The effects on growth performance, energy and nitrogen retention, and leucine metabolism of a subcutaneous combined implant of 140 mg trenbolone acetate (TBA)+ 20 mg oestradiol-17 beta (OE) have been examined in Hereford x Friesian beef steers (280-520 kg). Comparisons were made both with the same animals before implantation and with untreated control animals maintained under similar physiological and nutritional conditions. 2. Over a 10 week period the implanted steers showed an improvement in rate of live-weight gain (LWG) of 0.5-0.6 with an even greater proportional increase in N retention compared with control animals. Total energy retention was unaffected and thus the ratio, protein energy: total energy gain was 0.43 for implanted steers compared with 0.26 for untreated animals. 3. Estimates of protein synthesis and protein oxidation were obtained from the specific radioactivities of blood free-leucine and exhaled carbon dioxide during continuous infusions of [1-14C]leucine. Whole-body protein synthesis, based on metabolic size, and amino acid fractional oxidation remained similar for control steers throughout the experiment. Steroid-treated steers showed a slight decline in synthesis which was significant (P less than 0.05) at week +5 post-implant while amino acid oxidation was significantly lower at weeks +2 (P less than 0.01) and +5 (P less than 0.05) compared with control animals. The ratio, protein deposition: protein synthesis was 0.05 for control animals but 0.08-0.10 for steroid-treated animals after implantation. 4. There was a slight decrease in urinary N tau-methylhistidine elimination after implantation which suggested that muscle protein degradation may be reduced although the estimated decrease was insufficient to account for the total improvement in growth rate and N retention. 5. The results suggest that for both control and treated steers, less than 0.5 of total urine N elimination was derived directly from tissue catabolism of protein and amino acids. 6. The combined action of the exogenous steroids in the promotion of protein gain, primarily through a decrease in total protein degradation with little alteration of total energy retention, is compared with present understanding of the role of the endogenous sex hormones.
