Cow Talk
by Denise Mullinax, Dairy Advisor of the University of California Cooperative Extension

Manure Management

Determining the Value of Your Manure as a Source of Fertilizer Nutrients

Manure should be viewed as a renewable resource which may be used to increase farm efficiency and profitability in a time when agricultural producers are under extreme economic pressure. Manure can be conserved and utilized on-farm to increase overall profitability. It may be used as a fertilizer, a soil amendment, a material for energy generation, a bedding material, or a feedstuff.

The most common use for manure is as a fertilizer for agricultural crops in the form of manure water or dried solids. Although both manure water and dried manure are regularly applied to cropland, most producers often do not assign adequate value to manure as a source of fertilizer nutrients.

Manure contains numerous nutrients, however the most typically measured are nitrogen (N), phosphorus (P), and potassium (K). To determine the economic value of manure the N, P, and K content needs to be determined. This will vary among sources due to differences in handling and storage of the product, nutrition of the animal, stage of production, and weather. Benchmark values have been outlined by the Midwest Plan Service Livestock Waste Facilities Handbook, the American Society of Agricultural Engineers, the Environmental Protection Agency of Illinois, the Natural Resource Conservation Services, and others.

However, to get accurate values for the nutrient concentration of the manure at your facility is important to collect samples during manure application and have them analyzed. The value of N, P, and K is dependent on the nutrient needs of the crop relative to the nutrient status of the field, and the purchase costs of commercial fertilizer. In addition to sampling manure, soil samples should be collected prior to application to determine the soil nutrient status of the field. This will aid in proper estimation of application rates and accurate calculation of the economic value of manure nutrients.

Additional costs or benefits associated with manure use are very difficult to factor into an economic analysis. For example, increased soil salinity decreases manure’s fertilizer value, while manure’s organic matter and micro elements add value. Due to the decomposition of manure and subsequent release of plant-available nutrients over several years, nutrient value should be incorporated into the total value of manure in the year available. Additional costs to apply manure or needed herbicides in excess of normal application should be deducted from the total manure value (Follett and Croissantl 1990). Due to the nature of manure, some nutrients may be applied in excess of crop and soil requirements. In general, nutrients applied in excess of plant requirements should be given a value of zero.

The two most common means of quantifying the economic value of manure are replacement cost of synthetic fertilizers and increase in crop yield due to manure fertilization. The replacement value method uses the economic value of the replaced synthetic fertilizers to calculate the value of manure nutrients.

This method does not consider cost differences due to application method between manure and commercial fertilizers. However, the typical application rate for commercial fertilizers needs to be determined for each nutrient. In cases where excess nutrient application results in negative affects on crop production, negative values should be assigned to correctly quantify the value of the manure. The second method, determining the increase in crop yield, attempts to account for the additional "unquantifiable" benefits provided by manure. This method measures the increased crop yields resulting from manure application less the application expenses associated with manure use (Anonymous 1972). It accounts for the total value of manure to the soil including nutrient additive values and beneficial amendment qualities. The net effect on crop yield combines the manure’s beneficial amendment qualities (increased water holding capacity, improved soil tilth, and enhanced growth of soil microorganisms.) The weakness with this method is that the benefits are difficult to quantify in the short term, and may not be fully realized in the year of application. In addition, the use of both commercial fertilizers and manure in crop production makes it difficult to attribute an accurate value to manure as a source of fertilizer nutrients.

Both methods provide economic values for the use of manure as fertilizer in the production system. Incremental costs associated with using manure in place of purchased fertilizers should be considered. Accurate and reliable sources of plant and soil nutrient requirements as well as current manure nutrient contents should be used. This will aid in the precise assessment of manure value when justifying the cost of the manure management system.

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