SUMMARYThe introduction of sodium acetate-2-14C into a bovine rumen resulted in the in vivo labelling of the rumen gases and volatile fatty acids. The relative isotope concentration in substrate and products indicated that a maximum of 5.6 % of the methane and 11% of the carbon dioxide in rumen gas might have been derived from the methyl carbon of acetate when the substrate was added to the rumen 18 hr. after the animal had been fed a normal ration, A maximum of 3-53 % of the methane and 4.2 % of the carbon dioxide might have been derived from the methyl carbon of sodium acetate-2-14C when this substrate was introduced into the rumen immediately after the animal had been fed. The addition of sodium acetate-l-14C to the rumen 18 hr. after feeding indicated that 2 yo of the methane and 10 yo of the carbon dioxide was derived from the carboxyl carbon of the substrate. Most of the derived radioactivity of the volatile acids of the rumen was found in the butyric acid fraction, although smaller amounts appeared in propionic acid and the volatile fatty acids with a chain length of greater than 4 carbons.
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