Growth of Muscle and Fat in Beef Steers from 6 to 36 Months of Age

Abstract

Fifty-one Angus steers slaughtered at 6, 12, 18, 24, 30 and 36 months were used to study the growth of muscles, separable lean and separable fat in beef steers. These steers were randomly grouped as follows: group 1, full-fed; group 2, restricted to 0.34 to 0.50 kg of gain per day; group 3, fed the same as group 2 until 180 days (6 months) before slaughter then full-fed. All steers were individually fed the same diet. Three group 1 and three group 2 animals were slaughtered at 6 months of age. At 12, 18, 24, 30 and 36 months of age, nine animals were slaughtered, three from each group. Significant differences were found in slaughter and chilled carcass weights between the three groups of animals at each age period except 36 months; whereas, dressing percents were significantly different only at 6, 30 and 36 months, but not at 12, 18 and 24 months. The group 2 animals were lowest until 36 months of age, and the group 3 were intermediate except at 18 and 36 months of age. Rate of muscle growth varied among age groups regardless of feeding regimen. The psoas major and semitendinosus muscles tended to increase in weight more rapidly than the rectus femoris and adductor muscles. The weight of muscles in groups 2 and 3 increased more rapidly than in group 1. The increase in weight of separable lean was more rapid in groups 2 and 3 than in group 1. However, the increase in separable fat was more rapid in group 3, having increased 12.49 times in weight from 6 to 36 months. The most rapid increase in muscle weight occurred between 6 and 12 months for the three groups. The weights of the psoas major, biceps femoris and triceps brachii increased their proportion of total lean as the animals matured. The semitendinosus muscle had approximately the same proportion of the total lean throughout the growth period. The remaining muscles, longissimus, semimembranosus, rectus femoris and adductor, decreased in their proportion of total lean. The longitudinal and cross-sectional areas of the several muscles showed differences similar to those found for the individual muscle weights.