Weight gain in steer and heifer calves treated with zeranol or oestradiol 17β

Abstract

Male and female Hereford and Hereford cross beef calves between 2.5 and 4.5 months of age were implanted with 36 mg zeranol, 24 mg oestradiol 17 beta or left untreated, and then weighed every 4 to 5 weeks. Calves treated with zeranol or oestradiol 17 beta gained 9.6 kg (8.2%) and 12.9 kg (11.0%) more weight respectively than untreated calves. Steer calves gained more weight than heifer calves. There was no response to either growth promotant in heifers or in zeranol-treated steers beyond 35 days after implantation, and the only significant further gains in the next 31 days were made by steers treated with oestradiol 17 beta. After this time growth of treated and control calves was similar. The lack of a continued response to the growth promotants was not associated with significantly lower growth rates in any weighing interval.

Growth promotants in cattle in Australia

“…In 2.5to 4.5-month-old suckling calves, Sawyer (1987) reported that those treated with zeranol or oestradiol 170 gained 10 kg (8.2%) and 13 kg (11.0%) more weight, respectively, than untreated controls. An interesting feature of the results was the short duration of response recorded in the calves, compared to the claims of both manufacturers.…”

“…Several studies on repeated implantation with zeranol have generally demonstrated that a single implant increased weight gains above those of controls. Smaller and less consistent subsequent responses have been obtained by sequential treatment with up to 4 successive zeranol implants and in most cases these were not statistically significant (Moran 1972 Sawyer et at (1987) implanted 236-kg suckling steers with either oestradiol 17p or with zeranol or oestrogen-progesterone 6 weeks before weaning and again with the short acting anabolics before they were finished on feedlot for 4 months. Oestradiol 17p was the only agent to significantly increase weight gains compared to controls (29 vs 22 kg, 18.2%) UP to weaning and there was no significant response to any growth promotant in the next 6 weeks when the steers grazed oat stubble.…”