EFFECT OF DIETARY ENERGY SOURCE AND LEVEL ON SERUM GROWTH HORMONE, INSULIN-LIKE GROWTH FACTOR 1, GROWTH AND BODY COMPOSITION IN BEEF HEIFERS 1 K. L. Houseknecht 2 , D. L. Boggs 2 , D. R. Campion 3 , J. L. Sartin 4, T. E. Kiser2 , G. B. Rampacek 2 and H. E. Amos 2 The University of Georgia, Athens 30602 ABSTRACT Effects of fiber vs starch energy supplements on endogenous growth hormone (GH), insulin-like growth factor (IGF-1) and animal performance from weaning to breeding age were evaluated in 18, 9-mo-old beef heifers. Heifers had ad libitum access to wheat silage plus an average daily supplement intake of 1) 4.08 kg corn-soybean meal (SBM) (high energy-starch, HS), 2) 4.54 kg soyhulls (SH)-SBM (high energy-fiber, HF) or 3) 1.36 kg SH- SBM (low energy-fiber, LE). Serum samples were collected via jugular puncture every 10 d and were analyzed for IGF-1 by RIA. On d 45 and d 176, four heifers per treatment were fasted 18 h and serial blood samples .collected via jugular cannulas every 15 rain for 6.5 h. Arginine (.5 g/kg BW) was administered intravenously (ARG) to induce release of GH, and four additional samples of blood were collected. Samples were analyzed by RIA for GH. Mean fasted GH (6.4 -+ .4, 8.3 -+ .4 and 13.8 + .4 ng/ml, for HS, HF and LE, respectively) varied with energy source and level (P < .01). Mean GH following ARG was higher (P'< .01) in heifers receiving LE (46.2 + 4.7) than in those receiving HS and HF (23.5 + 4.4 and 24.1 + 4.6 ng/ml). Basal GH concentration and peak amplitude were higher (P < .05) in LE than in HS and HF treatments. Diet did not influence number or frequency of GH peaks. Mean IGF-1 values were similar for HS and HF heifers (P > .05) but were lower for LE heifers (P < .01). The LE heifers had lower (P < .05) ADG, backfat and ribeye area. We conclude that fiber supplements may overcome the depression in GH associated with grain feeding of heifers. (Key Words: Heifers, Energy, Somatotropin, IGF-1.) I ntroduction Beef heifers must reach puberty and become pregnant by 14 to 15 mo of age to calve at 2 yr 1Appreciation is expressed to J. Barton and the UGA Beef Center staff for care of the animals, to A. Fielding, D. Nisbet, K. McKissack and all others who helped in the collection of blood samples, to B. Barrett for his laboratory assistance, to A. Nelson for statistical assistance and to B. Mabry for preparation for manu- script. :Anita. and Dairy Sci. a USDA-ARS Richard B. Russell Res. Center, Athens. 4Dept. of Physiol. and Pharmacol., Auburn Uni- versity, Auburn, AL. Received January 19, 1988. Accepted May 13, 1988. of age and maximize lifetime productivity. How- ever, rapid growth may increase fat accretion and decrease lifetime milk production (Crich- ton et al., 1960; Swanson, 1960; Little and Kay, 1979; Sejrsen et al., 1983). In grain-fed heifers a decrease in serum growth hormone (GH) was associated with increased body fat and decreased mammary tissue development (Sejrsen et al., 1983; Johnsson and Hart, 1985), whereas increased concentrations of GH pro- moted protein accretion (Eisemann et al., 1986). Insulin-like growth factor 1 (IGF-1), which mediates some GH activity, was associated with increased cartilage and muscle growth in non- ruminants (Phillips et al., 1978; Ewton and Florini, 1980, 1981; Etherton and Kensinger, 1984). In ruminants, studies of the role of 2916 J. Anim. Sci. 66:2916-2923