Growth response of steer calves treated with zeranol, oestradiol 17 beta or progesterone-oestradiol benzoate implants before and after weaning.

This experiment compared the growth response of untreated steer calves with those given a long acting oestradiol-silicone rubber implant or implants of zeranol or oestradiol-progesterone pellets either singly, or repeated after 79 days. The experiment extended through the 6 weeks prior to weaning at 8 months of age, 5 weeks of grazing oat stubble and a 16-week finishing phase on a feedlot. The oestradiol-silicone rubber implant was the only product to significantly increase weight gains compared to controls (0.69 vs 0.52 kg/head/day, 18.2% advantage) in the pre-weaning phase. All anabolic agents produced higher weight gains ranging from increases of 0.16 to 0.19 kg/head/day (18.2-21.6%) above controls in the first 60 days of the feedlot phase. Responses did not differ significantly among the products and were the same whether or not steers had been previously implanted. Lower planes of nutrition in the late suckling and post-weaning periods were accompanied by smaller and non-significant responses to both the short acting anabolics, zeranol and oestrogen-progesterone, compared to the increased weight gains of steers given oestradiol-silicone rubber implants. Implanting with oestradiol in a silicone rubber matrix resulted in similar increases in weight gain both before and during the feedlot phase. This may have been due to the implant maintaining a continuously high level of circulating anabolic agent for the 190 days of the experiment.