A comparison of bulls and steers implanted with various oestrogenic growth promoters in a 15-month semi-intensive system of beef production

Abstract

AbstractTwo experiments, each involving 48 British Friesian cattle, were carried out to examine the performance and carcass quality of bulls and steers implanted with one of three oestrogenic growth promoters in a semi-intensive system of beef production in which the animals were slaughtered at 14 to 17 months of age. The treatments used in experiment 1 were: (1) control steers (no implant); (2) steers implanted with 45 mg oestradiol in silastic rubber at 4·5 months of age and with 300 mg trenbolone acetate at 10·5 and 13·5 months; (3) steers implanted with 20 mg oestradiol and 200 mg progesterone at 95-day intervals from 4·5 months and with 300 mg trenbolone acetate at 10·5 and 13·5 months of age; (4) bulls (no implant). For treatments 1 to 4 respectively dry-matter (DM) intakes were 6·74, 7·24, 7·50 and 7·16 (s.e. 0·092) kg/day; carcass weights were 260, 283, 298 and 290 (s.e. 5·4) kg; and fat depths over m. longissimus were 6·5, 5·3, 6·3 and 3·4 (s.e. 0·53) mm. The same treatments were used in experiment 2, except that 36 mg Zeranol® was used in treatment 3 instead of 20 mg oestradiol plus 200 mg progesterone, and the bulls in treatment 4 were implanted with 36 mg Zeranol at 7·5, 11 and 14·5 months. For treatments 1 to 4 respectively DM intakes were 7·37, 7·85, 7·76 and 7·42 (s.e. 0·092) kg/day; carcass weights were 262, 285, 284 and 296 (s.e. 7·1) kg and mean fat depths over m. longissimus were 7·1, 5·9, 7·0 and 5·2 (s.e. 0·54) mm. Implantation of steers increased food intake but also improved food conversion efficiency and did not consistently affect carcass fatness or conformation. Repeated implantation with 20 mg oestradiol plus 200 mg progesterone produced a greater response in performance than a single implantation with 45 mg oestradiol-17β in a controlled-release silastic-rubber implant, while repeated implantation with 36 mg Zeranol produced a similar response to the single silastic-rubber implant. Bulls produced leaner carcasses and converted food to carcass gain more efficiently than implanted steers.