Animals generate a significant amount of wastes (urine, feces, etc.) and the management and disposal of these wastes can be problematic. Currently, most livestock waste is applied to fields as fertilizer. However, animal wastes can potentially be used for bioenergy through the capture of biogas (from anaerobic digestion), syngas (from gasification), or bio-oils (from pyrolysis). Components of manure may also be recovered for use in producing bioproducts.
Beef production in the U.S. is divided into roughly two sectors: cow-calf operations and cattle feeding. Cow-calf operations are located throughout the country, are typically small, often operated by part-time farmers or retired persons, and involve letting the cows feed on range or pasture. Cattle feeding operations produce high-quality beef by feeding grain and other concentrates until the yearlings are ready for market. Cow-calf operations care for the calves until weaning or yearling stage, at which time the calves are usually sold to cattle feeding operations. About 70% of commercial U.S. beef operations use this system of production.
In 2004, there were 42.1 million head of beef cows, 5.6 million head of replacement beef heifers, and an additional 44.24 million head of cattle on inventory (combined beef and dairy steers, bulls, calves, and non-replacement heifers). Most cow-calf operations are small. About 91% of all farms with beef cattle had fewer than 100 head in 2002 (about 48% of the total number of cows) and 52% of beef farms had fewer than 20 head (about 12% of the total number of cows). Cow-calf operations widely distributed throughout the U.S. (figure 1).
The majority of cattle on feed are marketed by large feedlot operations. While feedlots with a capacity of less than 1,000 animals represented nearly 98% of all feedlots, they accounted for less than 15% of the cattle marketed in 2002. Feedlots with a capacity of greater than 16,000 head (0.28% of all feed lots) and 32,000 head of cattle (only 0.1% of the total feedlots) accounted for 60% and 42%, respectively, of the cattle marketed 2002. Large feedlot operations (capacity of ≥ 16,000 head) are concentrated in Arizona, California, Colorado, Kansas, Nebraska, and Texas.
The amount and characteristics of wastes produced per animal depends on a number of factors, including animal size, weight and type of feed. Forage is the principal feed source in cow-calf production. Cattle on feed get most of their nutrients from corn and concentrates, which are more digestible than forage. This reduces the solid content of the manure. Physical properties of manure that are 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.
A number of studies estimate the average amount of manure produced per animal. Estimated quantities differ by research source, due to different assumptions regarding feed ration composition and the efficiency of conversion of feed to meat, among other factors. Table 1 provides beef manure characteristics by animal type, as estimated by USDA-NRCS (1999), and are based on average amounts as excreted from the animal (i.e., don’t contain bedding or other materials).
The USDA estimates that in 1997, 12.2 million tons (dry matter) of manure was generated on beef feedlot operations. Of this total, 10.65 million tons were on operations where the cattle were confined for at least part of the year, and where sufficient amounts of manure accumulated to require regular removal. An additional 79 million tons were generated by non-dairy, non-feedlot cattle, of which about 8% of the wastes were from operations where the cattle were confined, with the remainder mostly from cow-calf operations on pasture.
Manure wastes generated from large beef feedlot operations are the highest environmental concern. The manure that is collected from such operations is mostly disposed of via field application as a fertilizer. Sometimes field application is in excess of the nutrient assimilation capacity of the soil, leading to problems associated with nitrogen and phosphorus runoff. Concentrated animal feeding operations (CAFO, which for beef feedlot operations means 1000 or more beef cows based on the average annual number of cows in inventory or sold) must be permitted under the National Pollutant Discharge Elimination System (NPDES), and must have a nutrient management plan for wastes applied to the field as fertilizer. A nutrient management plan is voluntary for non-CAFO operations.
Removal of manure in beef feedlot operations generally involves scraping using tractor mounted blades, although in some totally enclosed operations, flushing with water is also used. As a result, the manure is generally collected in either solid or semi-solid form. Solid forms of manure are inappropriate for anaerobic digestions, but can be used in other bioenergy technologies (such as gasification and pyrolysis).