The gas measured in an in vitro experiment is an indirect measurement of nutritionally important events. Feeds in the rumen are converted to short-chain fatty acids, to CO2, to CH4 and to microbial mass plus water. The pathways used for microorganisms for carbohydrate degradation will determine the amount of ATP available for microbial cell production and vary with the type of substrate, nature of inoculum and time of observation. The aim of this work was to study the relationship between gas volume and microbial growth regarding to the substrate quality.