Effect of time of starch supply to the rumen on the dynamics of urea and ammonia net flux across the rumen wall of sheep

Abstract

The effects of rumen starch fermentation on urea and ammonia net fluxes across the ruminal wall were studied in four Texel wethers (67 ± 1.5 kg BW) fitted with catheters in both ruminal veins and in a mesenteric artery, blood flow probes on ruminal arteries, and a ruminal cannula. They were fed 500 g of orchardgrass hay every 12 h. A dose of 120 g of starch was added directly to the ru- men either during the meal or 3 h later. On a daily basis, the time of starch supply did not modify the urea and ammonia net flux across the rumen wall. Within the feeding cycle, whatever the time of starch injection, the decrease in ruminal ammonia and pH and the increase in ruminal volatile fatty ac- ids and CO2 associated with starch fermentation were accompanied by a decrease in ammonia ab- sorption and an increase in urea net transfer across the ruminal wall. This study confirms the existence of a synchronisation between urea net transfer across the rumen wall and rumen carbohydrate fer- mentation within a feeding cycle. Factors acting in the regulation of urea and ammonia net flux in the ruminal veins are discussed. ammonia / urea / flux / rumen / sheep