The effects of implantation with different anabolic agents on growth rate, behaviour, carcase parameters and testicular size of bulls were evaluated in an experiment with 361 Friesian bulls about three months old on three farms. Animals were allocated to one of the following treatments: (a) Control; (b) repeated implantation with 36 mg zeranol at intervals of 95 to 124 days; (c) repeated implantation with 20 mg oestradiol benzoate plus 140 mg trenbolone acetate as in (b); (d) repeated implantation with 20 mg oestradiol benzoate plus 200 mg progesterone as in (b); (e) implanted once with 45 mg oestradiol-17 beta. Daily liveweight gain, carcase weight, testicular size and behaviour of the animals were the parameters measured. Repeated implantation of young bulls with hormones, beginning at eight to 10 weeks old, increases liveweight gain and carcase weight from 0 to 15 per cent, reduces aggressive behaviour and testicular size and, in some instances, improves conformation and increases fat cover of the carcase.
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