Summer 2002
Gaseous emissions from animal agriculture
by Wendy Powers, Department of Animal Science and Hongwei Xin, Department of Agricultural and Biosystems Engineering
Emissions from animal production systems originate from three primary sources: manure storage facilities, animal housing, and land application of manure. Other sources include feed production, processing centers, and silage storage. Many of the compounds that are emitted from animal production operations are by-products of anaerobic decomposition of livestock and poultry wastes. Aerobic decomposition generally produces fewer odorous by-products but can enhance volatilization of gaseous compounds. Moisture content and temperature affect the rate of microbial decomposition.
Animal waste. Animal wastes include manure (feces and urine), spilled feed and water, bedding materials (straw, sunflower hulls, and wood shaving), and wash water. The compiled waste includes carbohydrates, fats, proteins, and other nutrients that are readily degradable by microorganisms under a variety of suitable environments. As many as 200 volatile compounds have been identified as intermediate by-products of decomposition. Some of the principal classes of odorous compounds include amines, sulfides, volatile fatty acids, indoles, skatoles, phenols, mercaptans, alcohols, and carbonyls. If decomposition proceeds to completion, the resulting gases are carbon dioxide and methane, both of which are odorless. Some of the specific gases emitted (ammonia, methane, and carbon dioxide) have implications for global warming and acid rain issues. It has been estimated that one-third of the methane produced each year comes from industrial sources, one-third from natural sources, and one-third from agriculture (primarily animals and manure storage units). Although animals produce more carbon dioxide than methane, methane's contribution to the greenhouse effect is 15 times that of an equal amount of carbon dioxide.
Airborne emissions. Dust, pathogens, and flies are also airborne emission concerns from animal operations. Dust, a combination of manure solids, dander, feathers, hair, and feed, is difficult to eliminate from animal production units and tends to be more of a problem in buildings that have solid floors and use bedding as opposed to slatted floors and liquid manure. Dust emission rates are mostly unknown from animal production sites. Although pathogens are present in buildings and manure storage units, pathogens typically do not aerosolize but can be transported by dust particles. Flies can be a concern for some poultry and livestock operations, and large populations of flies can be produced relatively quickly in these operations.
Emission movement or dispersion. Dispersion of airborne emissions from an animal production facility is difficult to predict and is affected by topography, prevailing winds, and building orientation. Odor plumes decrease exponentially with distance, but long distances are needed if no odors, gases, or dust are to be detected downwind from a source. Recommendations exist for separation distances of animal production facilities from residential developments and other public and private areas where people live and work. A number of models are being developed to more accurately predict setback distances from livestock and poultry operations based on animal units or actual emission values. Prevailing winds should be considered so facilities are sited to minimize gaseous transport to close or sensitive neighbors. For many existing facilities, this consideration is impossible, and emission reduction techniques may be needed. Producers are encouraged to closely evaluate siting issues to better understand the potential for emission concerns. There is ample evidence that outdoor air quality issues have become a major concern in the siting of animal production facilities. In Michigan, North Carolina, and Missouri odors have been reported to reduce property values of residential homes near livestock production facilities. Another concern is the reduced value of land near livestock and poultry units for outdoor recreational activities. A siting assessment tool, such as that currently under development at Iowa State University and other institutions, can assist in identifying facilities or land application sites that present the greatest and least risk of causing concerns.
Research at Iowa State University. In addition to development of a siting tool, research is ongoing at ISU to quantify emissions, both at the sources (houses and storages) and downwind. These efforts address multiple species and management practices and represent interdisciplinary interactions within the university and across the United States. To learn more about ongoing air quality research efforts at ISU as well as other research efforts and to access ISU Extension publications regarding odor control, see the Animal Agriculture and Air Quality Web site at http://www.extension.iastate.edu/airquality/.
Materials contained in this article have been adapted form the Livestock and Poultry Environmental Stewardship Curriculum supported by CSREES, USDA; U.S. EPA, National Agriculture Assistance Center; and the University of Nebraska Cooperative Extension University of Nebraska-Lincoln under Cooperative Agreement Number 97-EXCA-3-0642.
