RECOMBINANT BOVINE SOMATOTROPIN (RBST) ADMINISTRATION TO CREEP-FED BEEF-CALVES INCREASES MUSCLE MASS BUT DOES NOT AFFECT SATELLITE CELL NUMBER OR CONCENTRATION OF MYOSIN LIGHT CHAIN-1F MESSENGER-RNA

Our objective in this study was to determine the effect of recombinant bovine somatotropin (rbST) on indices of muscle development in creep- fed beef calves. Crossbred steer calves were assigned to one of two tr eatment groups: control (sham-injected; n = 12) or rbST-treated (.09 m g.kg(-1).d(-1); n = 12). Calves were injected every 14 d starting at d 28 of age and were weaned at 205 d of age. Supplemental creep feed wa s supplied free access to all calves to compensate for an expected inc reased protein and energy requirement in calves given rbST. Biopsy (d 100) and slaughter (d 206) samples of semitendinosus muscle were evalu ated for satellite cell, myofiber nuclei numbers, and myosin light cha in (MLC-1f) mRNA quantification. Myofiber nuclei and satellite cell nu mbers per 100 myofibers and MLC-1f mRNA:rRNA ratios at 100 and 206 d o f age were not different (P > .10) between control and rbST-treated ca lves. Total gain, ADG, quality grade, femur length, percentage kidney, pelvic, and heart fat, dressing percentage, plasma IGF-I, and plasma urea nitrogen concentrations did not differ (P > .10) between control and rbST-treated calves. However, rbST-treated calves had larger longi ssimus muscle areas (P < .03), less marbling (P < .001), higher carcas s conformation scores (P < .04), greater mass of separated muscle (P < .03:, more ground meat (P < .01), and heavier carcass weights (P < .0 5) than control calves. Thus, rbST treatment increased muscle characte ristics while nuclei number and MLC-1f mRNA concentrations remained th e same, implying that the additional muscle growth was in a normal fas hion.