THE EFFECT OF PH ON RUMINAL METHANOGENESIS

Authors: VAN KESSELL, JO ANN - CORNELL UNIVERSITY; Russell, James

Submitted to: Federation of European Microbiological Societies Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Methane is a "green house" gas that has been implicated in global warming. Since a significant portion of this methane is derived from ruminant animals (cattle, sheep, goats, etc.), environmentalists have sought ways of inhibiting ruminal methane formation. The rumen is well buffered by saliva secretions, but the ruminal pH of grain-fed cattle can be as much as 1.0 pH unit lower (ten times more acidic) than that of forage-fed cattle. We found that ruminal methanogens (methane synthesizing microorganisms) are very sensitive to low pH and that this pH sensitivity is related to their method of intracellular pH regulation. Methanogenic bacteria create large pH gradients across the cell membrane at acidic pH and these pH gradients cause a toxic accumulation of acetic acid anion. Based on these results, starch feeding and mild ruminal acidosis may be a practical means of decreasing ruminant methane emission.

Technical Abstract: When a fistulated cow was fed an all forage diet, ruminal pH remained more or less constant (6.7 to 6.9). The ruminal pH of a concentrate-fed cow decreased dramatically in the period soon after feeding, and the pH was as low as 5.45. Mixed ruminal bacteria from the forage-fed cow converted CO2 and H2 to methane. When the pH of the ruminal fluid from the concentrate- fed cow was adjusted to pH 7.0, methane was eventually detected, and the absolute rate of methane production was as high as the one observed with ruminal fluid from the forage fed cow (0.32 h-1). Based on the zero-time intercepts of methane production, the concentrate-fed cow had approximately 10-fold fewer methanogens than the forage-fed cow. When the mixed ruminal bacteria were incubated in a basal medium containing 100 mM acetate, methanogenesis was pH-dependent, and no methane was detected at pH values less than 6.0. Because the removal of acetic acid completely reversed the inhibition of methanogenesis, it appeared that volatile fatty acids were causing the pH-dependent inhibition. Based on these results, concentrate diets that lower ruminal pH may provide a practical means of decreasing ruminal methane production.