EFFECTS OF SYNOVEX-S(R) AND RECOMBINANT BOVINE GROWTH-HORMONE (SOMAVUBOVE(R)) ON GROWTH-RESPONSES OF STEERS .2. MUSCLE MORPHOLOGY AND PROXIMATE COMPOSITION OF MUSCLES

Twenty crossbred beef steers (initial BW 182 +/- 1.8 kg) were used in a 2 x 2 factorial to determine the effects of implantation with Synove x-S(R) (SYN; 20 mg 17-beta estradiol benzoate + 200 mg progesterone, e ar implant) and administration of recombinant bovine growth hormone (S omavubove(R) SbV,.1 mg . kgBW(-1). d(-1), i.m.) on the morphology of s ix muscles (longissimus, psoas major, supraspinatus, triceps brachii, semitendinosus, rectus femoris) of growing steers. Implantation with S YN decreased the percentage distribution of FOG fibers and increased F G fibers in the supraspinatus and rectus femoris muscles (P <.05). Ste ers treated with SYN had a larger area of SO and FG fibers in the psoa s major muscle (P <.05). The administration of SbV decreased the perce ntage distribution of FOG fibers and increased FG fibers in the rectus femoris muscle (P <.05). Steers administered SbV had larger SO, FOG, and FG fibers in the psoas major muscle and SO fibers in the supraspin atus and semitendinosus muscles (P <.05). The combined administration of SYN and SbV had minimal, if any, effect on the percentage distribut ion of fiber types (P >.05) but increased (P < .05) the fiber areas of all muscles (18.5 to 54.8%) except the rectus femoris (P >.05). Proxi mate composition of the muscles was generally not affected (P >.05) by any of the treatments. The only observations were decreases in fat co ntent for psoas major and rectus femoris muscles as a result of the co mbined administration of SYN and SbV. These results indicate that both growth-promoting agents, SYN and SbV, have a potential to increase mu scle fiber size, but muscles respond differently to the administration s of SYN and SbV. However, when SYN and SbV are administered in combin ation, the combined effects result in an additive increase in muscle f iber hypertrophic response.