PHYSIOLOGICAL AND METABOLIC IMPLICATIONS OF CONVENTIONAL AND NOVEL METHODS FOR THE MANIPULATION OF GROWTH AND PRODUCTION

Growth-promoting strategies, both current and future, can only be most effectively introduced when the physiological consequences and nutritional requirements of the manipulation are understood. Beta-agonists increase protein gain, in part at least, by a reduction in protein breakdown, similar to the situation with anabolic steroids. Possibly as a consequence of this, the response is independent of feed quality. The specific myotrophic action of the beta-agonists is achieved at the expense of the growth of other tissues, including skin and fibre, and thus, while having potential for meat production, may have deleterious effects in other husbandry conditions. Alterations in the competitiveness of various organs for nutrients can produce marked anabolic and catabolic effects. The gastrointestinal tract represents a major contributor for both protein and energy dynamics in the animal, and manipulations which reduce the metabolic demands of the tissue (e.g. antibiotics, probiotics) increase nutrients available to other tissues, while stimulators of gastrointestinal secretions and hyperplasia (e.g. parasites, antinutritional factors) have an inhibitory effect on peripheral tissue anabolism. Growth promotion through stimulation of protein synthesis may be dependent on amino acid supply, and there is considerable evidence that effective use of growth hormone is achieved only at medium to high levels of dietary protein. This will have implications for future manipulation of the somatotrophic axis via immunological and biotechnological means. Protein accretion may also be enhanced by procedures which divert substrates away from fat. This has been demonstrated in rodents by the use of antibodies against fat cell membranes, although whether the protein anabolic response occurs by 'passive' (increased substrate availability) or 'active' (alterations in endocrine status) mechanisms is not known.