Impact of metabolism modifiers on protein accretion and protein and energy requirements of livestock1

Somatotropin (ST) and β-adrenergic agonists (β-agonists) have afforded scientists a significant opportunity to alter nutrient metabolism and, thereby, the partition of absorbed nutrients for lean tissue growth. The potential is well illustrated with growing swine treated with porcine ST during the 50 to 100 kg phase of growth. Relative changes and levels achieved for both protein and lipid deposition are unprecedented. Data presented herein also demonstrate that the biological response to bovine ST in growing cattle may have previously been constrained by inadequate provision of indispensable amino acids. The effects of both ST and β-agonists on protein (amino acids) and energy utilization are primarily postabsorptive. Somatotropin improved the partition of absorbed amino acids to protein deposition in growing pigs, cattle, and sheep fed conventional diets by as much as 20 to 40%. Accordingly, the opportunity exists to minimize a significant inefficiency in nutrient expenditure. Whether an increase in protein intake is required to accommodate the ST-stimulated increase depends on the partial efficiency of absorbed amino acid use for protein deposition. The dynamic natures of these and other components were considered using simple and complex models to estimate how selected indispensable amino acid and nitrogen needs would be altered for growing swine and cattle. The impact of β-agonists on the partial efficiency of amino acid use for protein deposition remains unclear because the data are not sufficiently quantitative. Of special interest, and in contrast to ST, is the differential muscle growth that characterizes the response to p-agonists. This suggests that protein input for muscle growth may be less because of conserved visceral growth and that the pattern of dietary amino acids needed to accommodate the tissue requirement for protein deposition in pigs treated with β-agonists may differ from that in pigs fed conventional diets or treated with ST. Neither metabolism modifier improves me efficiency of energy use. In fact, a subtle decrease may exist because the protein component of energy gain is inherently inefficient compared with lipid. This would increase maintenance energy expenditure. Thus, any opportunity for improvement in energetic efficiency may exist only to the extent that the daily maintenance requirement is diluted via more rapid growth.