Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 30, 2000
Publication Date: N/A
Interpretive Summary: Increasing dietary health concerns of consumers have forced producers to establish more efficient systems to produce leaner meat. In beef production, growth implants and bovine growth hormone (somatotropin) administration are expected to increase lean accretion and decrease fat deposition. This study was designed to determine the effect of recombinant bovine growth hormone (rbST) on muscle fiber morphology and muscle mass. rbST treated steer calves had more lean tissue mass than controls. This increase was synonymous with an increase muscle fiber size and an increase in fiber number of fast-twitch glycolytic (FG) fibers. FG fibers are the largest muscle fibers in skeletal muscle. Thus, larger muscles and more separable carcass lean tissue appears to be the result of a greater distribution and larger size of FG fibers in rbST treated steers.
The objective of this study was to determine the effect of recombinant bovine somatotropin (rbST) on indices of muscle fiber morphology. Crossbred steer calves were assigned to one of two treatment groups: control (sham-injected; n=12) or rbST treated (.09 mg/kg/d; n=12). Calves were injected every 14 d starting at d 28 of age and were weaned at 205 d of age. On d 100, semitendinous (ST) muscle biopsies and d 206, slaughter samples of ST muscle were collected for muscle fiber analyses. The rbST treated calves had (P<.04) larger ribeye areas, higher carcass conformation scores, and more separable lean tissue as previously reported by Vann et al. (1998). The rbST treated calves had larger (P=.045) FG fibers (2564 vs 2351 um2 cross-sectional area, respectively) than controls. No differences (P=.36) were detected for SO (1192 vs 1148 um2, respectively) or FOG fibers (1484 vs 1403 um2, respectively). The percentage distribution for FOG fibers was greater for the control calves compared to the rbST treated calves (38 vs 34 percent, respectively, P=.014) while the percentage distribution for FG fibers was greater (54 vs 48 percent, respectively, P=.03) in the rbST treated calves compared to control calves. The percentage distribution for SO fibers tended to be greater for the control calves (14 vs 12 percent, respectively, P=.07). The percentage of FG fibers increased (46 vs 57 percent, respectively, P=.001) while the percentage distribution of SO (14 vs 10 percent, respectively) and FOG fibers (40 vs 33 percent, respectively) decreased (P=.001) from d 100 to d 206. The increased separable lean in the rbST treated calves appears to be the result of a greater distribution of FG fibers which possess larger cross- sectional areas than the other fibers.