selective breeding after the introduction of gene constructs into the founder generation to ensure that animals display the desired phenotype with few or no undesirable side effects.
However, it is clear that serious welfare problems also have resulted from traditional breeding techniques. Broiler chickens are a case in point. Breeding for increased growth has led to serious physical disabilities, including skeletal and cardiovascular weakness. A large percentage of broilers have gait abnormalities (Kestin et al., 1992), and these might be painful, making it difficult for the birds to walk to feeders and waterers. In addition, broiler hens must be severely feed restricted to prevent obesity, and this feed restriction is associated with extreme hunger and a variety of behavioral problems, including problems with mating behavior and hyperaggressiveness (Mench, 2002; Kjaer and Mench, in press). Traditional selection of pigs for increased leanness has led to increased excitability during handling (Grandin and Deesing, 1998), and selection for high reproductive rates (either by shortening the interval between births or increasing the number of offspring born) or increased lactation (Chapter 1) also has led to welfare problems. In their report, The Use of Genetically Modified Animals, the Royal Society (2001) concluded: “Although genetic modification is capable of generating welfare problems…no qualitative distinction can be made between genetic modification using modern genetic modification technology and modification produced by artificial selection.” Several ethical frameworks for evaluating the animal welfare implications of biotechnologies applied to animals have been proposed in an attempt to resolve this difficulty. For example, Rollin (1995) has proposed the use of the “principle of conservation”, which states that transgenic and cloned animals developed for agricultural uses should not be worse off than the founder animals or other livestock of the same species under similar housing and husbandry practices.
Genetic engineering certainly has the potential to improve the welfare of farm animals. Decreasing mortality and morbidity by increasing resistance to diseases or parasites, or decreasing responses to ingestion of toxic plants, are obvious examples of welfare benefits, and an area in which some transgenic research is focused (Müller and Brem, 1994; Dodgson et al., 1999). It also has been pointed out that transgenic animals might receive a higher standard of care than nontransgenic animals because of their greater economic value (Morton et al., 1993). Cloning could be used as a strategy for breed preservation to maintain genes that are important for adaptation and resistance to disease, but equally could result in a further narrowing of the gene pool, with possibly deleterious effects on animal health (Chapter 2).