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Breeding

The breeding of farm animals seldom occurs spontaneously but is managed by farmers with a view to encouraging traits seen as desirable. These include hardiness, fertility, docility, mothering abilities, fast growth rates, low feed consumption per unit of growth, better body proportions, higher yields, and better fibre qualities. Undesirable traits such as health defects and aggressiveness are selected against.

Selective breeding has been responsible for large increases in productivity. For example, in 2007, a typical broiler chicken at eight weeks old was 4.8 times as heavy as a bird of similar age in 1957, while in the thirty years to 2007, the average milk yield of a dairy cow in the United States nearly doubled.