Submitted to: Journal of Animal Science
Publication Type: Proceedings
Publication Acceptance Date: 7/24/1998
Publication Date: N/A
Citation: Interpretive Summary: Increasing dietary health concerns by consumers have forced producers to establish more efficient systems to produce leaner meat. In beef production practices, growth-promoting agents are expected to increase lean accretion and decrease fat deposition. This study was designed to evaluate the effects of two types of growth-promoting agents either alone or in combination on skeletal muscle fiber morphology in growing steers. Revalor S implants (trenbolone acetate and estradiol) had minimal effect on fiber distribution but did increase fiber size. Administration of Posilac (bovine growth hormone) increased the percentage of fast-twitch muscle fibers with minimal effects on fiber area. The combination of Posilac plus Revalor-S had no effect on fiber type distribution, but increased fiber size. These results suggest that the administration of Revalor-S (REV) growth-promotant increased muscle hypertrophy relative to controls, but combining REV with Posilac caused no additive effect.
Technical Abstract: The objective of this study was to evaluate changes in muscle fiber characteristics in Angus crossbred feedlot steers treated with Posilac or Revalor-S, alone or in combination. Using a completely randomized design, treatments were: 1) CON (Control); 2) POS (Posilac, 320-mg recombinant bovine somatotropin s.c. injection/14 d); 3) REV (Revalor-S, 120 mg trenbolone acetate and 24 mg estradiol-17B implant); and 4) POS+REV. Muscl samples were removed from each steer immediately after slaughter on d 0 (BW 345 kg) and d 112 (BW 585 kg) from the longissimus (LM) and supraspinatus (SS) muscles. Transverse sections (8 um) were cut on a microtome mounted in a cryostat and stained for succinate dehydrogenase activity in combination with myosin ATPase (pH 9.4). Using an automated digital analyzer, muscle fiber area and fiber type percentage were evaluated. Fibers were classified as slow twitch oxidative (SO), fast twitch oxidative glycolytic (FOG), or fast twitch glycolytic (FG). Implantation with REV decreased (P<.10) the number of SO fibers in the LM and FOG fibers in the SS, while increasing (P<.10) the number of FG fibers in the LM and SO fibers in the SS. Steers treated with REV exhibited larger SO (P<.10), FOG (P<.05) and FG (P<.10) fibers in the SS and FG (P<.10) fibers in the LM. The administration of POS decreased (P<.05) the percentage of SO fibers and increased (P<.10) FG fiber distribution in the LM, and increased (P<.05) FOG fiber area in the SS. The combination of POS+REV had no effect (P>.10) on the distribution of fiber types in either muscle, but increased FOG (P<.05) and FG (P<.10) area in the SS. These results indicate that growth promoting agents POS, REV, and POS+REV increase muscle hypertrophy relative to CON, but POS+REV exhibited no additive effect.