MICROBIAL THIAMIN METABOLISM IN THE RUMEN SIMULATING FERMENTER (RUSITEC) - THE EFFECT OF ACIDOGENIC CONDITIONS, A HIGH-SULFUR LEVEL AND ADDED THIAMIN

The effects of acidogenic conditions, a high S level and the addition of thiamin on the rumen microbial metabolism of thiamin were investiga ted in vitro in a semi-continuous fermenter (RUSITEC), using a factori al design. Acidogenic conditions were obtained by simultaneously incre asing the starch:cellulose ratio and the amount of solid substrate fed , and by decreasing the buffering capacity of the liquid phase of the fermenter. S in the form of sulfate was supplied at two levels, one co rresponding to a control amount of S (2 g/kg dietary DM), the second t o an excess (5 g/kg DM) which is sufficient to trigger cerebrocortical necrosis (CCN) when used in vivo. Acidogenic conditions decreased the pH of the fermenters, CH4 production and cellulose digestibility, inc reased the short-chain fatty acid production, but had no effect on thi amin production. The high S level enhanced the production of sulfide c onsiderably, had no effect on the microbial metabolism of energy and N , and decreased thiamin production (326 v. 266 nmol/d). The added thia min was rapidly converted into phosphorylated compounds which largely decreased the apparent synthesis of this vitamin by the rumen microflo ra. The total thiamin flow was increased by added thiamin. In no case was thiaminase activity in the fermenter liquid phase significantly mo dified. The high level of S induced only a limited decrease of total t hiamin dow. Consequently, it is unlikely that the investigated factors could be considered to be high risk factors for the thiamin-dependent CCN.