The rumen microflora ecosystem adapts to the diet consumed by the animal. We tested the extent to which this adaptation facilitates or retards the digestion of plant-based forages. Following adaptation of sheep (Ovis aries) and red deer (Cervus elaphus) to diets containing different mixtures of alfalfa, grass, and heather (a dwarf shrub), an in vitro digestion technique was used to compare the ability of the rumen microflora to digest the mixtures of substrates to which they were adapted with their ability to digest different mixtures of the same substrates. In vitro digestion of different substrates was slightly greater in rumen liquor derived from sheep than in that derived from red deer for each of the different substrates, but the effect was not significant. Digestibility in sheep was independent of how the feed was presented (diet of equal proportions of alfalfa, grass, and heather in each meal (D-EQ): mean in vitro digestibility = 37.3%; alfalfa, grass, and heather presented sequentially on different days (D-SEQ): mean in vitro digestibility = 37.7%, SE of differences = 1.30%, p > 0.05). However, in red deer there was a significant effect of method of diet presentation (D-EQ: mean in vitro digestibility = 36.9%; D-SEQ: mean in vitro digestibility = 34.2%, SE of differences = 1.30%, p < 0.05), digestibility being substantially lower for D-SEQ than for D-EQ. Overall, the results demonstrated that whilst there were no species-specific differences in overall digestion efficiency, dietary adaptation had an effect on substrate digestion efficiency, with rumen microbes adapted to high-quality diets digesting these more efficiently than low-quality diets.