ABSTRACTThe acid‐detergent fibre (ADF) contents of 50 grass samples of knownin vivoandin vitrodigestibility were determined by hydrolysing with acid for 1, 2, 3,4, 5, and 6 h. The 50 samples used consisted of 6Lolium perenne, 9 Cenchrus ciliaris, 15Chloris gayana, 10Digitariaspp., and 10Setariaspp. Regression and residual standard deviations (RSD) were calculated for each hydrolysis time for each grass and for all 50 samples.Increasing the hydrolysis time decreased the RSD forLolium perenneandSetaria, had no effect onChloris gayanaandDigitariaand increased the RSD forCenchrus ciliarisregressions. For regressions based on all 50 samples die RSD varied between ±4.8 and 5.0 digestibility units with only minor differences between hydrolysis times. Correcting the ADF for contaminating ash generally increased the RSD.Changing the acid strength from the 10 N recommended by van Soest (8) did not lead to any major improvement in the RSD for regressions based onCenchrus ciliaris, but withSetariathe RSD was reduced from ±2.8 to 1.7 digestibility units when 2N acid was used and hydrolysis was extended to 2 h.It was concluded that unless the optimum conditions for determining ADF for each grass are known then thein vitrotechnique provides digestibility estimates with a lower er