Measurement in sheep of the quantity and composition of rumen digesta and of the fractional outflow rates of digesta constituents

Abstract

Knowledge of the fractional outflow rates of rumen digesta constituents is required for the quantitative prediction of rumen function. However, there are few data available for these parameters because of the problems associated with specific labelling of digesta constituents and with the need to ensure that samples are truly representative of rumen digesta. Methods are described here by which two markers can be used together to overcome the problem of representative sampling of the rumen, so eliminating the need for specific labelling, with 'ideal' markers, of digesta constituents. Thus, from the measurement of the amount and composition of true digesta in the rumen and the amount and composition of true digesta flowing from the abomasum, values for the fractional outflow rates (and its reciprocal, mean retention time) of rumen digesta constituents can be calculated. Appropriate application of these methods will allow the derivation of functions to predict fractional outflow rates. The results presented show that the fractional outflow rate of an adsorbable marker, 103Ru-P, was about half that of water but was about twice that of particulate matter; it was also greater than the net value for microbes. On the assumption that microbial material leaving the rumen is drawn from a free-floating microbial population and a population associated with particulate matter, it was calculated that 0.29 � SE 0.07 (range 0.09-0.49) of the rumen microbial population could have been free-floating.