PASSAGE THROUGH THE RUMEN AND THE LARGE-INTESTINE OF SHEEP ESTIMATED FROM FECAL MARKER EXCRETION CURVES AND SLAUGHTER TRIALS

External digesta markers (Yb-labelled diets and Co-EDTA) were given or ally as a pulse dose to four pairs of Rasa Aragonesa twin ewe lambs, f ed on either chopped or ground and pelleted lucerne hay, in order to e stimate slow (k(1)) and fast (k(2)) rates of passage of liquid and sol id phase from faecal marker excretion curves. After the faecal samplin g period daily doses of the same markers were infused continuously for 5 d and the animals slaughtered. Concentrations of markers in the dif ferent compartments of the gut were determined and used to calculate m ean retention times. The results showed that the rumen and the large i ntestine were the two main mixing compartments of the gut, accounting for more than 95 % of total mean retention time. Rates of passage esti mated from faecal marker excretion did not accurately represent marker kinetics in the compartments of the gut derived from slaughter data. Accuracy in the estimation of fractional outflow rate from rumen (k(R) ) by k(1) was higher for low values of k(R) whereas k(2) consistently overestimated large intestine outflow rate (k(LI)), especially for hig h values of k(R). The relationship between outflow rates from the main two mixing compartments was important in influencing the accuracy of prediction of faecal estimates.