The effects of a combined androgenic-oestrogenic anabolic agent in steers and bulls 2. Muscle weight distribution, partition of body fat and carcass value

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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-17β (20 mg) on muscle distribution, fat partition and carcass value. 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 and 300 days of age, and animals were slaughtered at 400 days following ad libitum consumption of a complete pelleted diet. The effects of castration and/or implantation on muscle distribution, fat partition and carcass value 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.Implantation of steers increased slightly the weight of muscles in the neck which in a preliminary study had been shown to be particularly heavy in mature bulls compared with steers. Implantation therefore produced a more bull-like muscle distribution in steers and the effect in bulls was to increase the weight of these muscles even further. In steers and bulls the partition of body fat was not affected by implantation. Both implanted steers and bulls were fatter than their respective control bulls and their fat partition was appropriate to this (higher subcutaneous to intermuscular fat ratio).Differences in carcass value between the groups, as evidenced by a retail cutting method applied to the right side of the carcass, reflected these differences in muscle distribution and fat partition. Thus, implantation of steers was judged to be economically advantageous despite the marginal reduction in value brought about by enlarged forequarter musculature. Implanted bull carcasses were less valuable than their respective controls mainly because they were fatter and required more trimming.