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.
Poultry is domesticated fowl raised for meat and eggs; in the U.S., poultry includes chickens, turkeys, ducks, geese, emus, ostriches, and game birds. The poultry industry consists mainly of chickens, but also includes significant production of turkeys (table 1), with limited production of other poultry. Chickens include those whose primary function is for egg production (layers) and chickens produced specifically for meat (broilers). Layers are sold as meat (such as roasting hens), following their egg producing years. Most turkey production is for meat.
Beginning in the 1960s, the poultry industry began to vertically integrate (i.e., a single firm controlling two or more successive stages of the production, processing, and/or marketing). The industry has evolved - from millions of backyard flocks where meat was a by-product of egg production, to the current industry with less than 50 highly specialized firms. In 1934, there were 11,405 chicken hatcheries, with a total egg incubation capacity of 276 million eggs (an average capacity of 24,224 eggs/hatchery). In 2001, there were only 323 hatcheries, with an incubator capacity of 862 million eggs (average capacity of 2.7 million eggs/hatchery). Broiler production occurs mainly in 16 states (figure 1) (USDA-NASS, 2002).
Nearly all broiler production is produced under contract with growers in which the contractor (or integrator) provides the baby chicks, feed, and veterinary services. The grower provides the chicken houses and labor in return for payment per pound of live broiler produced, adjusted for the grower’s production performance (Martinez, 1999). Nearly 93% of the broilers sold in 2002 were produced on farms with greater than 100,000 chickens; over half of the production was in operations with greater 500,000 chickens, though those operations represented only about 17% of the total broiler operations (table 2).
Almost 92% of operations with layer chickens (producing eggs) had fewer than 100 chickens (table 3). This production is mostly for personal use. Commercial egg production is highly concentrated, with only 0.5% of the operations having over 75% of the total layers. More than a third of all eggs are produced under production contracts, and about 60% of egg production is managed by firms who control both the production and processing of eggs (Martinez, 2002). Egg production occurs in every state, but is concentrated in 10 states (figure 2).
Turkey production is also highly concentrated, with 85% of production occurring on about 18% of the operations in 2002. These operations were large, having more than 60,000 turkeys sold, with half having more than 100,000 turkeys sold (table 4). About 56% of turkeys are raised under contract, and around 32% of turkeys are raised by firms controlling both the production and processing of turkeys (Martinez, 2002). Turkey production occurs in several states, but is most prominent in just six states (figure 3).
The amount and characteristics of wastes produced per animal depends on a number of factors including animal type, size, type of feed, and function. Feed rations consisting of feeds that are less digestible increase the total solid content of the waste, relative to more digestible feeds. Physical properties of waste 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, in treatment, and in 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).
Both 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 and characteristics differ 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 and eggs, among other factors. Table 5 provides poultry 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 do not contain bedding or other materials) and for systems that use a floor (litter) production system (USDA-NRCS, 1999).
The USDA estimates that in 1997, 17.86 million tons (dry matter) of manure were generated by poultry operations, of which 17.5 million tons were on operations where the chickens and turkeys were confined (USDA-ERS, 2001). The poultry litter 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 poultry production means at least 100,000 chickens or 55,000 turkeys - based on the average annual number of chickens or turkeys 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.
Broiler manure is usually collected and handled as solid litter from shelters where bedding has been placed on the floor. Typically, a grower will have five to six flocks of broilers per year. In between each new flock, the top 1-2 inches of litter material (referred to as cake) is removed and 2-3 inches of clean bedding material (e.g., wood, straw, etc.) is added. The houses are thoroughly cleaned once or twice a year (EPA, 2001; USDA-NRCS, 1999).
Some layer facilities (such as those using high rise houses for layer chickens, without the use of bedding) contain shallow pits to catch litter and store it in a semi-solid or liquid form. This type of manure management is most commonly used for laying hens. Lagoons are typically used only for layer chickens housed with flush-type pit systems, and in some duck production systems. This method of manure management is widely used in the Southeast (41% of farms), and to some extent in the West (12% of farms), but not in other regions of the U.S. Lagoons are also used to collect the egg wash water. Eggs from laying hen houses contain traces of manure on the shell; the water used to wash the eggs is collected in lagoons (EPA, 2001; EPA, 2006).