past animal agricultural practices, and particularly from those technologies that have reached fruition in the past half century.
Agricultural output of poultry and livestock in the United States exceeds $90 billion annually, of which around $11 billion consists of exports (USDA, 2001). There are currently about 9 million dairy cows, 5 million dairy heifers, and 85 million beef cattle and calves in the United States, and approximately 100 million hogs are slaughtered annually. However, trends in food consumption are changing. Even as the demand for red meat remains high, many consumers are changing their preferences from red meat to alternative protein sources. Americans consumed 82 pounds of chicken per capita in 2000 compared to 69.5 pounds of beef—a reversal of the situation a generation ago. Sales of farmed fish also have increased markedly as fish farming has become more productive and efficient. The main fish products traded domestically and internationally are shrimp (and prawns), Atlantic and coho salmon, and mollusks, but the market shares of tilapia, sea bass, and sea bream are increasing (Lem, 1999). Carp is, by far, the finfish type produced in largest quantity worldwide, with production about ten times that of salmon (FAO, 2000), but is primarily consumed domestically in Asian countries, rather than traded. Channel catfish constitutes the major species of finfish farmed in the United States (Lem, 1999). Per capita demand for high-quality meat and fish products is expected to increase both in response to rising world population and to improvements in the standard of living over the next 25 years (Pinstrup-Andersen and Pandya-Lorch, 1999). As a consequence of increased demand for meat and the deterioration and loss of agricultural land, there is pressure to utilize the potential for biotechnology to improve productivity in animal agriculture. As the techniques for producing transgenic animals become more efficient and as more is known about controlling how inserted genes are expressed, it is likely that the approaches soon can be integrated into agriculture. Indeed, the commercial production of transgenic fish, which is likely to occur worldwide, already is imminent.
Genetically engineered poultry, swine, goats, cattle, and other livestock also are beginning to be used as generators of pharmaceutical and other products, potential sources for replacement organs for humans, and models for human disease. The technology to produce foreign proteins in milk by expressing novel genes in the mammary glands of livestock already has advanced beyond the experimental stage, with some of the products currently in clinical trials (Colman, 1996; Murray and Maga, 1999). In theory, transgenic animals can provide milk that is more nutritious for the consumer, or that is enhanced for certain protein components that might be valuable for