EFFECTS OF GROWTH HORMONE-RELEASING FACTOR AND FEED-INTAKE ON ENERGY-METABOLISM IN GROWING BEEF STEERS - WHOLE-BODY ENERGY AND NITROGEN-METABOLISM

Effects of growth hormone-releasing factor (GRF) on energy and N metabolism in six growing Hereford x Angus steers were measured using a split-plot design with 4-wk injection periods within 8-wk intake periods. Steers were fed a 75% concentrate pelleted diet at two intakes (low: 50 g/BW.75 and high: 90 g/BW.75 as fed) and injected s.c. with saline or 10-mu-g/kg of BW of human GRF(1-29)NH2 twice daily for 3 wk. Measurements of energy and N balance were obtained during wk 3 of treatments. Diet DM digestibility (%) was decreased by greater intake (P < .05) and increased by GRF (P < .06). Treatment with GRF increased (P < .01) N retention by decreasing (P < .05) fecal and urinary excretion: N retention averaged 10.0 and 20.8 g/d at low intake and 25.9 and 46.7 g/d at high intake for control-and GRF-treated steers, respectively. Increased ME (P < .05) in GRF-treated steers also resulted from decreased fecal (P < .05) and urinary (P < .07) energy excretion but was countered by increased (P < .06) heat energy (HE). Tissue energy (TE), partial efficiency of ME use for TE retention, and estimated maintenance energy were not affected (P > .10) by GRF treatment. In summary, GRF treatment altered the partition of TE by increasing protein retention (108 and 80% for low and high intake, respectively) at the expense of fat retention.