Rumen microbial sequestration of [2-C-13]acetate in cattle

To investigate the impact of rumen microbial sequestration of VFA carbon on estimates of acetate availability based on intraruminal infusion of [2-C-1 3] acetate, three nonlactating or low-yielding dairy cows were continuously intraruminally infused with [2-C-13] acetate for 26 h. The C-13 content of ruminal VFA, duodenal carbon, and fatty acids (FA) and AA isolated from li quid-associated ruminal microbes and duodenal DM was measured by an isotope ratio mass spectrometer interfaced to an elemental analyzer or a gas-liqui d chromatograph. The ruminal gross production of acetate was 38 +/- 4 mol/d and could account for about 38% of the DE intake. Of the intraruminally in fused C-13 in [2-C-13]acetate, 7.6 +/- 0.9% was recovered at the duodenum. The C-13 content of ruminal propionate, butyrate, and valerate increased (P < 0.05) with intraruminal infusion of [2-C-13] acetate. It was estimated t hat about 28% of the C-13 intraruminally infused in [2-C-13]acetate could b e accounted for by duodenal C-13 flow and absorption of non-acetate VFA. A number of FA isolated from liquid-associated ruminal microbes (C-6, C-12, C -14, ante-iso C-15, and iso C-15) were enriched with C-13 (P < 0.05) at a l evel comparable to the enrichment of ruminal butyrate. Any absorption of th ese FA from the rumen would further contribute to non-acetate C-13 uptake. A maximum of 72% of the ruminal gross production of acetate represented ace tate absorption from the rumen in the present study. Consequently, previous ly used models using intraruminal isotope dilution techniques seem not to b e appropriate for measuring acetate availability in ruminants. The number o f metabolites exchanging carbon with acetate was found to be so high that a ssessments of the entire range of inter conversions seem to be practically impossible. Portal absorption studies are discussed as an alternative metho d of estimating VFA availability to the metabolism in ruminants.