Experiments must be conducted to establish the relationships between levels of dietary protein and energy intake in growing ruminants fed β-adrenergic agonists to empirically determine nutrient requirements. It cannot be assumed that the increased efficiency of protein use for muscle growth that is observed in lambs (Beermann et al., 1991) and steers (Houseknecht et al., 1992) treated with bST will be observed in ruminants fed β-agonists. Detailed characterization of diet formulations and factors that influence nutrient requirements must accompany the composition-of-gain data obtained across the potential application dose of the individual β-agonist being investigated. Until these studies have been completed, diets for ruminants should be formulated to take into account the energy requirements associated with increasing empty-body or carcass protein accretion to the magnitude expected with administration of the β-agonist. Whether additional nonprotein energy should be fed to these animals remains an open question.

Mineral requirements are not expected to be altered because neither bone mass nor length is altered in ruminants administered β-agonists (Beermann, 1993). Impact of environmental fluctuations on nutrient requirements of growing ruminants have not been evaluated but may be important (Fox et al., 1988). The Food and Drug Administration has not approved any of the β-adrenergic agonists for use in growing or finishing ruminants.

Larger increases in carcass protein deposition and skeletal muscle growth have been observed in growing cattle and lambs fed select β-agonists than have been observed with ST or GRF administration, without obvious differences in diets. Furthermore, visceral organ weights are not increased with β-agonists, suggesting that conservation of amino acids may occur and an altered pattern of dietary amino acids may be required for optimizing protein deposition with β-agonists as compared with ST or GRF. The question of whether β-agonists improve efficiency of protein utilization for protein deposition in growing ruminants, as has been demonstrated for ST and GRF, is unresolved. Although efficiency of energy use is unaltered, whether energy intake requirements are altered is also unknown. Systematic evaluation of energy and protein (amino acid) requirements in ruminants fed β-agonists must be conducted before meaningful conclusions can be drawn and accurate predictions can be made.

The same considerations given for determining effects of ST, GRF, or β-adrenergic agonists on nutrient requirements in growing ruminants must be extended to administration of anabolic steroid implants. Significant limitations exist in the data from lamb and cattle experiments in which nutritional manipulations were conducted. Either titration of energy or protein requirements involved too few intervals, or accurate determination of composition of gain was not conducted. It appears that anabolic steroids increase feed intake and increase the live weight at which a similar physiological maturity (percent body fat) is reached (Perry et al., 1991). Special diet formulations have not been required to achieve significant improvement in rates of gain or feed efficiency in growing ruminants implanted with anabolic steroids.

Preston and Burroughs (1958) demonstrated that diethylstilbestrol-treated lambs achieved the greatest improvement (32 percent) and absolute average daily gain when fed a high-energy diet containing 17 percent crude protein, compared to 13 and 9 percent crude protein diets. Feed conversion efficiency was also maximized with this diet, compared to other protein and lower energy combinations, but the lack of a plateau among protein levels suggests that nutrient adequacy was not unequivocally demonstrated. Variability in carcass composition also existed among the treatment groups. Although dressing percentage and rib eye area may be consistently increased by trenbolone acetate-estradiol implants (Bartle et al., 1992), carcass measurements of fat thickness, percentage kidney and pelvic fat, and longissimus (rib eye) area are insufficient indices of composition of gain to assess the question of altered nutrient requirements in ruminants treated with anabolic steroids.

Significant improvement in empty-body weight gain was observed in very young (119 kg live weight) British Friesian steers fed a silage diet substituted with increasing concentrations of fishmeal (0, 50, 100, and 150 g/kg diet) (Gill et al., 1987). Estradiol-17β implants increased daily gain, but only when fishmeal was supplemented in the silage diet (13.75 percent crude protein) at 100 or 150 g/kg diet. The interaction was significant and was associated with increased dry-matter intake in implanted steers (gain was 0.77 kg/day with silage alone). Fishmeal increased empty-body and carcass protein, and the effects appeared to be additive with the estradiol implantation. Fat content was not altered with estradiol implants or with fishmeal. These data suggest that improved balance of nutrients may be required to facilitate growth potential and response to anabolic steroids, if nutrients are limiting. Similar relationships were observed in fattening steers and heifers fed a silage diet with or without fishmeal (Lowman and Neilson, 1985).

Estimates of the effects of anabolic steroids on maintenance indicate there is little change. Lobley et al. (1985) observed no increase in heat production in steers administered a combined trenbolone acetate-estradiol implant that dramatically increased live weight gain and nitrogen retention. Lemieux et al. (1988) and Solis et al. (1989) reported that estimates of net energy for maintenance were decreased by only 1 to 3 percent in implanted cattle. They also reported

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)