An animal unit (AU) is a measure that represents 1,000 pounds of live weight of farmed animals. It can be used to measure the total weight of a single type of animal on a farm or on a group of farms. It can also be used to aggregate different types of farmed animals at different lifecycle stages.[1,2]
Gollehon, N. R., et al., (2016). Estimates of recoverable and non-recoverable manure nutrients based on the census of agriculture—2012 results. USDA Natural Resources Conservation Services, Washington, DC, p. 2.
Note that the EPA has also used the term “animal unit” in the past, with a different and less precise meaning. We think they have discontinued using the term. See, Gollehon, N. R., et al. (2001). Confined Animal Production and Manure Nutrients, USDA Economic Research Service, Bulletin No. 771, p. 7, fn 8. [“Our (the USDA’s) definition of an animal unit based on 1,000 pounds of live weight should not be confused with the Clean Water Act (CWA) specification of “1,000 animal units.” …The act specified an animal per animal unit conversion only for that purpose… These specifications of animals per animal unit have proven to be confusing because they are not complete and are not based on a common specification…”]
Kellogg, R. L., et al., (2014). Estimates of recoverable and non-recoverable manure nutrients based on the census of agriculture. USDA Natural Resources Conservation Service, Table 1, p. 4; Table 2, p. 6. [Tables have additional details on AUs of different species and at different lifecycle stages. Of the many estimates of AUs, we think these are the most credible – by USDA scientists that spent decades in this area of research. Naturally, average animal weights continue to change, but we are not aware of a more recent credible source.]
Kellogg, R. L., et al., (2000). Manure nutrients relative to the capacity of cropland and pastureland to assimilate nutrients: Spatial and temporal trends for the United States. USDA Economic Research Service, Table 1, p. 3. [We use this older report for “pigs for slaughter” only. The 2014 report breaks down AUs for pigs according to type of farm, i.e., “finish,” “farrow to finish,” etc., whereas the 2000 report gives an average for all “hogs for slaughter.”]
The concept of animal units is mainly used to simplify the process of evaluating amounts of manure, more particularly to analyze nutrients in manure. The measure can be applied across large numbers of animals categorized according to their functions, lifecycle stages, and degrees of confinement.[1,2]
Kellogg, R. L., et al., (2014). Estimates of recoverable and non-recoverable manure nutrients based on the census of agriculture. USDA Natural Resources Conservation Service, pp. 3-9.
U.S. EPA (2013) Literature Review of Contaminants in Livestock and Poultry Manure and Implications for Water Quality, EPA 820-R-13-002, Appendix 1, p. 109.
AUs based on question above.
Manure amounts based on Kellogg, R. L., et al., (2014). Estimates of recoverable and non-recoverable manure nutrients based on the census of agriculture. USDA Natural Resources Conservation Service, Table 5, p. 33. (Note: Hogs for slaughter based on Table 5 “Confined livestock types 1982–1997.”]
American Society of Agricultural Engineers (2003) Manure Production and Characteristics, ASAE D384.1 FEB03, Table 2, p. 684. [Though we have not relied on these figures, this report is also considered highly credible for amounts of manure generated by different types of animals.]
The EPA estimated that cattle and dairy cows generated ~83% of total manure in 2007. Pigs generated ~10% and poultry ~7%.[1] This is based on an analysis of the 2007 Census of Agriculture. Most likely, this is still a reasonably accurate figure, with pig and poultry figures now slightly higher. A large portion of the cattle manure and some of the dairy manure is generated on pasture.
U.S. EPA (2013) Literature Review of Contaminants in Livestock and Poultry Manure and Implications for Water Quality, EPA 820-R-13-002, p. 8 & Appendix 1, p. 109.