Influence of trenbolone acetate zeranol and oestradiol-17β implantation on growth performance and reproductive function in beef heifers

Twenty-four Hereford Friesia heifers were divided into four equal treatment groups and given subcutaneous ear implants as appropriate 14 days after prostaglandin treatment for the induction of synchronized cycles, Subsequently, they were observed for 75 days. One group was an untreated control, one was given 300 mg trenbolone acetate, on 36 mg zeranol and one 45 mg oestradiol-17β. Growth and ovarian cyclicity, assessed by periodic progesterone using radioimmunoassay measurement, were monitored. Trenbolone acetate-treated heifers grew faster (P<0.05) than other groups which did not differ significantly. Trenbolone acetate shortened the oestrous cycle in which treatment was strated in two of five heifers and lengthened it in two others. Subsequent cycles were either suppressed or of short duration. Zeranol and oestradiol-17β treatment had little effect on ovarian cyclicity apart from a small increase in the number of short cycles. It is concluded that, at the doses used, trenbolone acetate suppresses mechanisms leading to ovulation or pre-ovulatory follicular development. (author). 16 refs., 3 tabs., 1 fig