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 (grade select or higher) by feeding grain and other concentrates (70-90% of the total feed ration) for an average 140 days (range from 90 to 300 days, depending on the weight of the calves at the time they begin feeding). Cow-calf operations care for the calves until weaning or sometimes until they are yearlings (depending on economic conditions), 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 (ERS, 2005; Short, 2001).
In 2004, the inventory of beef cows and beef heifers available as replacement stock was 42.1 million and 5.6 million head respectively. The USDA identifies beef cows and milk cows, and beef and dairy heifers that will become replacement stock, but does not distinguish between dairy and beef for other cattle categories. Thus, there were an additional 44.24 million head of cattle (combined beef and dairy) on inventory in 2004 (table 1). Most cow-calf operations are small (table 2) and are widely distributed throughout the U.S. (figure 1).
The majority of cattle on feed are marketed by large feedlot operations, concentrated in only a few states. While feedlots with a capacity of less than 1,000 animals represented nearly 98% of the 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) accounted for 60% of the cattle marketed and feedlots with a capacity of more than 32,000 head of cattle (only 0.1% of the total feedlots) accounted for 42% of the cattle marketed in 2002 (table 3). Large feedlot operations (capacity of ≥ 16,000 head) are concentrated in Arizona, California, Colorado, Kansas, Nebraska, and Texas (231 of the 262 feedlots) (table 4) (NASS, 2004).
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 (NRCS, 1999).
Estimates of the average manure produced per animal and the composition of the manure can be found in a number of sources such as the National Resource, Agriculture and Engineering Service (NRAES), the American Society of Agricultural and Biological Engineers (ASABE), North Carolina State University, and the USDA National Resource Conservation Service (NRCS), among others. Estimated average quantities of manure per unit and manure characteristics differ by research source, due to different assumptions regarding feed ration composition, the use of feed additives such as phytases (used to reduce phosphorus), and the efficiency of conversion of feed to meat, among other factors. Table 5 provides beef 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. Wastes characteristics 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 (USDA-ERS, 2001).
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 (Ribaudo, 2003). 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 (Ribaudo, 2003).
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 (EPA, 2001). Solid forms of manure are inappropriate for anaerobic digestions, but can be used in other bioenergy technologies (such as gasification and pyrolysis).