Measurement of the rates of production of acetic, propionic and butyric acids in the rumen of sheep.

Numerous studies have been made of the quantitative aspects of the production of volatile fatty acids (VFA) in the rumen, and have been reviewed by Rook (1964), Warner (1964) and Annison (1965). Interest in rumen VFA production has recently been intensified because of the findings correlating body-weight increases with the concentration of rumen VFA in lambs on various pastures (Johns, Ulyatt & Glenday, 1963 ; Rae, Brougham, Glenday & Butler, 1963). Lambs grazing pasture supporting the greatest rate of growth had the greatest concentration of VFA in the rumen and also the highest proportions of propionate and butyrate to acetate. It is still not known, however, whether organic acid levels in the rumen are indicative of production or absorption rates since concentrations of individual acids in the rumen are a balance between production, absorption, passage along the intestinal tract and conversion into other compounds. Four main groups of experimental techniques have been used to measure production rates of rumen VFA and include: ( I ) methods based on production of organic acids in rumen fluid incubated in vitro; (2) methods based on changes in rumen concentrations after feeding; (3) analyses of the blood draining the rumen either in vivo or in isolated preparations ; (4) isotope dilution techniques applied either to the rumen or to the whole animal. Isotope dilution techniques probably offer the most accurate means of measuring production of organic acids in the rumen, since the measurements may be made in vivo and with a minimum of disturbance to the animal. Continuous intravenous infusions of r4C]acetate (Annison & Lindsay, 1961), and [14C]propionate and [14C]butyrate (Annison & Lindsay, 1962) have been used to measure the rates at which these compounds enter their body pools. The entry rates for propionate and butyrate have only been measured in experiments in which blood concentrations have been raised by infusion of carrier (Annison & Lindsay, 1962). This was found necessary since only traces of these acids are present in the peripheral blood of sheep (Reid, 1950; McClymont, 1g51a, b). In addition, the entry rate underestimates production in the rumen because there is considerable metabolism of propionate and butyrate in the rumen epithelium with the production of ketone bodies from butyrate (Pennington, 1952) and of lactate from propionate (Pennington & Sutherland, 1956). With acetate a variable amount of that entering the body pool arises from endogenous sources, especially from fat (see, for example, Annison & White, 1962). For these