A Histological Study of Bovine Longissimus Muscle Fiber Characteristics as Related to Performance and Carcass Traits

Thirty-four bulls, 25 Angus and nine Hereford, ranging in age from 15 to 17 months and varying in weight from 362 to 485 kg were fed to a constant subcutaneous fat thickness of 10.0 mm. Longissimus dorsi muscle biopsies were removed from the 12th and 13th rib region and stained histologically for ATPase activity to study the relationships of muscle fiber characteristics as related to performance and carcass traits. One-hundred muscle fibers from each sample were counted, measured and classified into type I, red, and type III, white, fibers. Breed muscle fiber means were compared using the T-test and observed to be non-significant (P > .01). Number of red fibers was significantly (P < .05) correlated with final fat thickness, initial weight, kidney fat and rib eye area per hundred weight of carcass. Number of white fibers was observed to be negatively correlated with the number of red fibers. Muscle fiber area measurements were not significantly correlated with performance traits. Mean red fiber areas increased significantly with rib eye area per day of age (0.43, P < .01) and mean white fiber areas increased significantly with rib eye area per hundred weight of carcass (0.40, P < .05), Muscle fiber area measurements significantly (P < .05) increased with increasing rib eye area. Yield grade was correlated (P < ,05) with total red fiber area (-.35) and total white fiber area was correlated with dressing percent (0.52). Percent red and white fiber areas were significantly correlated with yield grade, kidney fat, rib eye area per hundred weight of carcass and rib eye area per day of age, Muscle fiber characteristics as the independent variables were fitted in various regression models, but their predictive effectiveness, even though significant, was poor.