The Food, Conservation, and Energy Act of 2008 directed the Department of Agriculture (USDA) to prepare a study that would evaluate the role of animal manure as a source of fertilizer, and its other uses, including evaluating the effects on agriculture production contributable to the increased competition for animal manure use due to bioenergy production, including as a feedstock or a replacement for fossil fuels.
The report Manure Use for Fertilizer and for Energy assesses current efforts to use manure for energy production and evaluates the impact of bioenergy investments on manure’s use as fertilizer.
Below are highlights and key findings from the report:
Large livestock operations are increasingly required to have nutrient management plans, which require balancing nutrient applications with the nutrient utilization of crops. Compliance with the plans can raise farm costs. Estimated costs vary sharply with the degree to which excess manure needs to be disposed of and the willingness of nearby farmers to accept manure for application to their cropland. A low willingness to accept among nearby farmers means that livestock producers will need to transport excess manure much farther for crop application.
Manure-to-energy projects are not currently in widespread use. Digester systems, including those planned or in construction, cover less than 3 percent of dairy cows and less than 1 percent of hogs. The single operating combustion plant utilizes litter from 6.6 percent of U.S. turkey production, while an idled plant in California could utilize manure from about 3 percent of fed cattle.
Manure-to-energy projects may allow farmers to realize benefits from avoided purchases of electricity, from selling electricity, or from selling manure to generating plants, but few realize enough savings to justify the expense. But because such projects use existing resources, they could provide society with benefits if manure replaces newly mined fossil fuels in energy production, and if methane, a greenhouse gas, can be captured. Those societal benefits have led to proposals to support manure-to-energy projects through State utility mandates (to purchase electricity from farms and to invest in renewable production sites), through subsidies for capital costs, and through direct subsidies and credits for energy production.
Expanded support could lead to a substantial growth of energy applications for manure.
Currently envisioned manure-to-energy projects are not likely to impose substantive constraints on the use of manure as fertilizer. Many of the nutrients that are beneficial to crop growth remain after energy production. Nitrogen, phosphorus, and potassium nutrients remain in the effluent of the digester process, to be spread on fields. Combustion processes do consume the nitrogen nutrients in manure, but leave phosphorus and potassium in an ash residue that, because of its concentrated form, is less costly to transport than raw manure. In addition, manure-to-energy projects function in markets for fertilizer and energy, and will be most economic in those areas in which the acquisition costs of manure are lowest. In turn, manure acquisition costs will be lowest where manure is in excess supply, with the least value as fertilizer.