INFLUENCE OF GRASS MATURITY AND DIET ON RUMINAL DRY-MATTER AND NEUTRAL DETERGENT FIBER DIGESTION KINETICS

The effects of, and interactions between, the diet and maturity of gra ss were studied in cattle in a 6 x 6 Latin square experiment with a 2 x 3 factorial arrangement of treatments. Barn-dried grass or direct-cu t silage from the same award were fed together with 250 (L), 500 (M) o r 750 (H) g concentrate dry matter (DM) kg(-1) total DM. Six timothy-m eadow fescue grasses, cut at 7 d intervals, were incubated in nylon ba gs in the rumen for 6, 12, 24, 48, 72 and 96 h, and the kinetics param eters for DM and neutral detergent fibre (NDF) digestion were estimate d. Ruminal 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 o f DM and NDF digestion decreased with maturity. The changes were curvi linear with an increasing depression in the extent of digestion and a decreasing depression in the rate of digestion with maturity. Ruminal NDF digestibility averaged over the diets decreased from 61.2 to 39.7% with the maturity. DM and NDF disappearance and calculated NDF digest ibility decreased as the proportion of concentrate in the diet increas ed. This adverse effect of concentrate increased more rapidly as the p roportion of concentrate increased from M to H and rumen pH dropped fr om 6.2 to 6.0 than as the proportion of concentrate increased from L t o M with a consequent reduction in rumen pH from 6.4 to 6.2. The minim um 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 give n the dried grass diets than in those given the silage diets. The adve rse effect of the concentrate level on NDF digestibility increased wit h 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 concen trate, respectively.