Buffer systems in the rumen of sheep. II. Buffering properties in relationship to composition.

Abstract

Acid-base titration curves of ovine ruminal fluid confirmed the findings of Clark and Lombard (1951) that ruminal fluid is relatively well buffered against addition of acid, but relatively poorly against additions of alkali. However, considerable differences were observed between samples from different sheep. These were associated with differences in the interval after feeding, the nature of the diet, and the consumption of drinking water; and were correlated with differences in total and relative concentrations of bicarbonate, phosphate, and volatile fatty acids (VFA). The relative importance of the main buffering components in particular fluids was studied by examining the effects of the following procedures upon titration curves: ( a ) varying bicarbonate concentration by varying pCO₂; ( b ) removing, and restoring, bicarbonate, phosphate, and VFA; and ( c ) removing particulate material. Particulate material was relatively unimportant as a buffer. Within the usual pH range of the rumen, the important buffering components were bicarbonate and phosphate. In the fasting rumen, bicarbonate was more important than phosphate, and this was found to be also true in parotid saliva. As ruminal fermentation proceeded and as VFA accumulated, bicarbonate and pH decreased and the buffer value depended more and more upon phosphate. In actively fermenting rumens, in which VFA concentration was high and the pH less than 6, VFA contributed significantly to the buffer resistance against further additions of acid. It is pointed out that, because the acids and bases generated during ruminal fermentation are relatively weak, they alter ruminal pH less, mole for mole, than the strong titrants conventionally used in titrations.