As grazing ruminants rely almost entirely on mastication to disrupt plant tissues, a series of processes (mastication, bolus formation and ingestion) will impact on the viability and number of cells that remain intact, and consequently alive, after ingestion (Kingston-Smith and Theodorou, 2000). Preliminary work in our group has shown substantial variation in the degree of cell damage during mastication and ingestion between grass species, resulting in differences in the rate of release of cell contents (protein, sugars and lipids) into the rumen (E.J. Kim, unpublished). These differences may affect nutrient utilisation by ruminal micro-organisms. The aim of this study was to compare the extent of nutrient release from three contrasting grass species following ingestion of the fresh forage by dairy cows.