The method of sample preparation for the in situ measurement (nylon bag technique) Of dry matter degradation has been described by Dulphy . The procedure of the measurement was according to Michalet-Doreau and the data were fitted according to 0rskov & McDonald. Effective degradable dry matter was calculated using different passage rates. A passage rate of dry matter in the total tract of 3% h-' gave best results for estimating organic matter total tract digestibility. A ruminal passage rate of dry matter of 4.5% h-' is used in the Netherlands and 6% h-' in France In our calculations also a passage rate (kp) equal to rumen degradation rate (kd) was used. kp as function of kd improved the estimate of fermentable organic matter as calculated in the Dutch protein evaluation system. 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