EFFECTS OF DIETARY CONCENTRATE LEVEL ON NUTRIENT ABSORPTION, LIVER-METABOLISM, AND UREA KINETICS OF BEEF STEERS FED ISONITROGENOUS AND ISOENERGETIC DIETS

Six multicatheterized beef steers (421 +/- 21 kg BW) were used to pred ict the effect of dietary concentrate level on blood flow and net flux of urea and other metabolites across splanchnic tissues. Diets ranged from 0% (switchgrass hay) to 90% concentrate (10% switchgrass hay, 89 % cracked corn, 1% urea). Daily DMI varied from 8.01 to 5.34 kg/d. Nit rogen intake (99 g/d) and calculated ME intake (16.8 Mcal/d) were equa l among diets. As dietary concentrate increased from 0 to 90%, liver b lood flow decreased from 850 to 795 L/h, portal-drained visceral (PDV) blood flow decreased from 750 to 620 L/h, and mesenteric-drained visc eral (MDV) blood flow decreased from 270 to 250 L/h. Liver release of urea N was 94 mmol/h when dietary concentrate was less than 20%, then increased to 146 mmol/h at 55% concentrate. Urinary excretion of urea N was 13 mmol/h or less when dietary concentrate was 20% or less, incr eased to 53 mmol/h at 55% concentrate, then continued to increase to 7 6 mmol/h at 90% concentrate. Transfer of urea N to PDV ranged from 71 to 91 mmol/h and transfer to MDV ranged from 0 to 10 mmol/h among diet s. As dietary concentrate increased from 27 to 63%, VFA release by PDV decreased, net MDV and splanchnic release of glucose increased, and s planchnic tissues switched from net uptake to net release of L-lactate . Net PDV release or liver removal of ammonia or alpha-amino N and net liver release of glucose were not affected. We conclude that the live r responded to changes in the percentage of dietary concentrate by alt ering urea production and by altering the role of lactate in intermedi ary metabolism.