Abstract

Recombinant bovine somatotropins (rbST) have been synthesized and manufactured using recombinant DNA (rDNA) techniques. The rbST has increased milk production in dairy animals, including cow, sheep and goats. There are management factors including dosage of rbST, injection interval, genetic potential and environmental conditions which affect the magnitude of the animal-response to rbST. For dairy cows, the optimal dose of rbST treatment is between 25 and 50 mg/day. Daily injection of rbST may produce better response, but sustained-release formulations of rbST are more practical. The magnitude of milk production response to rbST range from 10 to 35% in dairy cows. The rbST has no or little effects on the milk composition, processing properties and taste. The rbST did not affect digestion of feeds or the efficiency of utilizing absorbed nutrients for milk synthesis, and it has no or little effect on rumen fermentation, flow of nutrients to the small intestine and total tract digestibility, but it improves the feed efficiency in dairy cows. The rbST induced significant increase in serum bST. It has a galactopoietic effect, since it increase the metabolic activity of mammary cells or slow their involution, thereby allowing more secretory cells to persist over time and contribute to increasing milk synthesis and yield. It has a great impact on mammary gland development and subsequently milk-producing capacity in dairy heifers. The rbST increase the hypothyroid status of lactating cows and maintain euthyroid condition in the mammry gland, thus enhancing its metabloic priority. The rbST did not adversely affect reproduction, and the observed decreases in reproductive performance in rbST-treated cows may be attributed more to the increases in milk yield than to direct effect of rbST. The increased production responses to rbST in heat-stressed dairy cows is less than that under more moderate conditions. The incidence of mastitis in rbST-treated cows is due more to increased milk yield than to any direct effects of rbST. The rbST was efficacious in increasing milk yields in sheep and goats without adverse effects.

Key words: Recombinant bovine somatotropin, dairy animals, physiological responses.