The Importance of pH in the Regulation of Ruminal Acetate to Propionate Ratio and Methane Production In Vitro

¹ Agricultural Research Service and Section of Microbiology, Cornell University, Ithaca, NY 14853

Grain feeding often causes a decrease in ruminal pH, and experiments were conducted to define the role of pH in regulating the acetate to propionate ratio and production of CH₄. Cows that were fed 90% concentrate had lower ruminal pH values (6.22 vs. 6.86), higher VFA concentrations (85 vs. 68 mM), and lower acetate to propionate ratios (2.24 vs. 4.12) than did cows that were fed forage only. When mixed ruminal bacteria from cows that were fed 90% concentrate or 100% forage were incubated (48 h) with hay (10 g/L) or cracked corn (5 g/L) in a medium containing bicarbonate (38 mM) and tricarballylate (50 mM), the final pH values were less than 0.3 units lower than the initial pH. At final pH values less than 5.7, hay fermentation was inhibited, the acetate to propionate ratio and CH₄ production declined more than twofold, and the inoculum source was without effect. Small amounts of H₂ were detected at pH values less than 5.5. Total VFA production from cracked corn decreased when pH declined, but only if the inoculum was obtained from cows that were fed 90% concentrate. The acetate to propionate ratio of cracked corn incubations declined from 1.2 to 0.6 when final pH was decreased from 6.5 to 5.3, and CH₄, as a percentage of total VFA production, also decreased. At pH values less than 5.3, the acetate to propionate ratio of cracked corn increased more than fourfold, and large amounts of H₂ could be detected. Over the final pH range of 6.5 to 5.3, CH₄ production was highly correlated with acetate to propionate ratio, which was dependent on pH and substrate (CH₄ = 0.02 + 0.05 pH; r² = 0.80). Calculations based on the differences between pH 6.5 and 5.8 indicated that as much as 25% of the decrease in acetate to propionate ratio could be explained by the effect of pH alone.

Key Words: forage • pH • acetate to propionate • methane

Submitted on March 26, 1998
Accepted on August 14, 1998

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