The effects of age at slaughter, genotype and finishing system on the biochemical properties, muscle fibre type characteristics and eating quality of bull beef from suckled calves

Muscle fibre characteristics and biochemical properties of muscle recovered from young bulls of two genotypes (Aberdeen Angus X and Charolais X), reared on two different diets (silage-based and barley-based) and slaughtered at varying ages between 10 and 19 months of age were established These analyses were restricted to samples of m. longissimus lumborum (Ll) recovered at 48 h post mortem, vacuum packed and stored at 2 degrees C for 14 clays. Biochemical measurements included intramuscular fat content, intramuscular collagen content and its solubility, haem pigment concentration, sarcomere length and myofibril fragmentation. Muscle fibre type was classified according to the contractile nature of the fibres and their metabolic properties.

Intramuscular fat content increased (P < 0.01) with age at slaughter and at a fixed age teas greater for Angus X than Charolais X bulls (211.5 v. 195.8 mg/g dry matter; P < 0.01). Total intramuscular collagen and its solubility tended to decrease with age (P < 0.01). Differences in haem pigment concentration in samples of Ll were detected between genotype (3.99 v. 3.59 mg/g for Angus X and Charolais X bulls; P < 0.01) and diet (3.97 v. 3.62 mg/g for bulls given barley and silage; P < 0.01), and increased with age at slaughter (P < 0.01). There was a significant increase in eye muscle cross-sectional area with increasing slaughter date (P < 0.01) and this was paralleled by an increase in cross-sectional area of individual muscle fibres (P < 0.001). Differences in cross-sectional area of individual muscle fibres between genotype and diet were small and inconsistent. Charolais X bulls had a greater percentage area of fast twitch glycolytic fibres than Angus X bulls (54.3 v. 49.3%; P < 0.01) and a smaller percentage area of slow twitch oxidative fibres (15.8 v. 18.9%; P < 0.05). Beef tenderness was positively correlated (r = 0.48; P < 0.01) with the frequency of slow twitch oxidative fibres and negatively correlated (r = -0.38; P < 0.05) with the frequency of fast twitch glycolytic fibres.