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 micrograms/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 less than .05) and increased by GRF (P less than .06). Treatment with GRF increased (P less than .01) N retention by decreasing (P less than .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 less than .05) in GRF-treated steers also resulted from decreased fecal (P less than .05) and urinary (P less than .07) energy excretion but was countered by increased (P less than .06) heat energy (HE). Tissue energy (TE), partial efficiency of ME use for TE retention, and estimated maintenance energy were not affected (P greater than .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.