The rates of production of volatile fatty acids in the rumen. III. Measurement of production in vivo by two isotope dilution procedures

(1) Production of volatile acid in the rumen of sheep fed at 12-hourly intervals on a mixture of wheaten hay and lucerne hay has been measured by two isotope dilution procedures involving continuous infusion of 14C-labelled fatty acid into the rumen and determination of the concentration of 14°C in the rumen acid throughout the feeding cycle. In the first procedure the varying rate of production was matched by a varying rate of infusion so that a constant concentration of label was maintained in the rumen acid. In the second procedure a constant rate of infusion was maintained and a mean value obtained for the concentration of label in the rumen acid during the cycle. Production values calculated from the data from both procedures were similar: approximately 5 moles of volatile acid were produced per kilogram of dry fodder. (2) This same rate of production was found for rations of 360, 450, and 675 g dry fodder fed at 12-hourly intervals.