Feed specific passage rates form an essential element within modern nutrient-based models to predict nutrient uptake of the dairy cow. The main objective of this project is to acquire knowledge on the passage behaviour of single feed components through different compartments of the digestive tract of dairy cows. Several animal trials will be conducted in which passage and degradation rates are determined for various grass and maize silages. Passage kinetics is studied by incorporating stable isotopes (13C, 15N) in the cell wall and non-cell wall fractions of the plant. The outcome of this study will contribute to the further development of more dynamic mechanistic feed evaluation models.