Knowledge on the rate of passage of feed particles and feed components through different compartments of the digestive tract is important for a correct description and estimation of nutrients digested in and absorbed from digestive tract in dairy cows. The objectives of this project are therefore: a) To measure, by using feedstuffs enriched with the stable isotopes 13C and 15N, the passage rate or residence time in compartments of the digestive tract (post-ileal, post ruminal and in the entire digestive tract) of feed particles or fractions within them in dairy cows. b). Testing the hypothesis that rate of passage of feed particles out of the rumen is largely related to rate of degradation and that different components (protein, starch, cell walls) in plants show a different passage behaviour through the digestive tract of dairy cows c) Track dowm other factors (level of feed intake, forage/concentrations ratio) inlfuencing rate of passage or residence time. d.) Construct a simulation model which describes passage behaviour of feed particles and feed components through the different parts of digestive tract of dairy cows.