The rates of production of volatile fatty acids in the rumen. IV. Individual and total volatile fatty acids

Abstract

(1) In sheep fed at 12-hr intervals on roughage diets (lucerne and wheaten hay) total volatile fatty acid production in the rumen has been measured by an isotope dilution procedure based on the infusion of a single ¹⁴C-labelled volatile fatty acid. (2) Total and individual acid production has been measured through the infusion of a mixture of ¹⁴C-labelled acids in which the proportions of individual acids were such that transfers of ¹⁴C by interconversions between the acids were balanced. (3) The findings support the view that the mixture of acids produced in the rumen is similar in composition to that present in the rumen fluid throughout the feeding cycle. Consequently it was possible to determine the production of individual acids by a relatively simple procedure requiring only the infusion of a single ¹⁴C-labelled acid and measurement of the concentration of ¹⁴C in a composite sample of the acids in the rumen fluid collected throughout the feeding cycle. This procedure is considered suitable for routine use; automatic sampling of rumen fluid still further reduces the work involved. (4) The infusion of certain mixtures of ¹⁴C-labelled fatty acids showed that the composition of the acid initially formed in the rumen was acetic 77–83%, propionic 15–18%, and butyric 1–7% according to the diet given and the time elapsed after feeding. From 50 to 80% of the butyric acid in the final volatile acid product was formed from acetic acid. Degradation of ¹⁴C-labelled butyrate formed from [1-¹⁴C] acetate in the rumen showed 93% of the ¹⁴C to be nearly evenly divided between carbon atoms C₁ and C₃.