Net flux of metabolites across the ruminal wall of sheep fed twice a day with orchardgrass hay.

Four Texel wethers (68 +/- 2.5 kg BW) fitted with catheters in the ruminal veins and a mesenteric artery, blood flow probes around ruminal arteries, and a ruminal cannula were used to determine meal-related variations and daily significance of net flux across the ruminal wall of urea and ammonia (NH3), VFA, D-beta-hydroxybutyrate (D beta HOB), lactate, and glucose. Sheep were fed every 12 h with orchardgrass hay (430 g of DM/meal; 611 g of digestible OM/kg of DM and 23.6 g of N/kg of DM). Apart from lactate and glucose, the fluxes of studied metabolites were significantly affected by time after morning feeding. Maximum absorption of VFA and NH3 were observed at the end of the meal; however, 5 h after the meal VFA absorption was still high, whereas NH3 absorption had decreased to the prefeeding level. Net release of D beta HOB was greater during the 2 h after the meal than during the rest of the time. Urea net transfer decreased during the meal, and thereafter it increased to the 5th h after feeding, at which time it was twofold higher than at prefeeding. The difference in net flux across the ruminal wall of urea and NH3 was linearly correlated with NH3 concentration in the ruminal fluid. Daily urea and NH3 net transfer were -2.10 and 3.76 g of N/d, respectively. The VFA net appearance in the ruminal veins was 1.167, .226, and .014 mol/d for acetate, propionate, and butyrate, respectively. Daily net release of D beta HOB, lactate, and glucose by the rumen wall was .153, .093, and -.012 mol/d, respectively.