The aim of this experiment was to examine the effect of rumen digesta flow parameters on effective degradability (EDG) and microbial nitrogen (MBN) yield in sheep fed diets of identical provision of both metabolizable energy and rumen degradable nitrogen (RDN). Ruminal degradation parameters of early-harvested perennial ryegrass hay (EH), late-harvested perennial ryegrasses hay (LH) and winter sown barley straw (BS) were determined by a nylon bag technique. Subsequently, three experimental diets (EHD, LHD and BSD) were formulated using the tested forages, sucrose and urea as supplements. An in vivo feeding study was conducted using four rumen cannulated sheep in a partial Latin square design. Digestibility, rate constants of rumen particle breakdown and passage, ruminal fermentation parameters and MBN supply to the small intestine were determined. Animals thoroughly consumed the forages fed at a restricted level. Digestion coefficients were greater for EHD than for LHD and were lowest for BSD (P < 0.05). The rate constant of large particle breakdown was 4.3, 5.9 and 6.7 %/h, respectively, and small particle passage was 5.3, 4.7 and 6.3 %/h for EHD, LHD and BSD, respectively. The estimates differed (P < 0.05) between the diets. The overall passage rate constant of total rumen particles was estimated to be higher for BSD than that for EHD or LHD (P < 0.05). Ruminal fermentation parameters were unaffected by dietary treatments (P > 0.05). Intake levels of rumen degradable organic matter (RDOM) and RDN were estimated to be greater for LHD than that for EHD or BSD (P < 0.05). Although dietary arrangements were made to give identical microbial efficiency, the estimated value was higher for EHD than that for LHD or BSD (P < 0.05); and the MBN yield for BSD was estimated to be lower than that for EHD or LHD (P < 0.05). Rumen kinetic parameters of degradation and particle flow of forage affected EDG values and MBN yield from forage-related RDN intake, although those had little effect on the efficiency of MBN yield from forage-related RDOM intake.