Feedlot performance and carcass characteristics were examined in bulls and steers with and without a subdermal Silastic capsule containing oestradiol-17-dipropionate. Castration and/or oestrogen treatment was imposed at approximately 8 months of age. The implants were removed 56 days before slaughter, i.e. 140 days after the trial commenced. Bulls gained more rapidly (129%), converted food to live-weight gain more efficiently (180%) and yielded leaner carcasses than did steers (
P< 005). The oestrogenic implant elevated serum oestradiol concentrations substantially (> ninefold) over that of bulls and steers and induced a castration-response in the former. Serum luteinizing hormone, testosterone and testicular growth were suppressed ( P< 005) during the first 140 days of the trial in bulls, but returned to normal following implant removal. This response was paralleled by a reduction in average daily gain ( P< 005) and food: gain ratio ( P< 010). Contrary to expectations, average daily gain and food: gain ratio were not significantly improved in implanted steers. Oestrogen treatment reduced hot carcass weight, loin eye area and quality grade in both bulls and steers; the last as a result of decreased marbling and internal fat deposition. Results of this study suggest that excessive oestrogen as provided in this study via the implant fails to improve feedlot performance and carcass merit in steers and may, in fact, diminish the inherent anabolic attributes of the intact male. Additional studies are required to understand the complex interactions of androgens and oestrogens in the regulation of body growth and composition of male cattle.