Effect of castration on bovine muscle composition

Samples of bovine muscles were cross-sectioned and stained for myofibrillar ATPase. Three fibre types were distinguished, I, IIA and IIB. In the longissimus dorsi (LD) muscles of steers the average occurrence of IIB fibres was 32%, but in bulls was only 8%. The difference was highly significant. In contrast, the average occurrence of I fibres in steers and bulls was not significantly different. IIB fibres were generally rare in splenius (Sp) muscle, and occurred in some steers but in no bulls. That IIB fibres were more common in steer muscle was confirmed by studies with pairs of identical twins. The increased occurrence of IIB fibres in steers was unrelated to carcass weight changes often associated with castration. Fibre type occurrence data in steer or bull muscles were highly variable. Also, muscles of twins remained macroscopically similar in spite of significant maleness differences between two animals of a pair. Together, these results indicate that castration exerted a relatively minor, but nonetheless significant, effect on the muscles studied. Other factors, such as breed, apparently exert greater effects: LD muscles of Sahiwal × Friesian bulls (a tropical-temperate cross) were very steer-like in composition. Areas of fibres increased with carcass weight. For LD, areas increased with similar slopes in steers and bulls, but for Sp, areas in bulls increased with steeper slopes. This difference related to the increased development of muscles about the neck in bulls. Further, in heavier bulls, areas of IIA fibres were significantly larger than areas of I fibres. Evidence is presented which suggests that the paucity of IIB fibres in bulls and the disproportionate area increases of IIA fibres in bull Sp, relate to the serum concentrations of androgens, which differ markedly in steers and bulls. Possible modes of action of androgens are discussed.