Five groups of six Hereford × Friesian steers, approximately 1 year old and weighing 363 kg at implantation, were individually fed according to body weight and slaughtered 100 days after implantation. Group 1 was sham-implanted subcutaneously in the ear. Group 2 was implanted with 20 mg of the steroid oestradiol-17β group 3 with 140mg of the steroid trenbolone acetate; group 4 with oestradiol-17β in one ear and trenbolone acetate in the other; and group 5 with oestradiol-17β and trenbolone acetate as a single implant in one ear.
Groups 1 to 5 had mean overall live-weight gains (±s.e.) of 0·79, 0·88, 0·89, 0·98 and 1 15±0·067kg/day; mean feed conversion ratios of 9·30, 8·78, 8·45, 8·13 and 6·92 + 0·61; and mean cold carcass weights of 236, 246, 250, 252 and 257 ± 6.4kg, respectively. Oestradiol-17/? reduced omental (P <0·05) and kidney (P <001) fat. The two steroids appeared to produce independent and additive effects except when used together as a single combined implant (group 5), when the effect on live-weight gain was more prolonged. The mean concentrations of oestradiol-17/i in plasma in groups 2, 4 and 5 were greater than in the control group at 28, 28 and 91 days after implantation respectively (P < 0·001). Trenbolone acetate was present in the plasma in groups 3, 4 and 5 throughout the experimental period.