The effects of a combined androgenic-oestrogenic anabolic agent in steers and bulls 1. Growth and carcass composition

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Abstract
Twenty-one pairs of male cattle twins, 10 of which were monozygotic, were used in a study of the effects of a combined implant of trenbolone acetate (140 mg) and oestradiol-17B (20 mg) on growth and carcass composition. There were three comparisons within pairs: (1) between bulls and steers (no implants); (2) between bulls and implanted steers; and (3) between bulls and implanted bulls.Implants were given twice, at 44 to 73 days and at 300 days of age. Animals were slaughtered at 400 days following recorded consumption of a complete pelleted diet given ad libitum. Carcasses were separated into tissues and lean and fat were chemically analysed. The effects of castration and/or implantation on growth and carcass composition were assessed by comparison with the untreated (control) bull values within each group and expressed as a ratio (treatedxontrol). These ratios were then compared between groups.Bulls were significantly heavier than steers at slaughter (29·6 kg, P < 0·05), but were not heavier than implanted steers. Implantation of steers tended to increase appetite; it proportionately increased lean and protein production by about 0·1, and partly reversed the effects of castration on the length of limb bones and the weights of reproductive organs and hide. The proportions of carcass tissues were very similar in the untreated and implanted steers. In bulls, implantation increased the weight of fatty tissue in the subcutaneous and intermuscular depots, but it had small effects on other carcass characteristics.