Impact of ruminal pH on enteric methane emissions.

The objective of this study was to determine the impact of ruminal pH on methane (CH4) emission from beef cattle. Ruminal pH and CH4 data were generated in 2 experiments using 16 beef heifers offered high-forage (55% barley silage) or high-grain (92% concentrate; DM basis) diets. Both experiments were designed as a replicated 4 × 4 Latin square with 4 periods and 4 dietary treatments. Methane was measured over 4 consecutive days using open-circuit respiratory chambers with each chamber housing 2 heifers. The ruminal pH of individual heifers was measured using indwelling pH loggers. The mean ruminal pH and CH4 emission (g/h) of 2 heifers in every chamber were summarized in 30-min blocks. Even though rumen methanogens have been described to be inhibited by a pH < 6.0 in vitro, in vivo CH4-production rates (g/h) did not decrease when ruminal pH declined to threshold levels for subacute (5.2 ≤ pH < 5.5) or acute ruminal acidosis (pH < 5.2; P > 0.05). Daily mean CH4 emission (g/d) and ruminal pH were only mildly correlated (r2 = 0.27; P < 0.05), suggesting that additional factors, such as increased propionate formation or passage rate, account for the lower CH4 emissions from cattle fed high-grain as compared to high-forage diets. Lowering ruminal pH alone is, therefore, not an effective CH4-mitigation strategy. Mechanisms permitting methanogens to survive episodes of low-ruminal pH might include changes in community structure toward more pH-tolerant strains or sequestration into microenvironments within biofilms or protozoa where methanogens are protected from low pH.