The effects of, and interactions between, the diet and maturity of grass were studied in cattle in a 6 × 6 Latin square experiment with a 2 × 3 factorial arrangement of treatments. Barn‐dried grass or direct‐cut silage from the same sward were fed together with 250 (L), 500 (M) or 750 (H) g concentrate dry matter (DM) kg−1total DM. Six timothy‐meadow fescue grasses, cut at 7 d intervals, were incubated in nylon bags in the rumen for 6, 12, 24, 48, 72 and 96 h, and the kinetics parameters for DM and neutral detergent fibre (NDF) digestion were estimated. Ruminai NDF digestibility was calculated using a value of 0.02 for the rate of passage. With increasing maturity of grass, crude protein content and in vitro digestibility decreased with associated increases in the contents of cell wall constituents. Both the rate and extent of DM and NDF digestion decreased with maturity. The changes were curvilinear with an increasing depression in the extent of digestion and a decreasing depression in the rate of digestion with maturity. Ruminai NDF digestibility averaged over the diets decreased from 61.2 to 39.7% with the maturity. DM and NDF disappearance and calculated NDF digestibility decreased as the proportion of concentrate in the diet increased. This adverse effect of concentrate increased more rapidly as the proportion of concentrate increased from M to H and rumen pH dropped from 6.2 to 6.0 than as the proportion of concentrate increased from L to M with a consequent reduction in rumen pH from 6.4 to 6.2. The minimum rumen pH explained more of the variation in NDF digestibility than mean pH, duration or summation (pH x time) of the pH depression below 6.2, 6.0 or 5.8. Predicted NDF digestibility was higher in cattle given the dried grass diets than in those given the silage diets. The adverse effect of the concentrate level on NDF digestibility increased with the maturity of the grass. The decrease was 0.54, 0.59, 0.72% units per one day delay in cutting for low, medium and high levels of concentrate, respectively.