Manipulation of Ruminal Fermentation IV. Effect of Altering Ruminal pH on Volatile Fatty Acid Production

Abstract

The effect of altering ruminal pH on volatile fatty acid production in vitro or volatile acid concentration in vivo was investigated. In Experiment 1 three lactating Holstein cows fitted with ruminal cannulae were fed a high-concentrate, restricted roughage ration. Continuous infusion of 9 to 12 moles of sodium bicarbonate into the rumen raised ruminal pH from 5.5 to 6.2 and increased the acetate:propionate ratio from 1.1 to 2.8. A series of in vitro incubations with pH controlled at 5.6, 6.2, or 6.8 showed that volatile acid production was largely unaffected between pH 6.2 and 6.8, but between pH 5.6 and 6.2 acetate production was inhibited. Propionate and butyrate production were increased at pH 5.6.In Experiment 2, two nonlactating Jersey cows fitted with ruminal cannulae and fed only alfalfa hay were infused with phosphoric acid to reduce ruminal pH from 6.8 to 6.2. There were no changes in the molar proportions of ruminal acids although total volatile acid concentration was significantly lower during acid infusion. Incubation of ruminal ingesta from these cows at pH 5.6, 6.2, and 6.8 indicated much the same pH effect as in Experiment 1. These experiments suggest that changes in volatile fatty acid production can take place in response to low pH conditions without necessarily changing the rumen microbial population. This would be important in the meal-eating ruminant whose rumen pH may temporarily drop from an otherwise higher prevailing pH.Sodium bentonite was fed with a high-concentrate ration to three ruminally fistulated cows in Experiment 3. In a switchback design with short periods, no difference could be detected between control and bentonite groups in ruminal pH or volatile fatty acid concentration. Dietary bentonite reduced rumination time and had no effect on the distribution of dry matter within the rumen. Incubation of ruminal ingesta with bentonite did not alter pH or production of volatile acid by the ingesta. It appears that the reported effect of bentonite on ruminal acid concentration is not mediated through the influence of pH but through means not readily detected in switchback experiments with cows fed almost continuously.