The in vitro gas production technique aims to characterise feeds in terms of both the amount and rate of supply of fermentable carbohydrate. Models are used to interpret the data collected and typically assume that there is one pool of fermentable carbohydrate which is used at a fractional rate although it has been shown that inclusion of a modifying rate improves the fit of the model to the data (France et al., 1993). Since forage carbohydrate is not homogenous and can, at the simplest level, be divided into soluble (sugars) and insoluble (fibre) components, the aim of the experiment was to determine the influence of soluble material on the parameters obtained using this technique.

Gas production from glucose: 12.5, 25 and 50 mg; cellulose: 50, 100, 150 and 200 mg; glucose/cellulose mixtures: 10/190, 20/180, 30/170, 40/160 and 50/150 mg and Brachiaria decumbens (Brach.), Cynodon nlenfluensis (Star) and Pennisetum clandestinum (Kikuyu) grasses: 200 mg, was measured in prewarmed 100 ml glass syringes using standard techniques with four or six replicates per sample.