Livestock production in the U.S. is dominated by beef, dairy, swine, and poultry production, but other livestock are produced as well (table 1). Sheep (produced for both wool and meat) represent the largest category. Goats are the source of angora and mohair, and are also produced for meat and milk. A substantial number of horses and ponies are maintained in the U.S. Most other livestock are produced in small quantities.
Sheep production ranks fifth behind beef, dairy, swine, and poultry production, both in terms of total number of animals and total value. While there are numerous small sheep producers, nearly half of the sheep and lamb inventory reside on the less than 1.5% of farms with flocks of greater than 1,000 head (table 2).
Sheep production has been declining over time. Peak production occurred in 1942 and totaled 56 million animals. In 1997, there were 8.08 million sheep and lambs on inventory. That number had fallen to 6.34 million by 2002. Unlike for the major livestock classes (in which the number of farms have decreased, but the number of animals per farm has increased), the number of sheep farms has declined in every size class, except the smallest size (1-24 animals) which showed a slight increase (from 43,586 farms to 44,562 farms) between 1992 and 1997. Figure 1 shows the distribution of the sheep and lamb inventory for 2002.
The amount and characteristics of wastes produced per animal depends on a number of factors including species, size, type of feed, and function. Feeds that are less digestible increase the total solid content of the waste, relative to more digestible feeds. Physical properties of interest include weight, volume, total solids and moisture content, as these properties describe the amount and consistency of the material that must be dealt with by equipment and in treatment and storage facilities. Chemical constituents (nitrogen [N], phosphorus [P], and potassium [K]) are important in use of livestock wastes as fertilizers, as well as for environmental considerations (NRCS, 1999).
Estimates of the average manure produced per animal and the composition of the manure can be found in a number of sources, including the National Resource, Agriculture and Engineering Service (NRAES), the American Society of Agricultural and Biological Engineers (ASABE), North Carolina State University, and USDA National Resource Conservation Service (NRCS), among others. Estimated average quantities of manure produced per unit and manure characteristics differ among studies, due to different assumptions regarding feed ration composition and the efficiency of conversion of feed to meat and milk, among other factors. Table 3 provides manure characteristics by animal type, as estimated by USDA-NRCS (1999). Waste characteristics are expressed as pounds per day per 1,000 pounds of livestock live weight (lb/d/1000#) for weight, total solidsa, N, P, and K; as cubic feet per day per 1,000 pounds of livestock live weight (ft3/d/1000#) for volume; and percent (wet basis) for moisture and total solidsb. Waste characteristics are based on average amounts as excreted from the animal (i.e., wastes don’t contain bedding or other materials) (USDA-NRCS, 1999).
Information regarding the collection and management of manure for these livestock species is lacking. It seems likely that a substantial number of the animals spend a significant portion of their time on pasture, rather than pens or barns, and thus much of the waste may not be collected. For veal production, sanitation is an important factor; therefore, collection facilities need to be able to accommodate at least three gallons of wash water per day per calf. For horses, large amounts of bedding are typically used in stables, so the qualities and quantities of wastes are generally dominated by the kind and volume of bedding used (USDA-NRCS, 1999).