Chemicals are used extensively in animal industries, from production of the feed base through to processing of animal products. Concerns over chemical use arise from the possibility of residues in animal products, resistance of some organisms (most notably bacteria and parasites), environmental consequences and carcase damage. Many traits targeted by chemicals are heritable, so breeding can sometimes achieve similar ends. In many applications though, chemicals used for improved production will be augmented by, rather than replaced by genetic improvement. Breeding for disease resistance is an exception, as it should often be possible to reduce reliance on veterinary chemicals. The optimal emphasis to place on resistance in breeding programs depends on a number of factors, some of which are difficult to measure. Genetic correlations between disease resistance and productivity appear to be slightly unfavourable in a number of species, so that if resistance is ignored in breeding programs, reliance on other means of control, including chemical therapy, may increase. Public acceptance of transgenic animals that produce abnormal levels of chemical substances, even those naturally occurring, should not be taken for granted.

R. R Woolaston

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 27: Reproduction; fish breeding; genetics and the environment; genetics in agricultural systems; disease resistance; animal welf, , 145–152, 1998
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