An in vitro technique for measuring the production rate of volatile fatty acids in the rumen under dynamic conditions

A one-year-old wether fitted with a permanent ruminal cannula was used to examine an in vitro techniques potential for estimating the rate of VFA production in the rumen. Ruminal contents were sampled 4-h post-feeding and incubated under anaerobic conditions for 0, 10, 15, 20, 30, 70, 110, 160, and 240 min. The rate of VFA production was calculated from the incremental increases in acid concentration occurring over time. An exponential model, Y = a − b exp−ct (where Y is the concentration of VFAat time t, a the asymptote, b the fermentation potential, exp the base of natural logarithms, c the fractional rate of fermentation or rate constant and t the time) was used to describe VFA production in vitro. It was assumed that the rate of in vitro VFA production at time zero was equivalent to the rate of in vivo VFA production at the time the sample was removed. To ensure that the incubation intervals used in this study were appropriate, sampling times were estimated for future studies. The average coefficient of multiple determinations for the model used to describe in vitro VFA production was 0.96 and 0.93, respectively, for the high concentrate and high forage diets which suggests that it was appropriate. Estimates of daily VFA production for the high concentrate diet averaged 7.0 ± 1.87 mol/day while estimates for the high forage diet averaged 4.0 ± 0.5mol/day. Rates of VFA production from both diets were similar to rates reported in vivo from sheep feed similar diets using isotope techniques. Estimation of appropriate sampling times using data from the present study indicated that the in vitro fermentation times for the high forage diet should be extended to 4.5 h using sampling intervals of 0, 15, 30, 50, 75, 100, 135, 185, and 275 min. In vitro fermentation for 3 h at 0, 10, 20, 30, 45, 60, 85, 115, and 170 min appeared appropriate for the high concentrate diet. An in vitro procedure and mathematical approach were applied to estimate ruminal VFA production in vivo. This method provides a practical technique to measure the effect of different diets on ruminal fermentation. © 2003 Published by Elsevier B.V.