first 7 weeks of treatment in a study by Lobley et al. (1985). Estimates of whole-body protein synthesis rate (based on metabolic body size) were similar throughout the 10-week experiment, while amino acid oxidation was lower in treated steers at 2 and 5 weeks. Urinary 3-methyl histidine excretion was slightly less and total energy retention was unaffected in treated steers, indicating that reduction of protein degradation rate may account for most of the improvement in daily gain and nitrogen retention. Heat production was not increased in steers treated with the TBA-estradiol combination.

A recent experiment indicates that one possible mechanism responsible for TBA's ability to stimulate skeletal muscle hypertrophy may be through enhanced proliferation and differentiation of satellite cells as the result of increased sensitivity to IGF-I and fibroblast growth factor (Thompson et al., 1989).

Few data are available regarding effects of anabolic steroid implants on lipid metabolism in growing ruminants. Prior et al. (1983) demonstrated that lipogenic enzyme activity and fatty acid synthesis in vitro were elevated in subcutaneous adipose tissue from bulls implanted with estradiol. This may account for the increase in fat content of carcasses reported in some studies. St. John et al. (1987) found that TBA implants had no effect on lipogenesis in intact heifers and only tended to reduce lipogenic enzyme activities in ovariectomized heifers treated with TBA.

Assessment of indirect effects is difficult because few studies have reported determinations of circulating hormone or metabolite concentrations in animals treated with anabolic steroid implants. Treatment of lambs (MacVinish and Galbraith, 1988) or cattle (Henricks et al., 1988) with TBA and estradiol caused reduced plasma urea nitrogen concentrations, which would be expected with decreased rates of amino acid oxidation; but a causative effect has not been demonstrated. Galbraith (1980) observed no significant changes in blood hormone or metabolite levels in heifers treated with 300 mg TBA for 60 days. The review by Buttery and Sinnett-Smith (1984) notes the lack of any consistent change in ST, prolactin, insulin, or other metabolic hormones in a total of 15 studies summarized. Recent data obtained from a study in which TBA alone, estradiol alone, and TBA plus estradiol treatments were compared in yearling steers (Hayden et al., 1992) supports this conclusion.

The consistent net effect of anabolic steroid implant use in growing ruminants appears to be that of increasing the live weight at which carcass or empty-body fat content or concentration equals that of nonimplanted animals, thus increasing their potential mature size. Increased growth rate is usually accompanied by an increase in feed intake. Data suggest that protein metabolism is altered toward increased rates of protein synthesis and reduced rates of amino acid oxidation and/or lesser rates of protein degradation. There may be little direct effect on lipid metabolism and associated fat deposition. Indeed, as is noted by the lack of information in the literature, little is known about direct effects of anabolic steroids and combinations of same on lipid metabolism in growing ruminants.

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)