About 20 % of ruminal microbial N in dairy cows derives from purines and pyrimidines; however, their intermediary metabolism and contribution to the overall N metabolism has sparsely been described. In the present study, the postprandial patterns of net portal-drained viscera (PDV) and hepatic metabolism were assessed to evaluate purine and pyrimidine N in dairy cows. Blood was sampled simultaneously from four veins with eight hourly samples from four multi-catheterised Holstein cows. Quantification of twenty purines and pyrimidines was performed with HPLC–MS/MS, and net fluxes were estimated across the PDV, hepatic tissue and total splanchnic tissue (TSP). Concentration differences between veins of fifteen purine and pyrimidine nucleosides (NS), bases (BS) and degradation products (DP) were different from zero (P≤ 0·05), resulting in the net PDV releases of purine NS (0·33–1·3 mmol/h), purine BS (0·0023–0·018 mmol/h), purine DP (7·0–7·8 mmol/h), pyrimidine NS (0·30–2·8 mmol/h) and pyrimidine DP (0·047–0·77 mmol/h). The hepatic removal of purine and pyrimidine was almost equivalent to the net PDV release, resulting in no net TSP release. One exception was uric acid (7·9 mmol/h) from which a large net TSP release originated from the degradation of purine NS and BS. A small net TSP release of the pyrimidine DP β-alanine and β-aminoisobutyric acid ( − 0·032 to 0·37 mmol/h) demonstrated an outlet of N into the circulating N pool. No effect of time relative to feeding was observed (P>0·05). These data indicate that considerable amounts of N are lost in the dairy cow due to prominent intermediary degradation of purines, but that pyrimidine N is reusable to a larger extent.

The objective of this study was to determine the pattern of energy metabolites net flux across the portal-drained viscera (PDV) and total splanchnic tissues (TSP) in mature sheep fed varying levels of lucerne hay cubes. Four Suffolk mature sheep (61.4 ± 3.6 kg BW) surgically fitted with multi-catheters were fed four levels of dry matter intake (DMI) of lucerne hay cubes ranging from 0.4- to 1.6-fold the metabolizable energy (ME) requirements for maintenance. Six sets of blood samples were simultaneously collected from arterial and venous catheters at 30-min intervals. With increasing DMI, apparent total tract digestibility increased linearly and quadratically for dry matter (P < 0.05), quadratically (P < 0.05) with a linear tendency (P < 0.1) for organic matter and tended to increase quadratically (P < 0.1) for NDF. PDV release of volatile fatty acids (VFA) and β-hydroxybutyric acid was relatively low at 0.4 M and then linearly increased (P < 0.05) with increasing DMI. Net PDV flux of non-esterified fatty acids showed curvilinear decrease from 0.4 to 1.2 M and then increased at 1.6 M. The respective proportions of each VFA appearing in the portal blood differed (P < 0.05) with DMI and this difference was more obvious from 0.4 to 0.8 M than from 0.8 to 1.6 M. Heat production, as a percentage of ME intake (MEI), decreased linearly (P < 0.05) with increasing DMI accounting for 37%, 21%, 16% and 13% for PDV and 62%, 49%, 33% and 27% for TSP at 0.4, 0.8, 1.2 and 1.6 M, respectively. As a proportion of MEI, total energy recovery including heat production, decreased linearly with increasing DMI (P < 0.05) accounting for 113%, 83%, 62% and 57% for PDV and 140%, 129%, 86% and 83% for TSP at 0.4, 0.8, 1.2 and 1.6 M, respectively. Regression analysis revealed a linear response between MEI (MJ/day per kg BW) and total energy release (MJ/day per kg BW) across the PDV and TSP, respectively. However, respective contributions of energy metabolites to net energy release across the PDV and TSP were highly variable among treatments and did not follow the same pattern of changes in DMI.

