Modern breeds of cattle, pigs, and sheep have been selected for production of muscle mass and against production of fat mass. Within a species, there is considerable diversity among breeds in body composition. Much of the diversity results from different rates of growth and the age at which mature body composition is achieved. Thus, a comparison of breeds at a given weight may compare breeds at different stages of maturity.

Genetic selection has been and continues to be very effective in changing the composition within a breed. The high degree of heritability of fat growth is a major contributing factor. There are extremes of muscling in several breeds of cattle and in sheep. Several experimental strains of pigs have been genetically selected for extreme fatness.

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