The objective of this experiment was to compare measurements of fractional clearance rates obtained by using an unlabeled valerate-CoEDTA technique with measurements obtained by using a 13C-labeled volatile fatty acids (VFA) technique. The exponential decay rate of the 13C/12C ratio after pulse-dosing 13C-acetate, 13C-propionate, or 13C-butyrate into the rumen was compared with the decay rate of rumen valerate concentration following a simultaneous pulse dose. The unlabeled valerate, CoEDTA, and each labeled VFA, one at a time, were concurrently mixed with the evacuated ruminal content of 6 lactating cows in two 3 × 3 Latin squares. The clearance of VFA by passage to the omasum was assumed to be equivalent to the decay in ruminal Co concentration and was around 50% of the total clearance. Acetate, propionate, and butyrate had similar fractional clearance rates (31.2, 33.4, 30.4%/h, respectively), but propionate had a higher absorption rate (19.2%/h) than butyrate (14.2%/h). Linear regression determination coefficients using the valerate clearance rate as an estimator for acetate, propionate, and butyrate rumen clearance were 0.51, 0.56, and 0.99, respectively. In a second experiment, the 13C-valerate fractional clearance rate estimate (33.7%/h) was similar to the estimate obtained with unlabeled valerate (35.0%/h) by the valerate-Co technique. No 13C enrichment of rumen microbes was noted 4 h after the intraruminal infusion of 13C-valerate. Fractional VFA absorption rate estimates obtained in both techniques were similar, although both were lower than estimates reported in the literature by other methods.