Crossbred sheep (n 16,8·5 months of age and 33 (SE 0·9) kg) were used in a 21 d experiment (2x2 factorial) to determine effects on net flux of nutrients across the portal-drained viscera (PDV) and liver of ad libitum consumption of bermudagrass (Cynodon dactylon; B) v. ryegrass (Lolium multiflorum)-wheat (Triticum aestivum; RW) hay, coarsely chopped (CC) or finely ground and pelleted (GP). Crude protein concentrations were 86, 81, 113 and 119g/kg and neutral-detergent fibre concentrations were 710, 688, 654 and 672 g/kg (dry matter basis) for B-CC, B-GP, RW-CC and RW-GP respectively. Digestible energy intake (6.0,9.6·, 10·2 and 13·8 W/d) Mered (P < 0·01) with grass source and form, and digestible N intake values were 4·4, 7·0, 8·4 and 14.1 (SEM 0·82) g/d for B-CC, B-GP, RW-CC and RW-GP diets respectively. Consumption of O2 by the PDV (118,165,144 and 155mmol/h) and splanchnic bed (196,273,247 and 266 mmollh for B-CC, B-GP, RW-CC and RW-GP respectively) was greater (P=O·O7) for GP than for CC. The ratio splanchnic heat energy production: digestible energy intake was greater (P=0·06) for B than for RW (0·374,0·300,0·278 and 0·219 for B.CC, B-GP, RW-CC and RW-GP respectively). α-Amino-N release by the PDV (P< 0·01; 11·6, 12·8, 23·0 and 18·7 mmoyh) and uptake by the liver (P=0·07; 15·2, 6·1, 17·0 and 19·3 mmol/h for B-CC, B-GP, RW-CC and RW-GP respectively) were greater for RW than for B. Release of NH3-N by the PDV was greater (P=O·02) for CC than for GP (12·5, 6·2, 15·7 and 8·9 mmol/h), and hepatic urea-N release differed between grass sources (P=O·O3) and physical forms (P=0·07; 22·6, 12·7, 31·4 and mmol/h for B-CC, B-GP, RW-CC and RW-GP respectively). In conclusion, decreases in forage particle size elicited by grinding and pelleting did not affect the difference between grass sources in splanchnic tissue heat energy production relative to digestible energy intake.

Fifteen sheep (53 kg), with catheters in a hepatic vein, the portal vein and a mesenteric vein and artery, were offered a bromegrass (Bromus inermis) hay (104 g crude protein (CP), 700 g neutral-detergent fibre and 65 g acid-detergent lignin/kg dry matter (DM)) ad lib. with different supplements to determine the effects on net flux of oxygen and nutrients across the portal-drained viscera (PDV) and liver. The sheep were unsupplemented (Control) or received 5 g DM/kg body weight (BW) of ground maize (M), 7 g DM/kg BW of soya-bean hulls (H) or 0.73 g DM/kg BW of a mix of feedstuffs high in rumen-undegradable protein (P). Apparent digestible energy (DE) intakes were 5·3, 10·4, 10·6 and 6·7 (SE 0·74) MJ/d and apparent digestible CP intakes were 37, 50, 79 and 68 (SE 4·3) g/d for Control, M, H and P treatments respectively. Splanchnic tissue oxygen consumption rates were 0·23, 0·32, 0·30 and 0·27 (SE 0·054) mol/h, and oxidative metabolism accounted for 0·46, 0·31, 0·33 and 0·47 (SE 0·051) of DE intakes for Control, M, H and P treatments respectively. Supplements increased (P 0·05) release of α-amino nitrogen (AAN) by the PDV (4·2, 17·5, 19·6 and 18·1 mmol/h for Control, M, H and P treatments respectively). Splanchnic net flux of AAN was not affected by supplement treatments. Hepatic release of urea-N was increased (P 0·05) by supplement treatments (27, 40, 46 and 44 mmol/h for Control, M, H and P respectively); the P treatment increased (P 0·05) and the H treatment tended (P = 0·10) to increase splanchnic release of urea-N (7, 10, 20 and 27 mmol/h for Control, M, H and P treatments respectively). Net flux of glucose across the PDV was -4·6, 1·4, -5·6 and -7·2 (SE 1·65) mmol/h for Control, M, H and P treatments respectively. Hepatic glucose released averaged 23 (SE 2·0) mmol/h and was not affected by treatment. Treatments M and H increased (P 0·05) PDV release of propionate compared with the Control treatment (4·5, 15·5, 16·8 and 7·7 mmol/h for Control, M, H and P treatments respectively). Release of acetate by the PDV was 43, 97, 118 and 67 (SE 23·9) mmol/h for Control, M, H and P treatments respectively. In summary, different supplements of low-quality grass did not increase the efficiency of N metabolism by splanchnic tissues. Treatment P had little effect on net flux across splanchnic tissues of glucose, L-lactate. β-hydroxybutyrate and volatile fatty acids (VFA). Overall, treatments M and H had similar effects on splanchnic net fluxes of VFA and L-lactate whereas butyrate and β-hydroxybutyrate releases by the PDV were increased by treatment M.