1982; Vanderwert et al., 1985; Calkins et al., 1986; Fisher et al., 1986).

Few studies investigating the effects of anabolic steroids on growth in ruminants in the United States include direct measurement of carcass or empty-body composition, a necessity for understanding the mode of action and defining nutrient requirements. Total lean carcass (muscle) increased 9.5 and 10.4 percent in steers implanted twice with 300 mg TBA/36 mg resorcylic acid lactone over the live-weight range of 250 to 400 kg (Griffiths, 1982). Separable fat was reduced 2 percentage points (P < 0.05) and efficiency of gain was greater in implanted cattle fed at the higher of two levels of energy intake. Dressing percentage was higher in implanted cattle, which implies that neither organ weights nor gut fill were increased with treatment. Nitrogen balance was increased 31 and 70 percent, respectively, in two subsets of animals used in metabolism trials in this study. Nitrogen intake was not different between implanted and nonimplanted cattle, and urinary nitrogen excretion was reduced 25 and 29 percent, respectively.

Nitrogen balance of Holstein bull calves indicate that TBA-estradiol combinations increase cumulative nitrogen retention 47 percent compared with 28 percent increase in controls treated with estradiol alone (VanderWal et al., 1975). When administered separately, zeranol, progesterone, testosterone, and TBA also did not lead to a significant increase in nitrogen retention. Galbraith (1980) observed a 106 percent increase in daily nitrogen retention in beef heifers treated with a 300-mg implant of TBA for 60 days.

Long-term administration of TBA and resorcylic acid lactone to heifers and steers fed to 491, 612, or 731 days of age exhibited greater absolute and relative amounts of carcass lean and lesser absolute and relative amounts of carcass fat than nonimplanted cattle (Keane and Drennan, 1987). Implanted cattle had 23.6 kg heavier side weights, 23.1 kg more lean, 3.2 kg more bone, and 2.6 kg less fat than nonimplanted cattle. Sex-by-implant interactions were not significant. Cattle implanted the longest exhibited a greater (P < 0.05) increase in carcass lean than cattle implanted for shorter periods. The increase in carcass weight was accounted for entirely by the increase in carcass lean observed in implanted heifers and steers, and the decrease in fat accretion was offset by increased bone growth. Dressing percentage was increased in the implanted cattle.

Choice of the experimental end point for cattle growth trials can influence the outcome and interpretation of results. Compositional differences are influenced by live weight, as dictated by normal allometric growth patterns in farm animal species. However, industry grading and pricing systems may dictate what end point is most appropriate in a commercial production system. Such is the case for fed cattle for which the degree of marbling in the longissimus muscle measured at the twelfth rib determines quality grade. Few studies on the effects of anabolic steroids in fed cattle have been conducted with degree of marbling as the end point. Growth performance and composition of gain responses to TBA-estradiol implants were compared in three breeds of steers representing different frame sizes—Holstein, Angus, and Simmental-crossbred—using rib dissection and the comparative slaughter technique at a common degree of marbling end point (Perry et al., 1991). Daily empty-body protein gain was increased 25 to 27 percent in all three breed groups with little effect on carcass composition, carcass quality grade, or retail cut distribution. However, live weight required to reach the common degree of marbling end point was increased 25 to 45 kg with the TBA-estradiol implant. Fat gain was increased by 19 percent on average in these cattle. These results suggest that anabolic steroids stimulate growth without dramatic effects on composition of gain and increase the weight at which a common carcass intramuscular fat concentration is achieved.

Studies using combined implants in growing and finishing lambs confirm the results observed in cattle. Lambs fed diets containing 16 percent crude protein ad libitum and implanted with TBA and estradiol-17β exhibited significant increases in intake (14 percent), live-weight gain (16 percent), and carcass lean mass (8 percent), as well as significant reductions in subcutaneous and total carcass fat (Sulieman et al., 1986). A subsequent study with similar lambs at two initial weights (24 and 37 kg) demonstrated even more substantial growth performance and carcass composition effects that were equivalent in both weight groups (Sulieman et al., 1988).

The consistent improvement in rate of protein deposition observed in growing ruminants indicates that anabolic steroid implants exert their primary influence through altering protein metabolism, with lesser effects on lipid metabolism. They appear to increase feed intake in most cases.

Few data are available that describe the effects of anabolic steroids on protein metabolism, but even fewer data exist for assessment of direct effects of anabolic steroids on lipid metabolism in growing ruminants. Protein metabolism studies suggest that both fractional protein synthesis rate and protein degradation rate might be reduced by TBA, with the degradation rate reduced to a greater extent, resulting in an overall increased protein accretion rate in rats (Vernon and Buttery, 1976, 1978a) and lambs (Vernon and Buttery, 1978b; Sinnett-Smith et al., 1983). Combined TBA-estradiol (140 and 20 mg, respectively) implant treatment increased daily live-weight gain 50 to 60 percent at similar feed intakes and increased daily nitrogen retention 100 and 146 percent in Hereford-Holstein steers during the

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)