Effect of Breed and Time on Feed on the Size and Distribution of Beef Muscle Fiber Types

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Abstract
Thirty-six steers, divided equally between Angus and Charolais breed types, were slaughtered after 153 (Group I) and 233 (Group II) days on a standard finishing ration. A 3.81 cm steak from the longissimus from the seventh thoracic rib region was removed from the left wholesale rib 48 hr postmortem and used for histological studies. Transverse sections (16 µm) from the medial portion of the muscle were cut on a freezing microtome and subsequently reacted with DPNH-TR, myo-fibrillar ATPase at alkaline pH and Sudan Black B to determine fiber type. The three fiber types identified were βR (red), αR (intermediate) and αW (white) based on their reactions to the aforementioned tests. Enlarged photomicrographs were made of each section, and from these all fibers covering a 10 cm square were measured for diameter and total area using a Zeiss Particle Size Analyzer and a Compensating Polar Planimeter, respectively. Time on feed appeared to have a preferential effect on fiber type development which was independent of breed effects. For example, the Group II steers had significantly (P < .05) larger βR fiber diameters and somewhat larger βR fiber areas than the Group I steers. Many of these large βR fiber diameters were three or more standard deviations above the group mean diameter, which resulted in a significant right skew to the distribution curve with time. Breed effects were primarily confined to the Charolais steers which had larger fiber type diameters and areas than the Angus for all fiber types with the area of the Charolais αW fibers being significantly (P < .05) larger. The large βR fibers observed in this study did not have the same characteristics of giant fibers reported in porcine muscle but were thought to be hypertrophied fibers undergoing a unique phase of muscle development. Copyright © 1975. American Society of Animal Science . Copyright 1975 by American Society of Animal Science.

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