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.
Unlike the poultry and swine industries, which are characterized by production contracts and vertical integration (a single firm controls two or more successive stages of the production, processing, and/or marketing), the dairy industry is composed primarily of independent farmers who make the milk production decisions. In 2002, almost 85% of dairy farms were either individual or family-operated businesses. Milk production is the sole or most important activity on most dairy farms.
The traditional milk producing states of Minnesota, Michigan, New York, Pennsylvania, and Wisconsin continue to be large producers, but other states, particularly in the western U.S. (California, Idaho, New Mexico, Texas, and Washington) represent an increasing share of production (figure 1). Milk production in the Southeast is declining.
The total number of milk cows have been declining over time, but total milk production has increased due to higher milk production per cow. In 2004, there were 9 million milk cows on dairy farms, and an additional 4 million head of heifers (juveniles who have not yet started to produce milk) kept as replacement stock for older milk cows when they are retired from production.
Dairy production is shifting to fewer farms with larger herds. In 2006, the number of farms with greater than 1,000 and 2,000 milk cows was 1443 and 573, respectively, and accounted for 37.3 and 63% of milk production, respectively. Dairy operations in the western U.S. tend to be much larger than those in other parts of the country.
The amount and characteristics of wastes produced per animal depends on a number of factors including breed, size, type of feed, and whether or not the animal is producing milk (lactating or dry). Dairy production is shifting from pasture/forage feeds to the use of corn and other grains, which affect the solid content of manure. 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.
A number of studies estimate the average manure produced per animal and the composition of the manure. Estimated quantities and characteristics differ by study, due to different assumptions regarding feed ration composition, the use of feed additives to reduce phosphorous levels, and the efficiency of conversion of feed to milk, among other factors. Table 3 provides dairy manure characteristics by animal type, as estimated by USDA. Estimated waste quantities and characteristics are based on average amounts as excreted from the animal (i.e., don’t contain bedding or other materials). Dairy stalls are often covered with bedding, and the quantities and characterization of bedding influences the composition of the manure.
The USDA estimates that in 1997, 27 million tons (dry matter) of manure were generated by dairy operations, of which 21.3 million tons were on operations where the cattle were confined for at least part of the year.
Dairy 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 dairy production means 700 or more dairy 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.
Dairy manure is collected in both solid and liquid form (EPA, 2001). Collection in solid form generally involves scraping the manure to the end of the barn for temporary storage. In barns equipped with an alley way, the manure may be flushed and deposited in a storage pit or lagoon. Milking parlor wastes (i.e. the area where the cow is milked) are dilute and contain little manure. Liquid storage systems are more common in the southern U.S. (66% of operations in 2000) relative to the northern U.S. (29% of operations).
The problem of excessive application of manure for fertilizer is
particularly important for large dairy operations that store manure wastes as liquids, due to the large volumes of manure produced and the frequency of manure collection. The USDA identified 68 counties where manure nitrogen levels exceed the soil nutrient assimilative capacity of all the county’s crop and pasture land (primarily in North Carolina, northern Georgia, Alabama, central Mississippi, western Arkansas, and California) and 152 counties (concentrated in eastern North Carolina, northern Georgia, northern Alabama, western Arkansas, central California, and western Washington) where the manure phosphorus levels exceed the county assimilative capacity. Additionally, 155 and 337 counties were identified where manure nitrogen and phosphorus levels, respectively, exceed half of the county soil nutrient assimilative capacity. These counties are most in need of alternative waste management methods.
The EPA estimates that as of 2005, approximately 2,623 dairy farms were good candidates for biogas collection and bioenergy production. Nearly 38% of these farms were located in California, and about 80% of the candidate farms were located in the top 10 dairy production states.