Use of compartmental analysis to study ruminal volatile fatty acid metabolism under steady state conditions in Holstein heifers

Four rumen-cannulated Holstein heifers were used to quantify volatile fatty acid (VFA) kinetics in the rumen using ¹³C stable isotopes. Heifers were fed either a low (L) or high (H) concentrate diet with level of concentrate of 250 or 700 g/kg dry matter, respectively. Isotopic tracers, Na-1-¹³C-acetate (Ac), -propionate (Pr), or -butyrate (Bu) were infused as a bolus into the rumen. Three days prior to sampling, cows were fed equal meals every 4 h for 2 days followed by 2 h feeding intervals 1 day prior to and on the day of sampling. The rumen was sampled 26 times (3 times prior to tracer infusion; background samples and 23 times during 8 h after tracer infusion). Isotopic enrichment was expressed as Tracer/(Tracee + Tracer) or F value. F values were corrected for skewness and graphed against time. A three-compartment model was adequate to describe VFA kinetics in the rumen. It was estimated that there was a flux of Ac to the Pr pool [0.05 g/min (L) versus 0.03 g/min (H)], resulting in an average conversion fraction of Ac to Pr of 0.051 in both groups. The model predicted that the fraction of Ac which contributed to the Pr pool was 0.198 (L) versus 0.145 (H). Although the fraction of the Ac conversion to Bu ranged from 0.367 (L) to 0.448 (H), only a small fraction [0.0137 (L) and 0.0160 (H)] of the Bu was converted to Ac. A major portion of the Bu was coming from Ac in both diet groups. The portion of Bu coming from Ac in the rumen varied between 0.95 and 0.65 in (L) and (H) diets, respectively. Turnover time of all VFA was calculated to be longer in H. Results indicate that Ac and Bu exchange, and that Ac contributes to the Pr pool. However, Pr does not contribute to the Ac or Bu pools. This study demonstrates the usefulness of ¹³C isotopes to study VFA kinetics and metabolism in cattle.