NET TRANSFER OF UREA AND AMMONIA ACROSS THE RUMINAL WALL OF SHEEP

Texel wethers (68 +/- 2.5 kg BW) fitted with catheters in the ruminal veins and a mesenteric artery, blood flow probes on ruminal arteries, and a ruminal cannula were fed 500 g of orchardgrass hay every 12 h. D uring the last third of the feeding cycle, intraruminal injections wer e performed to evaluate the effect of urease activity, osmolality, and concentrations of NH3, butyrate, and CO2 in the rumen on urea and NH3 fluxes across the rumen wall. At pH 6.7, NH3 absorption increased wit h NH3 and butyrate concentrations in the rumen, and to a lesser extent with CO2 concentration. The increase in ruminal blood flow associated with CO2 and butyrate increase was always greater than the increase i n NH3 absorption. Increasing ruminal osmolality slightly decreased NH3 absorption. Ruminal NH3 concentration and ruminal blood flow seemed t o be the main determinant of NH3 absorption. Decreasing urease activit y in the rumen decreased urea net transfer. The net transfer of urea t o the rumen was stimulated by CO2. High concentrations of NH3 (330 mg of N/L) and butyrate (25 mM) in the rumen decreased urea net uptake, w hereas osmolality (up to 420 mOsmol/L) did not affect it. Modification s in ruminal blood flow or water net movement across the ruminal wall did not seem to account for the effect Of CO2, NH3, and butyrate on ur ea net uptake.