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Effect of Breed and Time on Feed on the Size and Distribution of Beef Muscle Fiber Types¹

University of Kentucky, Lexington 40506

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 ßR (red), R (intermediate) and W (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 ßR fiber diameters and somewhat larger ßR fiber areas than the Group I steers. Many of these large ßR 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 W fibers being significantly (P < .05) larger.

The large ßR 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.

¹ Published with the approval of the Directors of the Kentucky Agricultural Experiment Station as Journal Article 74-5-100.

² Kroger Company, Roanoke, Virginia.

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