Volatile fatty acid production in the rumen of the grazing sheep: its use as an indicator of pasture value.

I . An isotope dilution method, known to be satisfactory for measurement of volatile fatty acid (VFA) production in the rumen under steady-state conditions, was tested for its applicability where the VFA pool in the rumen could vary irregularly. The method indicated mean production rates closely similar to those obtained when the pool size was virtually constant. Tests carried out in a model rumen confirmed this finding and the method was accepted as suitable for measurement of VFA production in grazing sheep. z. The method, modified to allow continuous sampling of rumen acids by dialysis, was applied to sheep grazing a mixed pasture. Mean rates of VFA production determined in seven sheep during fourteen 3-day periods throughout winter, spring and summer months ranged from 3.4 to 5’3 moles daily, production increasing during the period of growth of the pasture, and declining again with its drying-off. 3. The pasture was at all times abundant; the findings are therefore considered to represent the pasture’s full potential yield of VFA to the sheep and so to indicate its changing value in this respect as the seasons progressed. 4. Results from sheep given roughages and from the grazing sheep indicated that, despite irregular intake, good predictions of mean VFA production rates could be made from mean VFA concentrations in the rumen fluid.