Exogenous Somatotropin Growth

Somatotropin alters nutrient partitioning to improve growth performance and body composition, and pST has been approved commercially in 14 countries.[6'7] Exogenous pST results in dose-dependent increases in lean deposition and reductions in fat (Fig. 1). It is effective in increasing protein deposition and decreasing fat deposition in all sexes and genotypes. Although the greatest responses occur in finisher pigs (60 120 kg), exogenous pST also improves growth in younger pigs (30 60 kg). As a result of the reduction in fat deposition, there is a corresponding reduction in feed intake.[8] Since pST stimulates protein deposition in all tissues, there are increases in visceral and skin mass, and reductions in dressing percent. In general, lean tissue responses in growing ruminants have been less than in pigs, although much of these differences may relate to the difficulty of ensuring an adequate balance and quantity of amino acids to maximize response. Also, reductions in fat deposition and feed intake appear less in young ruminants compared to pigs.

There is little effect of pST on digestibility, so effects are due to an increase in the efficiency of use of dietary protein and/or an increase in the requirement of dietary protein to support the increased protein deposition. In grower pigs (30 60 kg), pST has little or no effect on dietary protein requirements, but there is an improvement in the efficiency of amino acid use. In finisher pigs (60 120 kg), pST has little effect on the efficiency of dietary protein use, but there is an increase in protein requirement commensurate with the increase in protein deposition.[6] As a consequence of the increased protein mass and protein synthesis, there is also an increase in maintenance requirement. That increase, when combined with the reduced intake, means that dietary energy may often limit the response to pST. Protein deposition in growing ruminants is virtually always limited by dietary energy consumption, and this may explain why ruminants treated with ST do not decrease feed intake, since energy spared from the reduction in lipid synthesis is partitioned toward protein deposition (or milk secretion in lactation, as discussed subsequently). Therefore, if the full benefits of exogenous ST are to be achieved, feed intake needs to be maximized regardless of species or physiological state.[9]

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