The objective was to evaluate the ability of in situ and in vitro gas production methods in determining the variation in grass and grass silage organic matter digestibility (OMD) compared to in vitro pepsin-cellulase solubility, which was considered as a reference method. The variation in OMD was obtained by varying grass species, stage of maturity at harvest and preservation method (dried vs. ensiled). Five grass species (Dactylis glomerata, Phleum pratense, Lolium perenne, Festuca arundinacea and Lolium multiflorum . Festuca arundinacea) harvested from primary growth at four dates in 2005 were analysed for chemical composition and OMD using in vitro pepsin-cellulase solubility (OMDPC), in vitro gas production (OMDGP) and three in situ methods. When using the in situ methods, the OMD was derived from traditional sequential nylon bag incubations and calculated using one-compartment or two-compartment models (OMDIS1 and OMDIS2, respectively), or from indigestible neutral detergent fiber (iNDF).