Five grass species (Dactylis glomerata L., Phleum pratense L., Lolium perenne L., Festuca arundinacea L. and the hybrid Felina (Lolium multiflorum L. × Festuca arundinacea L.)), commonly used in roughages for ruminants, were harvested at different maturities of primary growth (n = 60) and evaluated for contents of dry matter (DM), crude protein (CP), ash, crude fat, neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), indigestible neutral detergent fibre (INDF) and digestible neutral detergent fibre (DNDF). INDF and DNDF contents were determined by in sacco rumen degradation of grasses for 12 days in non-lactating cows. ADL content was found to be highly correlated (P < 0.05) with DNDF (r = –0.87) and presented a reliable (R2 = 0.78; residual mean square error of 17.65 g/kg DM; P < 0.0001) parameter to predict INDF contents. Over a six-week period of maturation INDF contents increased (P < 0.0001) in all studied grasses. It was confirmed by this study that the INDF contents of grasses, which markedly increased during maturation, could be effectively predicted from ADL contents.