No abstract

1998). The effects of age at slaughter, genotype and nishing system on the biochemical properties, muscle bre type characteristics and eating quality of bull beef from suckled calves. AbstractMuscle fibre characteristics and biochemical properties of muscle recovered from young bulls of two genotypes (Aberdeen Angus Xand 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 ofm. longissimus lumborum (LI) recovered at 48 h post mortem, vacuum packed and stored at 2°C for 14 days. 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 was greater for Angus X than Charolais X bulls (211-5 v. 295-8 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 LI 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.

Eighteen pure-bred steers (live weight 350 kg) from each of two breeds, Aberdeen Angus (AA) and Charolais (CH), were split into three equal groups (six animals each) and offered three planes of nutrition during a 20-week period. The same ration formulation was offered to all animals with amounts adjusted at 3-week intervals to give predicted average weight gains of either 1·0 kg/d (M/M group) or 1·4 kg/d (H/H group). The remaining group (M/H) were offered the same amount of ration as the M/M group until 10 weeks before slaughter when the ration was increased to H. Data on animal performance, carcass characteristics and fibre-type composition in skeletal muscle are presented elsewhere (; ). On three occasions (17, 10 and 2 weeks before slaughter) the animals were transferred to metabolism stalls for 1 week, during which total urine collection for quantification of Nτ-methylhistidine (Nτ-MeH) elimination was performed for 4 d. On the last day, animals were infused for 11 h with [2H5] phenylalanine with frequent blood sampling (to allow determination of whole-body phenylalanine flux) followed by biopsies from m. longissimus lumborum and m. vastus lateralis to determine the fractional synthesis rate of mixed muscle protein. For both breeds, the absolute amount of Nτ-MeH eliminated increased with animal age or weight (P < 0·001) and was significantly greater for CH steers, at all intake comparisons, than for AA (P < 0·001). Estimates of fractional muscle breakdown rate (FBR; calculated from Nτ-MeH elimination and based on skeletal muscle as a fixed fraction of live weight) showed an age (or weight) decline for M/M and H/H groups of both breeds (P < 0·001). FBR was greater for the H/H group (P = 0·044). The M/H group also showed a lower FBR for the first two measurement periods (both at M intake) but increased when intake was raised to H. When allowance was made for differences in lean content (calculated from fat scores and eye muscle area in carcasses at the end of period 3), there were significant differences in muscle FBR with intake (P = 0·012) but not between breed. Whole-body protein flux (WBPF; g/d) based on plasma phenylalanine kinetics increased with age or weight (P < 0·001) and was similar between breeds. The WBPF was lower for M/M compared with H/H (P < 0·001) based on either total or per kg live weight0·75. Muscle protein fractional synthesis rate (FSR) declined with age for both breeds and tended to be higher at H/H compared with M intakes (intake × period effects, P < 0·05). Changing intake from M to H caused a significant increase (P < 0·001) in FSR. The FSR values for AA were significantly greater than for CH at comparable ages (P = 0·044). Although FSR and FBR responded to nutrition, these changes in protein metabolism were not reflected in differences in meat eating quality (Sinclair et al. 2000).