Nutrients intake of grazing sheep based on net flux of nitrogen and oxygen by splanchnic tissues

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The objectives were to investigate energy intake and nitrogen (N) metabolism of grazing sheep with net flux of N and oxygen consumption by the portal drained viscera (PDV) and liver through three experiments over two years. Wethers with chronic indwelling catheter in the portal, hepatic and mesenteric veins and the abdominal aorta were used under housing with restricted feeding (Exp. 1 and 2) and grazing with adlibitum intake (Exp. 3) conditionsExp. 1 : The sheep were fed a diet consisting of forage (70) and concentrate (30) at four levels, 0.5 to 1.76 times maintenance energy (M). Digestible energy and N intakes and N retention of sheep linearly increased with the feeding level. Ammonia N absorption by PDV also increased linearly, while α-amino N absorption, starting from null at 0.5M, increased quadraticallyExp. 2 : The sheep were fed forage (1.0M) plus four amounts of barley supplement (0 to 1.0M). Digestible energy and N intakes increased linearly, but N retention quadratically responded to the supplement level. The PDV release of α-amino N increased linearly with the supplement level, although the a -amino N release by the total splanchnic tissues (TSP) was almost constant due to the increase in the liver uptake. The increased supplement linearly increased the remove of urea N by PDV, but did not affect urea N synthesis by the liver and ammonia N absorption by PDVExp. 3 : The sheep were grazed on a pasture during May and June. The α-amino N release by TSP increased with barley supplement, and was less in June compared with May. Digestible energy intake, estimated with a linear regression between the PDV oxygen consumption and energy intake in Exp. 1 and 2, was greater in June, and not affected by barley supplement