Nutrient availability during the early stages of colonization of fresh forage by rumen micro-organisms

• During grazing, it is unclear to what extent rumen conditions cause immediate physical damage to intact, ingested plant cells and whether this coincides with microbial degradation of plant constituents. • An in vitro model that included or excluded rumen microorganisms was used to investigate the extent of membrane damage and release of cell constituents from model and forage plants in relation to fermentation parameters. • Significant ion release after 2 h indicated that plant cell membranes were intolerant of rumen conditions. Linear release of soluble carbohydrates was observed when timothy grass (Phleum pratense) was incubated in the presence or absence of rumen fluid for 6 h. Up to 50% of the betanin was released from vacuoles of red beet (Beta vulgaris) tuber at least 6 h before microbial fermentation had attained maximal rates; 20% of this release was due to nonmicrobial action. • We conclude that release of small molecules from ingested, viable plant cells is limited by cell wall porosity. Thus plant structure may influence the colonization and degradation of plant cells by the microbial population.