Recombinant growth hormone : potential interest and risks of its use for bovine milk production. A review

Abstract

The administration of recombinant bovine growth hormone (rbGH) increases milk yield by 2-6 kg/d, according to cow age, lactation stage and nutrional status, injected dose and slow-release formulation of the hormone. Effects on milk composition and body reserves depend on the duration of trials and the kind of diets used. GH increases activity and/or longevity of mammary secretory cells, probably via IGF-1 produced by the liver and/or the mammary gland. Simultaneously, GH orients adipose tissue and muscle metabolism towards increased fatty acid mobilization and oxidation, and glucose sparing. The administration of rbGH has few direct effects on the reproductive function, but tends indirectly to delay it, when starting before fecondation, due to the transient decrease in energy balance of the cows.

The milk processing ability does not seem to be changed per rbGH, nor the increase in milk IGF-1 secretion to create a risk for consumers. GH is probably stimulating immunological responses of animals, and hence increasing the milk cell count. On the other hand, a trend towards increased mastitis frequency seems to result indirectly from the increased milk yield. GH at high concentration can stimulate viral yield in some "in vitro" models in monogastric species, but there are until now only few data obtained in dairy ruminants. Furthermore, the clearance rate of xenobiotics (antibiotics..) by the liver could be decreased.

A putative commercial use of rbGH would decrease the efficiency (or increase the cost) of genetic selection, and would be of limited economic value for farmers in a milk quota system. However, its use in other countries without quotas could decrease the cost of milk yield and increase the competition on the world market of dairy products, despite the risks of decreasing the image of these products for the consumers.