Passage through the rumen and the large intestine of sheep estimated from faecal marker excretion curves and slaughter trials.

External digesta markers (Yb-labelled diets and Co-EDTA) were given orally as a pulse dose to four pairs of Rasa Aragonesa twin ewe lambs, fed on either chopped or ground and pelleted lucerne hay, in order to estimate slow (k1) and fast (k2) rates of passage of liquid and solid phase from faecal marker excretion curves. After the faecal sampling period daily doses of the same markers were infused continuously for 5 d and the animals slaughtered. Concentrations of markers in the different compartments of the gut were determined and used to calculate mean retention times. The results showed that the rumen and the large intestine were the two main mixing compartments of the gut, accounting for more than 95% of total mean retention time. Rates of passage estimated 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 (kR) by k1 was higher for low values of kR whereas k2 consistently overestimated large intestine outflow rate (kLI), especially for high values of kR. The relationship between outflow rates from the main two mixing compartments was important in influencing the accuracy of prediction of faecal estimates.