sense would say that the first step in manure application planning should be to soil test and determine the nutrient needs of the planned crop. NDSU Extension has bulletins available on soil sampling and testing, choosing realistic yield goals, and calculating the appropriate fertilizer application rates. When all or some of the fertilizer is to be supplied by manure, it becomes more complicated.
Manure can be sampled and tested in a manner similar to the soil. About 10 to 20 sub-samples should be taken from a solid manure stockpile. Liquid manure should be thoroughly agitated and mixed before sampling. Getting a representative sample is very difficult, as manure can be very non-uniform. The best sample can likely be taken when the manure is being spread. Thus, as a practical matter, the manure test results may not be available until after the manure has been spread. However, this will indicate if nutrients have been over or under applied, and if additional fertilizer need be applied at planting. It is also valuable information for next year, as manure characteristics tend to be stable under given management and feeding situations, but will likely change if management practices change.
Meanwhile, in the absence of actual test results, there are table values such as those in Table 1 that can be used to get started and be reasonably in the ballpark.
Another complication in using manure is the fact that much of the N is tied up in organic form that must be converted to nitrate N to be taken up by plants. Therefore, only a percentage is available the first year after application, another percentage the second year, etc. Table 2 shows percentages of N that can be assumed to be available the first, second, and third years for various types of manure and under various schemes of incorporation of the manure into the soil. When calculating the rate of desired manure N application, do not forget to credit the N available from any manure applied the previous year. P has no gaseous phase, reacts differently, and is simply assumed 80% available the first year.
Nitrogen vs. Phosphorus
A third complication with manure fertilizer is the fact that it is not a plant nutrient balanced product. Usually, the ratio of N to P in manure is such that applying enough manure N for most crops will result in excess P application. There are situations in the country where this has resulted in soil P levels high enough to cause potential for P to move off site and get into surface waters. There are two strategies to deal with this problem.
1. Apply manure to meet the P requirements of the crop. Make up the difference in N requirement, if necessary, with commercial fertilizer N.
2. If soil test P is at or below the “very high” category, as most North Dakota soils are, apply enough manure to meet the crop requirement for N. If that results in excess P application, do not apply manure to that field until following years’ cropping uses the excess P.
In any case, if soil test P is at 150 ppm or above, do not apply manure. Never exceed the N requirements of the crop, as N has a gaseous phase and a soluble phase, and the excess is sure to be lost.
Once a proper manure application rate has been calculated, it is necessary to calibrate the manure spreader. If a scale is available, it is simplest and best to weigh a loaded spreader to determine the tons per load. As that is usually not the case, one can measure the length, width, and average depth of the spreader box load and calculate the cubic feet. Then weigh a 5-gallon bucket full of a representative manure sample. As there are approximately 7.5 gallons in a cubic foot, multiply the weight in pounds of 5 gallons of manure by 1.5 to find the weight of one cubic foot, and then multiply that by the cubic feet in the whole load to determine the pounds per load. Divide by 2000 to get tons per load.
Use a measuring wheel to measure the area covered by a spreader load. (There are 43,560 square feet per acre). Divide the tons per load by the acres per load and you now have tons per acre. Then you can adjust the unloading time of the spreader and/or the tractor speed as necessary to achieve the desired rate.
A good manure utilization plan includes soil tests, manure tests, calibration of application equipment, and good records. Assistance with developing manure utilization plans is available from the NDSU Extension Livestock Waste Management Specialist.
Dean and Director for Agricultural Affairs
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