requirements for energy and protein would result with ST treatment. If intake is influenced by ST dose, nutrient density of diets may have to be increased to compensate. It remains to be demonstrated whether amino acid availability or balance (or both) limit the protein deposition response to ST in growing lambs and cattle fed conventional complete, mixed concentrate diets. Neither stepwise restriction of energy intake nor protein intake have been reported in lamb ST studies. Significant increases in weights of major organs such as liver, kidneys, and heart (Muir et al., 1983; Zainur et al., 1989; Beermann et al., 1990) indicate that small increases in maintenance requirement may be present. This increase is associated with proportional increases in organ and lean tissue weight, which resulted in more total lean tissue or protein mass per unit body weight. There is no apparent increase in maintenance requirement per unit of lean tissue in growing ruminants administered ST.

Limited data are available that describe long-term effects of GRF administration on growth and composition in growing cattle (Ringuet et al., 1988; Enright, 1989) and lambs (Wise et al., 1988; Byrem et al., 1989; Beermann et al. 1990). Multiple daily subcutaneous administrations of 5 µg hGRF/kg body weight increased average daily gain 13 percent and decreased feed: gain 18 percent in growing wether and ewe lambs treated for 6 or 8 weeks (Beermann et al., 1990). Doubling of the dose reduced feed intake 6 percent and resulted in no improvement in ADG. Carcass protein accretion rate was increased 30 to 35 percent, lipid accretion rate decreased 21 to 28 percent, and ash accretion rate increased 30 percent. Lambs in this study were fed a diet containing 16 percent crude protein and adequate energy. Subcutaneous infusion of a hGRF analog into lambs for 28 days increased rate of gain 16 percent and improved feed conversion 18 percent without effect on feed intake, wool growth, or carcass weights (Godfredson et al., 1990). Treated lambs contained less fat. A similar response was obtained with subcutaneous infusion of hGRF in wether lambs for 5 weeks (Byrem et al., 1989). Because the data indicate that responses nearly equal those achieved with ST administration, similar conclusions must be drawn for effects on nutrient requirements in lambs.

Effects of GRF on energy and nitrogen metabolism were recently investigated in growing beef steers fed a 75 percent concentrate diet at two levels of intake for 3 weeks (Lapierre et al., 1992). GRF treatment increased nitrogen retention 108 and 80 percent at the low (approximately 88 g/day) and high (approximately 159 g/day) levels of nitrogen intake. A significant increase in digestibility was observed, but most of the improvement resulted from reduction in nitrogen excretion and from the significant (approximately 50 percent) increase in efficiency of nitrogen utilization that was observed at both levels of intake. Measurements of energy and nitrogen metabolism in the portal drained viscera and liver in these steers demonstrated reduced amino acid extraction ratio and reduced net uptake of amino acids by the liver (Reynolds et al., 1992). GRF decreased the amount of energy lost in the urine and feces, but this was countered by increased heat production. Total tissue energy retention was not altered, but energy retained was repartitioned toward 67 and 19 percent less body lipid at the low and high intakes, respectively. The authors concluded, based on calculations, that maintenance energy costs and the efficiency of ME use for tissue deposition were not altered by GRF.

Taken together, results from recent studies suggest that protein (amino acid) availability and amino acid profile may be important factors influencing the magnitude of protein deposition response to ST or GRF administration in growing ruminants. The large absolute increases in protein synthesis and deposition rates observed in ST-or GRF-treated animals have occurred when protein nutriture was considered in the design of the experiment. Enhancing amino acid availability enhances the protein deposition response, and the expected increase in dietary protein or amino acid requirement is offset by increased efficiency of nitrogen utilization. Houseknecht and Bauman (1992) used data from five separate studies to calculate the biological value of consumed protein (grams of nitrogen retained per gram of nitrogen absorbed) in cattle or lambs treated with ST or GRF. The biological value was increased by 20 percent to as much as 70 percent in treated animals. This increased biological value appears to result from reduced amino acid oxidation and a major site of reduction appears to be the liver. Carefully designed studies will be required to elaborate the integrated effects of species, stage of growth, genotype and gender, and dose of ST or GRF on nutrient requirements of growing ruminants.

Well-designed experimental approaches to address the question of whether nutrient requirements are altered with exogenous ST administration in cattle, lambs, or other ruminants need to be conducted before meaningful recommendations can be made. Dose-response relationships between growth performance and composition of empty-body gain have not been comprehensively evaluated to determine influences of genotype, gender, and stage of growth. Data from several studies suggest that the smaller protein accretion rate and nitrogen balance responses observed in ruminants administered ST or GRF, compared with responses in swine, are caused by constraints on quantity or balance of amino acids available at the site of absorption. However, ST or GRF administration in ruminants effectively increases the calculated biological value (gram of retained nitrogen

Front Matter (R1-R12)

Executive Summary (1-2)

1. Introduction (3-4)

2. Mechanisms of Action of Metabolic Modifiers (5-22)

3. Effect of Somatotropin on Nutrient Requirements of Dairy Cattle (23-29)

4. Effect of Metabolic Modifiers on Nutrient Requirements of Growing Ruminants (30-37)

5. Effect of Metabolic Modifiers on Nutrient Requirements of Growing Swine (38-51)

6. Effect of Metabolic Modifiers on Nutrient Requirements of Poultry (52-58)

References (59-73)

Authors (74-74)

Index (75-82)