Genetic engineering of micro-organisms to produce mammalian protein hormones promises to make the hormonal treatment of ruminants for the stimulation of lactation, growth and wool growth commercially viable. Treatment of lactating dairy cows with growth hormone (GH) commonly elevates milk production by about 4 kg milk/d. In growing beef and sheep GH increases nitrogen retention, reduces carcass fat content and may increase growth rate. Wool growth increases following the cessation of GH treatment. Many of the actions of growth hormone are likely mediated by a 'family' of hormones, notably insulin-like growth factors I and II and epidermal growth factor. The efficacy of these materials in promoting production has not been investigated. Sustained-release implants have been developed which will deliver protein hormones over prolonged periods. However, the ability to promote production by direct manipulation of the animal genome may be the most economic approach. For example, injection of rat GH genes into fertilised mouse eggs resulted in mice that grew 2 to 4 times faster than normal.
Proceedings of the New Zealand Society of Animal Production, Volume 44, , 83-86, 1984
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