Validity of using accumulated gas pressure readings to measure forage digestion in vitro: a comparison involving three forages.

Gas output from in vitro digestion of forage can be used to measure both forage digestibility and the kinetics of microbial digestion. Two different approaches are used to measure gas volumes: 1) either the gas is collected at atmospheric pressure and its volume determined directly, or 2) the gas accumulates in a fixed volume container, and the volume is calculated from pressure changes. We have used the latter technique to study the in vitro digestion of alfalfa, timothy, and corn silage by ruminal microorganisms in bottles of different sizes. We measured pH changes, NDF disappearance, and VFA concentrations and characterized the digestion profiles using a single- or dual-pool logistic model. Bottle size had a slight effect on pH but was not associated with changes in any of the other characteristics measured including the kinetic parameters. Gas measurements based on pressure increases in a fixed volume container appear to offer a valid alternative to methods based on gas collection at atmospheric pressure. We present an equation to adjust pressure sensor calibration values for use in different sized containers.