THE EFFECT OF PH ON RUMINAL METHANOGENESIS

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-fe d 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 H-2 to methane, but the ruminal fluid from the conc entrate-fed cow did not produce methane. When the pH of the ruminal fl uid from the concentrate-fed cow was adjusted to pH 7.0, methane was e ventually detected, and the absolute rate constant of methane producti on 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 pro duction, it appeared that the concentrate-fed cow had fewer methanogen s than the forage-fed cow. When the mixed ruminal bacteria were incuba ted in a basal medium containing 100 mM acetate, methanogenesis was pH -dependent, and no methane was detected at pH values less than 6.0. Be cause the removal of acetic acid completely reversed the inhibition of methanogenesis, it appeared that volatile fatty acids were causing th e pH-dependent inhibition. Based on these results, concentrate diets t hat lower ruminal pH may provide a practical means of decreasing rumin al methane production.