MUSCLE-FIBER CHARACTERISTICS IN 4 MUSCLES OF GROWING MALE CATTLE - II- EFFECT OF CASTRATION AND FEEDING LEVEL

The impact of androgen status and energetic feeding level on metabolic and contractile muscle fibre differentiation was evaluated. Sixty mal e Montbeliard cattle, half of them castrated at 2 months of age, were assigned at 9 months of age to feeding groups of ad libitum or restric ted, followed at 12 months of age by ad libitum feeding for all. Slaug hter dates were scheduled at 4, 8, 12 and 16 months of age, and muscle samples were collected from m. semitendinosus (ST), m. biceps femoris (BF), m. longissimus dorsi (LD) and m. triceps brachii (TB). It was h ypothesised that the absence of testicular androgens would delay the p rocess of metabolic and contractile muscle fibre maturation occurring with age. In castrates aged 4 months the differentiation of hybrid fib res was retarded: IIC fibre percentage was 9.2 and 4.5 in ST of steers and bulls, respectively. Steers were less prone to hypertrophy and sh owed a preference for glycolytic metabolism and type IIB fibre recruit ment, 56.5% vs. 50.1% for bulls aged 12 months (P < 0.05, LD). Althoug h the effects were minor, energetic feeding restriction showed a sex-s pecific response. Restricted bulls increased(P < 0.05) oxidative enzym e activity as compared with non-restricted bulls. Compensatory growth rates could re-establish the physiological : chronological balance wit hin the two sex groups. Both castration and feeding level affect muscl e fibre diversification, whereby the individual growth pattern of a gi ven muscle seems to play a fundamental role in deciding upon the natur e and the size of the effect. (C) 1998 Elsevier Science B.V.