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Effect of pH on Fermentation Characteristics and Protein Degradation by Rumen Microorganisms In Vitro¹

Animal Research Centre, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6

ABSTRACT

The artificial rumen was used to study the effect of pH on rate of fermentation of a pelleted concentrate and corn silage diet. The pH of the artificial rumen contents was maintained by infusion of artificial saliva and automated addition of sodium carbonate. Characteristics of fermentation were measured for 5 days after a 14-day equilibration. Rate of volatile fatty acid production decreased from 80 to 50 mmoles/day when pH was decreased from 7 to 5. Rates of production of acetic, butyric, isobutyric, and isovaleric acids decreased, whereas those of propionic, valeric, and caproic increased when pH decreased from 7 to 5. The ratio of acetic to propionic acid was 4.3 at pH 7 and .9 at pH 5. Methane production was low at pH 5.5 and ceased at pH 5. Net hydrogen production increased from a trace at pH 5.5 to 20 mmoles/day at pH 5. The ammonia concentration in culture fluid was 4 to 6 mmoles at pH 6 and 7 but was .2 mM at pH 5.5 and 5. Low ammonia concentrations were accompanied by decreased free amino acid concentrations and by decreased protease and deaminase activity of washed bacterial cells. Proteolytic organisms, in concentrations of .5 to 1 X 10⁸ cells per milliliter at pH 6 and pH 7, could not be detected (less than 10⁴ cells/ml) at pH 5.5. Proteolytic organisms apparently were washed out of continuous culture at pH below 6.

¹ Animal Research Centre Contribution No. 1006.

² Department of Animal Science, University of Manitoba, Winnipeg, Canada, R3T 2N2.

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