HEPATIC UREA SYNTHESIS AND SITE AND RATE OF UREA REMOVAL FROM BLOOD OF BEEF STEERS FED ALFALFA HAY OR A HIGH CONCENTRATE DIET

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Abstract
Five Hereford × Angus steers, 240 ± 6 kg liveweight with chronic indwelling catheters in various sites in the splanchnic bed were used to evaluate effects of dietary energy source and nitrogen (N) level on urea synthesis and site and rate of urea removal from blood. Alfalfa hay (162 g N d−1) or a high concentrate diet (95 g N d−1) were fed at similar metabolizable energy intake (54.8 MJ d−1). Compared with the alfalfa, the high concentrate, low N diet decreased (P < 0.05) blood concentrations of urea N, hepatic urea synthesis, urinary urea N excretion, transfer of urea N to poststomach tissues, and salivary transfer of urea N. Similarly, the high concentrate diet decreased net absorption of ammonia N and increased net absorption of glucose from portal-drained viscera decreased hepatic removal of ammonia N and α-amino N, and increased release of α-amino N by splanchnic tissues (P < 0.01). Blood flow (dilution of para-aminohippurate) through the liver and gut was not affected (P > 0.10) by diet. A greater (P < 0.05) portion of urea synthesized in the liver was transferred to the gut when the high concentrate diet was fed. Site of total urea transfer to the gut (from blood plus saliva) shifted from 37% of total transfer into poststomach (alfalfa) to 75% into stomach (high concentrate diet). Increased intake of readily fermentable carbohydrate at the expense of N and fiber increased the percentage of liver urea synthesis recycled to the gut and focused that recycling into the stomach (rumen) in these steers. Key words: Bovine, energy, nitrogen, urea flux, gut, liver