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Prepubertal Friesian heifer calves (n = 24, initial BW = 195[+ or -]5 kg) were assigned to a 2 x 2 factorial block design and used to evaluate the effects of daily GH treatment (0 or 15 mg/d) at either a low or a high feeding level in a 5-wk treatment period on endocrine measurements, hormone receptors, muscle growth, and overall performance. In the pretreatment period, a low feeding level was employed for all calves. During the treatment period, animals at the low feeding level had free access to a roughage-based mixture, whereas animals at the high feeding level had free access to a concentrate mixture and were offered 2 kg/ d of the roughage-based mixture. Blood samples were collected weekly starting 3 wk before treatment. Longissimus (LM) and supraspinatus (SS) muscles were obtained at slaughter. Metabolizable energy intake was 81% higher, digestible CP intake was 140% higher, and ADG was 115% higher (all P < 0.001) at the high vs. low feeding level. Feed (DMI, ME, and protein) intake was not affected by GH treatment, but ADG was 18% higher (P < 0.13) in GH-treated than in control heifers at both feeding levels. Although of different magnitudes, the muscle anabolic effects of GH treatment and high vs. low feeding level were additive, and both treatments increased carcass weights (P Key Words: Binding Proteins, Biochemical Receptors, Heifers, Insulin-Like Growth Factor, Plane of Nutrition, Somatotropin

Sixteen prepubertal Friesian heifers were used to examine the effect of bovine growth hormone (GH) and ovariectomy (OVX) at 2.5 mo of age (2 x 2 factorial design) on growth, carcass quality, and fiber types, capillarization, and metabolic potentials of the longissimus muscle, and serum concentrations of estradiol-17[Beta] ([E.sub.2][Beta]), insulin, GH, IGF-I, and IGF-binding proteins (IGFBP). Treatment with GH (15 mg/d) started at 147 [+ or -] 3 kg BW and lasted for 15 wk. Heifers were fed a mixed roughage-based diet. Growth hormone increased ADG (P < .001), improved gain:feed (P < .007), and had a small but positive influence on lean accretion. Growth hormone reduced fat thickness (P < .009), carcass fat trim (P < .009) and i.m. fat (P < .09). Ovariectomy did not affect performance but increased dressing percentage (P < .03), full rib weight (P < .003), and fat thickness (P < .04). Ovariectomy reduced [E.sub.2][Beta] (P < .001) and insulin (P < .02), and increased the 32-kDa IGFBP (IGFBP-2) (P < .09). Growth hormone treatment increased GH, IGF-I, the 28-kDa IGFBP, and the 40- to 43-kDa IGFBP (IGFBP-3) (P < .004 or P < .001). Neither GH nor ovariectomy affected the proportion and relative area of the individual muscle fiber types, but GH tended to increase type I fiber area (P Key Words: Ovariectomized Females, Somatotropin, Muscle Fibers, Capillaries, Insulin-like Growth Factor, Binding Proteins

The objective of this study was to determine the effect of the beta-adrenergic agonist cimaterol (CIM) on fiber characteristics, capillary supply, and metabolic enzyme activities in muscles of young Friesian bulls. Four pairs of monozygotic twins in each of three live weight groups (WG) were used (initial average live weight [LW]: 162, 299, and 407 kg, respectively). Within each pair, one twin was fed .06 mg of CIM.kg LW-1.d-1 for 90 d. The other twin served as control (C). Needle biopsies were obtained from the longissimus (LM) and semitendinosus (ST) muscles at d 82 to 84 of treatment, and muscle fibers were identified as slow-twitch (Type I) or fast-twitch (Type IIA or Type IIB) by the myosin ATPase stain. In LM, the proportion of Type I (C: 24.0%, CIM: 20.4%; P < .07) and Type IIA fibers (C: 24.2%, CIM: 8.6%; P < .001) decreased, whereas the proportion of Type IIB fibers increased (C: 51.7%, CIM: 71.1%; P < .001). Cimaterol increased the cross-sectional area of Type I (P < .02) and Type IIB fibers (P < .001), with no change in Type IIA fibers. Overall, the mean fiber area increased (C: 2,363 microns 2, CIM: 3,934 microns 2; P < .001). The number of capillaries per fiber did not change, but the number of capillaries per square millimeter decreased (P < .001) after CIM treatment. Cimaterol changed metabolic enzyme activities toward lower oxidative capacity of the muscle (lactate dehydrogenase: +22%, hydroxyacyl-CoA dehydrogenase: -33%, and citrate synthetase: -34%; all P < .001) and reduced the glycogen content by 25% (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)