THE EFFECTS OF AGE AT SLAUGHTER, GENOTYPE AND FINISHING SYSTEM ON THEBIOCHEMICAL-PROPERTIES, MUSCLE-FIBER TYPE CHARACTERISTICS AND EATING QUALITY OF BULL BEEF FROM SUCKLED CALVES

Muscle fibre characteristics and biochemical properties of muscle reco vered from young bulls of two genotypes (Aberdeen Angus X and Charolai s X), reared on two different diets (silage-based and barley-based) an d slaughtered at varying ages between 10 and 19 months of age were est ablished These analyses were restricted to samples of m. longissimus l umborum (Ll) recovered at 48 h post mortem, vacuum packed and stored a t 2 degrees C for 14 clays. Biochemical measurements included intramus cular fat content, intramuscular collagen content and its solubility, haem pigment concentration, sarcomere length and myofibril fragmentati on. Muscle fibre type was classified according to the contractile natu re of the fibres and their metabolic properties. Intramuscular fat con tent increased (P < 0.01) with age at slaughter and at a fixed age tea s 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 ten ded to decrease with age (P < 0.01). Differences in haem pigment conce ntration 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 w ith age at slaughter (P < 0.01). There was a significant increase in e ye muscle cross-sectional area with increasing slaughter date (P < 0.0 1) and this was paralleled by an increase in cross-sectional area of i ndividual muscle fibres (P < 0.001). Differences in cross-sectional ar ea of individual muscle fibres between genotype and diet were small an d inconsistent. Charolais X bulls had a greater percentage area of fas t 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 n egatively correlated (r = -0.38; P < 0.05) with the frequency of fast twitch glycolytic fibres.