The gas production (GP) technique (e.g. Theodorou et al., 1994; Cone, 1994) is increasingly used as an indirect measure of substrate fermentability in the rumen. However, there are few data concerning the effect of making changes to the various integral steps of the GP procedure. This experiment was designed to study the effect of the host diet on the gas production profiles of grass hay and high temperature dried grass (HTDG).

Two cows were given either a silage:barley diet (80:20 dry-matter (DM) basis) or barley straw (ad libitum). Four experimental periods with an adaptation period of 3 weeks were used so that each animal experienced each diet type twice. Rumen fluid and solids were collected at the end of each 3-week period, prepared and used to inoculate the cultures. Duplicate samples (milled, 1-mm screen) of either hay or HTDG were accurately weighed (̴ 0-5 g DM) into 250 ml bottles, 85 ml anaerobic buffer (as Theodorou et al., 1994) were then dispensed into each bottle before being connected to the automated GP data logger (as Cone, 1994). After 16 h bottles were inoculated with rumen fluid (15 ml) according to the experimental design and incubated at 39°C for 48 h. The GP data were fitted to the model suggested by France et al. (1993). A representative sample (8 ml) of final culture fluid was preserved with 1 mol/1 orthophosphoric acid (2 ml) and stored frozen (-20°C) to await analysis for volatile fatty acids.