Growth and muscle development characteristics of purebred Angus and Brahman bulls

Thirty-eight purebred bulls (10 to 17 mo of age) were used to determine the effects of breed (Angus or Brahman) and slaughter weight (60, 80, 90 or 100% of the average mature dam's weight for the respective breed) on growth and muscle development characteristics. Angus bulls grazed summer forage after weaning whereas Brahman bulls were fed to simulate gains achieved on forage by Angus. Bulls were then placed in a confinement feedlot for finishing to their appropriate slaughter weight (293, 369, 411, and 469 kg for Angus and 307, 427, 464 and 520 kg for Brahman). No major differences due to breed were found for predicted carcass composition. The LD muscle from Brahman bulls contained more total DNA (2.27 v. 1.19 g), more total protein (768.22 v. 593.59 g) and generally less total lipid (70.56 v. 101.26 g) when expressed on a total muscle (wet tissue) basis. The percentages and areas for all three muscle fiber types were not affected by breed. As carcass weight increased, muscle weights, total protein, lipid, protein:DNA and muscle fiber size for the three fiber types increased. Total DNA content increased only up to the 90% weight group and then leveled off. The percentage of alpha R fibers decreased while the percentage of alpha W fibers increased with increasing carcass weight. These data suggest that slaughtering animals based on a percentage of their dam's mature weight seems to be a practical method for making comparisons of animals on an equivalent compositional basis. Moreover, it appears that histochemical and biochemical evaluations of skeletal muscle can successfully identify what point in the growth cycle an animal is in.