The effect of a single implantation (on d 1) with one or two long-acting, biodegradable estradiol implants (1E or 2E) on plasma estradiol concentrations in beef heifers was determined. The growth rates of these (2E) heifers, and of heifers repeatedly implanted with trenbolone acetate (TBA) or zeranol (Z) on d 1, 84, 168, and 252 of the trial, were compared to growth rates of controls. Trenbolone acetate alone was compared to TBA + 2E, and 2E was compared to 1E. At a mean age of 84 d (d 1 of experiment), 81 Hereford x Friesian heifers were allocated at random to the following treatments: Control (n = 15); TBA (n = 15); 1E (n = 12); 2E (n = 15); Z (n = 13); or TBA + 2E (n = 11). Mean live weight (kg) prior to slaughter on d 368 and hot carcass weight (kg) for heifers assigned to treatment Groups 1 to 6, respectively, were 366 and 200, 391 and 212, 374 and 201, 386 and 207, 387 and 210, and 391 and 208 (residual SD = 30.3 and 20.2). Heifers assigned to both the 2E and Z treatments were heavier on d 368 (P less than .05) and had longer teats on d 279 (P less than .05), less pelvic fat (P less than .05), and heavier kidneys (P less than .005) than control heifers. Heifers assigned to the TBA treatment had shorter teats on d 279 (P less than .001) but greater final live weight (P less than .05) and carcass weight than control heifers. Heifers given TBA alone had more pelvic fat (P less than .05) and lighter kidneys (P less than .05) than those given TBA + 2E. Mean estradiol concentrations in both the ipsilateral and contralateral jugular veins of heifers assigned to the 2E and TBA + 2E treatments, and in the ipsilateral jugular veins of heifers given 1E, were greater (P less than .05) than those in control heifers; concentrations did not decline during the experiment.

No abstract

The effects of a number of steroid hormone treatments on growth were examined in a trial involving 204 Friesian-type steers which was carried out over an 11 month period from May to April. The animals were at pasture from May until October and were over-wintered indoors on grass silage and supplementary concentrates. Thirty-four animals were used as untreated controls, and there were four treatment groups: 43 steers were implanted with pellet-type implants containing 20 mg oestradiol benzoate and 200 mg progesterone on days 1, 105 and 187; 47 steers were implanted with a single silastic rubber implant containing 45 mg oestradiol-17 beta; 36 steers received treatment (1) and in addition were implanted on the same days with 300 mg trenbolone acetate; 44 steers received treatment (2) and were also implanted with 300 mg trenbolone acetate on days 1, 105 and 187. The mean liveweight gains (+/- sem) of the steers during the first 249 days of the trial were 201.7 kg for the controls and 236.8, 219.4, 254.4 and 247.8 (+/- 6.1) kg for the steers assigned to treatments 1, 2, 3 and 4, respectively. The corresponding values for the carcase weights (+/- sem) were 300.0 kg for the controls and 318.4, 312.0, 327.9 and 321.6 (+/- 3.5) kg for the treated groups. Although all the treatments increased the liveweight gains and carcase weights significantly compared with the controls, the responses to the silastic rubber implants were smaller owing primarily to an apparently high rate of loss of the implants.(ABSTRACT TRUNCATED AT 250 WORDS)