Since grass will likely be a dominant feedstock for on-farm anaerobic digestion in Northwest Europe, changes in the chemical composition of five common grass species with advancing harvest date in the primary growth were investigated and related to specific CH4 yields. The increase in fibre components with advancing harvest date had a negative impact on the specific CH4 yield (253 and 225 NI CH4 kg(-1) VS for 12 May and 7 July harvests, respectively), and this impact was similar across the five grass species. At common growth stages, only small differences in herbage digestibility was observed between the grass species and this was reflected in similar specific CH4 yields: however, the 26% lower area-specific CH4 yield of the cocksfoot variety (Dactylis glomerata L var. Pizza) would make it the most expensive of the five grass species to produce and the least suitable for anaerobic digestion. (C) 2012 Elsevier Ltd. All rights reserved.