Net portal flux of metabolites following a meal in calves fed a grain-rich diet supplemented with urea or meat meal

The present experiment was carried out to evaluate the effect of protein supplementation of grain-rich diets on the net absorption of some metabolites following a meal by calves, estimated from their net portal flux. The study was conducted on eight Holstein male calves with an average liveweight of 141 +/- 1.4 kg, fitted with permanent catheters in a mesenteric vein and in the portal vein and with a temporary catheter in an auricular artery. The animals were assigned random to two treatments: T1, hay + concentrate containing urea and T2, hay + concentrate containing meat meal. Portal blood flow (PBF) was estimated by an indicator dilution and net metabolites flux was calculated as the difference between venous and arterial concentrations times portal blood flow. Feed was offered for 50 min and, immediately after, infusion of the indicator started and blood samples were taken at 30 min intervals during 4 h. Net portal flux of glucose and urea N was negative and similar for the two treatments (P > 0.10). Ammonia N, beta-hydroxybutyrate and acetoacetate flux was positive and higher in calves fed diet containing urea (P < 0.01, P < 0.07, P < 0.07, respectively). The urea N portal flux was quadratically (P < 0.08, r(2) = 0.64) and ammonia N portal flux cubically (P < 0.06, r(2) = 0.81) related to time after the meal.