Agricultural wastes include the inedible portions of food crops, animal manures, and unused portions of trees harvested for paper or lumber. The total amount of energy contained in such wastes is much more uncertain than the energy content of available municipal wastes. Neither the gross availability of such materials (the total amount produced by U.S. agriculture and forestry) nor their real availability (the amount that could be economically collected, transported, and fed into a conversion process) can be precisely determined. The Solar Resource Group estimates that the gross annual availability of these wastes is about 8 quads and that their real availability is about half that value.
The energy available as fuel from agricultural wastes depends on the conversion process. The decision on which process to use in a given situation will be based on the character of the wastes, the ease with which they can be brought to large central conversion plants or converted in smaller plants on site, and the local prices of various fuels and residues from the processes. The decision will sometimes, but not always, be made in favor of the process with the highest net energy yield. The Solar Resource Group estimates that in about a decade collection and conversion of agricultural wastes (with a recovery factor of 25 percent, to take account of both real availability and conversion losses) could yield about 2.2 quads of methane or some equivalent fuel. This value sets the order of magnitude for the following discussion.
The U.S. food system consumes significant amounts of energy, only a fraction of which is used in the actual production of food. Most is used in transporting and processing food after production. The amount of energy used on farms for producing food has been estimated to be about 2.1 quads/yr.36 Of this amount, about half is actually consumed as fuel on the farm, and about half is used in producing fertilizer, pesticides, and other farm chemicals. The amount of energy used in U.S. agriculture has increased rapidly over the past few decades. This increase has been associated with a decrease in the labor inputs to agriculture but reflects also a trend to centralized activities. Cattle feedlots and “chicken factories” are but two examples of this centralization.
This trend has had two major consequences. One is increased consumption of energy; the other is a growing problem of waste disposal from large central facilities. For example, the control of water pollution from chicken factories has been a major goal of the Environmental Protection Agency for a number of years. Disposal of these wastes is costly, and the wastes contain both energy and nonenergy (nutrient) values. Thus, the economic situation is similar to that of municipal waste conversion, except that