Rumen microbial sequestration of [2-(13)C]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-(13)C] acetate, three nonlactating or low-yielding dairy cows were continuously intraruminally infused with [2-(13)C]acetate for 26 h. The 13C content of ruminal VFA, duodenal carbon, and fatty acids (FA) and AA isolated from liquid-associated ruminal microbes and duodenal DM was measured by an isotope ratio mass spectrometer interfaced to an elemental analyzer or a gas-liquid 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 infused 13C in [2-(13)C]acetate, 7.6 +/- 0.9% was recovered at the duodenum. The 13C content of ruminal propionate, butyrate, and valerate increased (P < 0.05) with intraruminal infusion of [2-(13)C]acetate. It was estimated that about 28% of the 13C intraruminally infused in [2-(13)C]acetate could be accounted for by duodenal 13C flow and absorption of non-acetate VFA. A number of FA isolated from liquid-associated ruminal microbes (C6, C12, C14, anteiso C15, and iso C15) were enriched with 13C (P < 0.05) at a level comparable to the enrichment of ruminal butyrate. Any absorption of these FA from the rumen would further contribute to non-acetate 13C uptake. A maximum of 72% of the ruminal gross production of acetate represented acetate absorption from the rumen in the present study. Consequently, previously used models using intraruminal isotope dilution techniques seem not to be appropriate for measuring acetate availability in ruminants. The number of metabolites exchanging carbon with acetate was found to be so high that assessments of the entire range of inter conversions seem to be practically impossible. Portal absorption studies are discussed as an alternative method of estimating VFA availability to the metabolism in ruminants.