Introduction

Animal agriculture in the form of cattle, pig, sheep, poultry and fish farming represents a major aspect of food production worldwide. While many of these animals are produced for their meat alone, others contribute to human nutrition by way of milk and egg production, and yet others produce products such as wool and leather. In the developed world animal production is highly intensified and technologically driven. Animal production will reflect quality of feed, availability and need of growth hormones, pesticides, antibiotics and vaccines, good animal husbandry and welfare, and increasingly selective breeding, molecular biology, embryo manipulation and gene transfer.

As with plant agriculture early animal breeders sought to identify worthwhile properties in animals and to perpetuate them into future offspring. Selective breeding aims to increase the frequency of a large number of genes that work together with the remainder of the animal's genes or genome to produce the desired phenotype (selective breeding pre-dates our understanding of genes and the science of genetics). Between 1945 and 1993, selective breeding increased milk production of the average dairy cow by a factor of three. Further, when we consider the huge variety of dogs it should be remembered that they are all one species, capable of easily interbreeding, and the present varieties have arisen by carefully controlled selective breeding programmes. For many farm animals conventional breeding has already achieved high-producing animals, but it is apparent that the increases in productivity possible by this means now seem to be approaching a plateau.