Adeola et al. (1990) observed that ractopamine depressed ADG and gain: feed 9 and 10 percent, respectively, when the diet contained only 13 percent crude protein. However, ractopamine increased ADG and gain: feed 10 to 12 percent and 12 to 25 percent, respectively, when the diet contained 17 percent crude protein. These studies do not resolve the question of whether the efficiency of protein utilization is changed or whether protein requirement is increased.

If protein intake limits the rate of protein accretion, either the amount or the profile of amino acids supplied in the diet may account for the restriction of protein deposition. Therefore, in nonruminants protein intake titration experiments must be conducted using diets in which the amino acid profile is matched to the amino acid profile of deposited protein. These types of data are limited. Dunshea et al. (1991) demonstrated in gilts fed diets ranging from 8.3 to 23 percent crude protein concentration that carcass protein accretion rates were not increased by ractopamine at crude protein concentrations below 14 percent, and that additional crude protein was required (16.8 versus 14 percent) to accommodate the greater protein accretion rates achieved with 20 mg ractopamine/kg diet. No evidence was observed for ractopamine increasing the efficiency of protein use for growth. In ruminants, enhanced availability of amino acids does enhance skeletal muscle growth. Substitution of fishmeal for an equal amount of soy protein enhanced skeletal muscle mass in ram lambs by 15 to 19 percent, and effects were additive with cimaterol (Beermann et al., 1986a). Proximal hind limb muscles in lambs fed fishmeal and cimaterol were 40 to 45 percent larger than those in lambs that received no fishmeal or cimaterol.

An increase in skeletal muscle mass and in the basal metabolic rate of treated animals (see, e.g., MacRae et al., 1988; Kim et al., 1989) may increase the maintenance requirement of animals fed β-agonists. These increases would be offset only minimally by the reduction in fat deposition, but the shift of protein synthesis and accretion away from tissues with higher turnover rates (small intestine and liver) and toward skeletal muscle may help minimize changes in maintenance requirements. Although it is possible to speculate on the magnitude of any changes in nutrient response following treatment with β-agonists, it is doubtful that data are available to do this with any precision. In extrapolating from the data available for ST-treated animals, caution must be exercised because although both ST and β-agonists have similar effects on fat and skeletal muscle deposition, they have different effects on the relative growth of other tissues, especially the liver, kidneys, and other visceral organs. Different mechanisms of action could also dictate different effects or influences on nutrient requirements. This may result in differences in the response to changes in nutrient availability or to dietary manipulations.

Chronic dietary administration of select β-adrenergic agonists markedly influences protein and lipid metabolism in farm animals, leading to marked increase in skeletal muscle protein accretion rate and, in most cases, significant reduction in lipid deposition rates. Significant improvement in carcass composition results without effect on growth of bone and with little effect on mass of visceral tissues and other organs. Improvements in growth performance appear to be greatest within the first few weeks of administration and diminish to a varying extent with continued administration. Ruminants appear to be more responsive than swine, and poultry respond least. Significant influences of diet and genotype on magnitude of response have been observed, but the nature of these interactions varies across species.

Naturally occurring and synthetic estrogens and androgens have been safely used to improve efficiency of growth and carcass composition of meat animals for more than 40 years. Historically, the first commercial use of an estrogen was in poultry, but this lasted only a short time. Anabolic steroids are not approved for use in growing swine in the United States; however, both estrogens and androgens are extensively used in growing cattle produced for beef. Several anabolic steroid implants are currently approved for use in beef cattle in the United States. Only one compound, zeranol, is approved for use in lambs. These approved steroid implants include the naturally occurring hormone, estradiol, the hormone progesterone in combination with estradiol or estradiol benzoate, the fungal metabolite with estrogenic properties, zeranol, the synthetic progestin, melengestrol acetate (MGA), testosterone propionate in combination with estradiol benzoate, and a synthetic testosterone analog, trenbolone acetate (TBA). Structures of estradiol, progesterone, zeranol, testosterone, TBA, and MGA are shown in Figure 2-4.

Classification of the anabolic agents previously or currently in use is based on their chemical structures and associated actions. A review of the biosynthesis and metabolism of the naturally occurring estrogens and androgens has recently been published (Hancock et al., 1991). Descriptions, approval dates, and recommended doses of the commercial products are found in papers by Schanbacher (1984), Muir (1985), and Hancock et al. (1991). Efficacy of these anabolic steroid implants is summarized in several reviews (Galbraith and Topps, 1981; Schanbacher, 1984; Muir, 1985; Roche and Quirke, 1986; Beermann, 1989; Hancock et al., 1991).

The literature on growth-performance responses to anabolic steroids indicates great variability, ranging from no

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)