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 rum inal veins and a mesenteric artery, blood flow probes around ruminal a rteries, and a ruminal cannula were used to determine meal-related var iations and daily significance of net flux across the ruminal wall of urea and ammonia (NH3), VFA, D-beta-hydroxybutyrate (DbetaHOB), lactat e, and glucose. Sheep were fed every 12 h with orchard-grass 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 w ere significantly affected by time after morning feeding. Maximum abso rption 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 absorptio n had decreased to the prefeeding level. Net release of DbetaHOB was g reater during the 2 h after the meal than during the rest of the time. Urea net transfer decreased during the meal, and thereafter it increa sed to the 5th h after feeding, at which time it was twofold higher th an at prefeeding. The difference in net flux across the ruminal wall o f urea and NH3 was linearly correlated with NH3 concentration in the r uminal 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, respec tively. Daily net release of DbetaHOB, lactate, and glucose by the rum en wall was .153, .093, and -.012 mol/d, respectively.