Improving disease resistance to decrease pain and suffering is an application of transgenic technology that has clear animal welfare benefits. But it should be stressed that animal welfare is multifaceted, and this needs to be taken into account when assessing welfare impacts of the application of any technology—not just biotechnology. Important elements of animal welfare include freedom from disease, pain, or distress; physiologic normality; and the opportunity to perform normal behaviors (Broom, 1993). While reducing disease clearly is beneficial, if this also permits animals to be confined more closely, and thus decreases the opportunity for them to perform their normal behaviors, then the net effect on welfare could be negative.

Genetic engineering also could be used to deal with non-disease related welfare problems. It might be possible, for example, to engineer hens that produce only female offspring (Banner, 1995). This would eliminate the problems associated with surplus male chicks, which are killed at the hatchery. The need for the so-called standard agricultural practices like castration and dehorning also could be reduced or eliminated by genetic engineering. Pigs are castrated to prevent boar taint in the meat, but this trait is strongly linked (genetically) and thus is amenable to genetic manipulation. Similarly, horns on cattle, which are removed because they cause injuries to humans and other cattle, are the result of a single gene that could be knocked out by genetic manipulation without affecting other desirable performance traits; genetically polled (hornless) breeds of cattle already are available, and are produced by selective breeding.

In making assessments about the production of genetically engineered animals for farming, costs and benefits need to be weighed carefully. When expression of growth hormone is regulated appropriately in transgenic pigs, for example, the increases shown in growth and feed efficiency are modest, and are similar to the increases that can be attained simply by injecting pigs with porcine growth hormone (Pursel et al., 1989; Nottle et al., 1999). Pursel et al. (1989) suggest that centuries of selection for growth and body composition might limit the ability of the pig to respond to additional growth hormone. Indeed, it is possible that we already have pushed some farm animals to the limits of productivity that are possible by using selective breeding, and that further increases only will exacerbate the welfare problems that have arisen during selection.

The potential for reduction in genetic diversity in agricultural species also is posed by inappropriate application of certain biotechnologies (Chapter 1). Transgenesis raises such concerns because each transgene integration event results in a genetically unique potential founder and only one founder normally