The study investigated in vitro simulated rumen metabolism of freshly cut and wilted cocksfoot [Dactylis glomerata; high polyphenol oxidase (PPO)] and tall fescue (Festuca arundinacea; low PPO). A 16-vessel RUSITEC was used with the four treatment combinations: cocksfoot wilted (Cw); cocksfoot fresh (Cf); tall fescue wilted (TFw) and tall fescue fresh (TFf). Rumen liquor was collected from four fistulated dairy cows maintained on permanent pasture. The experiment ran for 12 d with sampling of effluent for rumen parameters [volatile fatty acids (VFA) and NH3-N] on days 10 and 11 at 0, 1, 2, 3, 4, 6, 8 and 24 h intervals. On days 9?12, gas production and composition were measured using a plastic gas collection bag attached to the effluent bottle. At the end of day 12, the grass residue and effluent from the vessels were collected and analysed for digestibility and lipid fractionation. Lipolysis was calculated as the proportional loss of glycerol-based membrane lipid between the forage and residue. Polyphenol oxidase was higher (P