Increased specialization has accompanied increased productivity. There has been little change in the number of pigs in the United States since 1920. The number of cows being milked peaked at 25 million in 1944 and has since dropped to about 9 million. Milk production per cow increased markedly from 2073 kilograms per year in 1944 to more than 8,000 kiograms per year in 2001 (USDA, 2002c, 2002d). Annual production of livestock and products has risen steadily over the past century, although production cycles are evident in the data. Also evident is a steady increase in livestock productivity (defined here as the quantity of meat, milk, and eggs produced annually from a given inventory of livestock). Productivity gains arise from an increased number of animals born and raised per breeding animal per year, increased growth rates and market weights of animals intended for slaughter, and increased milk or egg production per animal per year. In addition to producing more from a given inventory of animals, livestock farmers have greatly decreased the quantity of feed required to produce a pound of meat, milk, or eggs. Productivity gains have been accomplished through genetic selection, as well as through improvements in diet formulation and processing, housing and environmental controls (e.g., improved buildings, manure removal, and ventilation), veterinary medical care and medications, and management. Havenstein and colleagues (2002) demonstrate that a 2001 strain of broiler chicken fed a current diet requires about one-third the feed and one-third the time to produce a 4.0 pound (lb) live broiler as a 1957 genetic strain chicken fed a diet used in 1957. Since modern broilers are grown to heavier weights, the actual efficiency gains are altered. The modern broiler raised to 5.9 lbs in six weeks requires about 27 percent of the time and 42 percent of the feed per pound of live bird that the 1957 strain required. The 1957 strain required about 103-105 days to produce a 4.0-pound bird. These productivity gains are consistent with those cited by Martinez (2002, Table 3). Note that reduced feed consumption per pound of product results in a proportionally larger reduction in the quantity of excreta on a dry weight basis. For example, if feed consumption is reduced to 42 percent of the original quantity, and if 15 percent of the original quantity was and is retained in the product, then the dry weight excreta would be 31.7 percent of the original quantity excreted ([0.42 − 0.15] / [1.0 − 0.15]).

Dairy. In the United States, there were 79,318 dairy farms with more than three milk cows reported in the 1997 census of agriculture (Kellogg, 2002). Of these, 16 percent were very small (<35 USDA animal units [AUs]), 33 percent were small (35 to 70 USDA AUs), 40 percent were medium sized (70 to 210 USDA AUs) and 9.8 percent were large (>210 USDA AUs). USDA animal units differ from Environmental Protection Agency (EPA) animal units (Appendix E) and are equivalent to 454 kg (1000 pounds) live weight accounting for all animals on the farm. In contrast to other food animal industries, the dairy industry is not