Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics

A descriptive model is presented that can explain changes in the amount of methane (CH4) formed in the rumen in relation to passage rate, feed type, and the effects of pH and inhibitors of methanogenesis. The model is based on methanogen growth kinetics in continuous systems. The growth rate of hydrogen (H2) utilising methanogens in the rumen and the prevailing H2 concentration are dynamically linked. Higher H2 concentrations are required to permit a growth rate sufficient to prevent washout of methanogens from the rumen at higher ruminal passage rates, at suboptimal ruminal pH values, or in the presence of inhibitors. Lower H2 concentrations are possible when the passage rate is lower, when the pH is near optimum, or when methanogens are less affected by inhibitors. Analysis of the literature confirms that increased particulate passage rate is associated with higher rumen H2 concentrations, less CH4 formation, and increased importance of propionate as a fermentation endproduct. Published data also show that partial inhibition of methanogens results in higher H2 concentrations, less CH4 formation, and more propionate formation. The model suggests that the prevailing H2 concentration influences the thermodynamics of rumen fermentation. H2 producing fermentation pathways are favoured at low H2 concentrations. Therefore, feeds and conditions that result in low H2 partial pressures will result in more H2 formation, and less propionate formation, and so more CH4 is formed per mole of feed monomer fermented in the rumen. Conversely, feeds and additives that favour high H2 concentrations result in less H2 formation per mole of feed monomer fermented in the rumen, and so result in production of less CH4 and more propionate.