Effect of zeranol on bull muscle fibre composition

Two trials are reported in which bulls were implanted with zeranol and slaughtered at 18 months. There was no significant change in carcass weight due to zeranol in either trial. Samples of the longissimus dorsi (LD) and splenius (Sp) were cross-cryosectioned, stained for myofibrillar ATPase and examined by conventional light microscopy. The stain resolved three fibre types: I, slow contracting with an oxidative metabolism, IIA, fast-oxidative-glycolytic and IIB, fast-glycolytic. In the LD from treated bulls, the occurrence of IIB fibres was increased, apparently at the expense of IIA fibres, as the occurrence of I fibres was unchanged. These occurrence changes parallelled previously reported effects of castration. Changes in Sp were less dramatic. In one trial IIB fibres were absent from all Sp muscles, but, in the other, some of the Sp muscles from treated animals contained these fibres whereas they were absent from the Sp of the control group. If the changes in fibre type occurrence due to zeranol extend to skeletal muscles other than LD and Sp, there is likely to be a subtle change in the musculature's metabolism. Fibre areas were largely unaffected by zeranol. In this respect zeranol did not mimic castration.