Joint sessions and study commission reports, Vienna 1997

The impact of androgen status and energetic feeding level on metabolic and contractile muscle fibre differentiation was evaluated. Sixty male Montbéliard cattle, half of them castrated at 2 months of age, were assigned at 9 months of age to feeding groups of ad libitum or restricted, followed at 12 months of age by ad libitum feeding for all. Slaughter 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 hypothesised that the absence of testicular androgens would delay the process of metabolic and contractile muscle fibre maturation occurring with age. In castrates aged 4 months the differentiation of hybrid fibres 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 showed a preference for glycolytic metabolism and type IIB fibre recruitment, 56.5% vs. 50.1% for bulls aged 12 months (P<0.05, LD). Although the effects were minor, energetic feeding restriction showed a sex-specific response. Restricted bulls increased (P<0.05) oxidative enzyme activity as compared with non-restricted bulls. Compensatory growth rates could re-establish the physiological : chronological balance within the two sex groups. Both castration and feeding level affect muscle fibre diversification, whereby the individual growth pattern of a given muscle seems to play a fundamental role in deciding upon the nature and the size of the effect.