Twelve (Exp. 1) and 24 (Exp. 2) young growing bulls were used to compare the effects of long-acting,
cholesterol-based implants with short-acting,
lactose-based implants. Each implant contained 200 mg of
trenbolone acetate and 40 mg of
17 beta-estradiol. The long-acting,
cholesterol-based implants were applied once either 18 wk (early implantation) or 12 wk (late implantation) before slaughter. The
lactose-based pellets were implanted once (6 wk before slaughter), twice (12 and 6 wk) or three times (18, 12 and 6 wk) before slaughter. Treated animals had higher live
weight gains, a lower feed conversion ratio, a higher proportion of lean meat and a lower proportion of adipose tissue than control animals. In Exp. 1 live
weight gain of bulls implanted early with a single
cholesterol-based implant was similar to the gain of animals treated three times with a
lactose-based implant, but in Exp. 2 the effects of the long-acting implants given early were not sustained. In Exp. 3, 24 young growing bulls were used in a 3 X 2 factorial design to compare the effects of three dose levels of
trenbolone acetate (100, 200 and 300 mg) and two levels of
17 beta-estradiol (40 and 60 mg). There were no significant differences between treatment groups, but highest live
weight gains were observed in animals implanted with 300 mg of
trenbolone and 60 mg of
estradiol. This group also yielded the highest proportion of lean meat and the lowest proportion of adipose tissue. There was a decrease in plasma
urea concentration after each implantation. It appeared also that plasma
trenbolone was higher with implants containing 60 mg of
estradiol. There were no differences between groups in
growth hormone response to
arginine stimulation.