A passage rate of DM in the total tract of 3% h-' gave
best results for estimating OMD (Gosselink et a!., 2004). A ruminal passage rate of
DM of 4.5% h-' is used in the Netherlands (Tamminga et a!., 1994) and 6% h-' in
France (Verite et a!., 1987). In our calculations also a passage rate (kp) equal to rumen degradation rate (kd) was used. kp as function ofkd improved the estimate ofFOM as
calculated in the Dutch protein evaluation system (Van Vuuren, 1993).
Two incubation series were carried out per forage type. The two series were incubated
at the beginning and at the end of the week. Each forage type was incubated in
three cows that were fed a ration of 70% forage and 30% concentrates.