Cumulative gas production: how to measure it, and what it can and cannot tell you

Summary The three most important factors describing the nutritive value of ruminant feeds are: feed intake, rumen fermentation, and enzymatic digestion. Animal experiments are usually expensive and time-consuming, so many in vitro techniques have been developed which are supposed to assist in the prediction of nutritive value, usually of rumen fermentation. One of these is the measurement of gas production from the substrate when incubated with a specific inoculum. The cumulative gas production technique measures gas at regular intervals in time, as an indicator of the kinetics of fermentation. The resultant cumulative gas profiles, fitted to specific equations, and in combination with other end-products (i.e. VFA and NH3) can indicate both the rate and extent of fermentability of the test substrates. This information can be used to compare substrates with each other. However, the fact that the technique has not yet been standardized, and that inoculum varies according to time and place, means that comparison both between runs and locations, can be very difficult. Also, there are a number of aspects of the technique which will always mean that its resemblance to the in vivo situation must be limited. Some of these limitations and other important considerations of the technique are described in this review.