ESTIMATION OF MARKER PASSAGE IN THE DIGES TIVE-TRACT OF DAIRY-COWS BYDIFFERENT MODELS - INFLUENCE OF STAGE OF LACTATION, AMOUNT OF INTAKE AND DIET COMPOSITION

12 dairy cows were used to examine the influence of stage of lactation . amount of intake and diet composition on digesta passage from the re ticulorumen Cows were fed on mixed diets, containing on average 34% co ncentrate and 66% roughage (dry matter basis). The roughage portion co nsisted of maize silage and either extensively or intensively produced grass silage in the ratio of 40:60 (% as fed). Titanium(IV)-oxide (Ti O2) was used as a marker and faecal marker concentrations were determi ned for 120 h after oral dosing. Samples were taken in four periods - ante partum (1 period) and in the first half of lactation (3 periods) -, each period representing a certain physiological state. Cows were f ed according to milk yield with mean dry matter intakes of 12.0, 16.8. 22.6 and 21.2 kp in periods 1 to 4. Passage parameters were estimated by a model with gamma time-dependency (G4G1), by an exponential model and by a model calculating total tract mean retention time as mean va lue of all points on the marker-excretion curve. Results showed an obv ious acceleration of marker passage from the reticulorumen from ante p artum, to post partum. Total tract mean retention time calculated as m ean values of all points on the marker excretion-curves were in good a greement with total tract mean retention times calculated with G4G1. P assage rates from the reticulorumen showed poor agreement between G4G1 and the exponential model. Effects of period and diet post partum wer e observed for rate of passage from and mean retention time in the fas t compartment only, both estimated by G4G1. TiO2 appears to be a suita ble marker to estimate passage rates. Values for passage rates were in the range of those reported for concentrate particles by others.