Six steers implanted with 300 mg trenbolone acetate and six steers not implanted were fed on low protein, low-quality-roughage diets ad lib. in two experiments. The steers were Hereford (Bos taurus) x Brahman (Bos indicus) crossbreds (50:50), initially of about 400 kg mean live weight (LW). In the first experiment of 8 weeks duration roughage was given alone. In the second experiment of 6 weeks duration the diet was supplemented with 100 g urea and 4.6 g sulphur daily. The same steers were implanted in each experiment. At the conclusion of each experiment metabolic rate was measured after a 72 h fast. 2. In the first experiment control and implanted steers had similar rates of LW loss (0.57 and 0.59 kg/d respectively). Implanted steers had significantly (P less than 0.01) lower feed intakes (12.8 v. 10.9 g dry matter (DM)/kg LW), significantly (P less than 0.01) lower fasting metabolic rates even after adjustment for intake (83.3 v. 74.5 kJ/kg per d) and significantly (P less than 0.01) lower plasma insulin concentrations (24 v. 19 mu units/ml). Differences in plasma concentrations of free 3,5,3'-triiodothyronine (T3), non-esterified fatty acids and urea-nitrogen were not significant. 3. In the second experiment intake of the supplemented diet was similar in both control and trenbolone acetate-treated steers (19.5 and 20.0 g DM/kg LW respectively). LW gains were 0.23 and 0.41 kg/d for control and implanted steers respectively, the difference being significant (P less than 0.05). Fasting metabolic rate (76.9 v. 70.7 kJ/kg per d) was significantly (P less than 0.05) lower in implanted steers.
