Effects of oestradiol, zeranol or trenbolone acetate implants on puberty, reproduction and fertility in heifers.

The effects of anabolic agents on reproduction in beef heifers were determined by using 300 mg trenbolone acetate (TBA), 36 mg zeranol and 19 mg oestradiol-17 beta in a biodegradable pellet (1E: American Cyanamid, USA), or two such pellets (2E). On Day 1 of experiment, 81 Hereford x Friesian heifers (mean age = 84 +/- 1.2 days) were allocated at random to the following treatments: (1) controls (N = 15); (2) TBA (N = 15); (3) 1E (N = 12); (4) 2E (N = 15); (5) zeranol (N = 13); (6) TBA + 2E (N = 11). The 1 (1E), or 2 (2E) oestradiol implants were administered on Day 1 of the experiment only. Heifers assigned to receive TBA and zeranol were implanted on Days 1, 84, 168 and 252. Blood progesterone concentrations and oestrous activity were monitored from Days 137 and 200 respectively. Mean age (days) and weight (kg) at puberty (first ovulation), for heifers that reached puberty in Groups 1-6 respectively were 352 and 308, 419 and 356, 373 and 325, 381 and 331, 400 and 353, 423 and 383 [residual standard deviation (r.s.d.) = 43.8 and 39.4 for age and weight respectively]. Heifers in Group 4 were older (P less than 0.05), but not heavier (P greater than 0.05), while those in Groups 2 and 5 were both older (P less than 0.005) and heavier (P less than 0.005) than the controls at puberty. Age and weight at puberty were not different in heifers assigned to Groups 3 and 4, or to Groups 2 and 6. The proportion of heifers showing oestrus before puberty (prepubertal oestrus) were 3/15, 12/15, 6/12, 7/15, 10/13 and 11/11 in Groups 1-6 respectively. Heifers in Groups 2 and 5 had higher incidences of prepubertal oestrus than controls, while those in other treatment groups were not different. There was no treatment effect on the incidence of silent ovulations, but the incidence of non-ovulatory oestrus, after puberty, was increased from 4/48 in Group 1 to 26/40 (P less than 0.001), 15/56 (P less than 0.05) and 34/57 (P less than 0.001) in Groups 2, 4 and 5, respectively. Heifers in Group 6 had a higher incidence of non-ovulatory oestrus (P less than 0.05), but not of prepubertal oestrus, than did those in Group 2.(ABSTRACT TRUNCATED AT 400 WORDS)