Although methane production represents a loss of feed carbon, this loss is balanced to some extent by increased bacterial biomass that occurs in mixed cultures containing co-cultured methanogens. The greater bacterial biomass results from the improved growth rate of the bacteria that live in syntrophic relations with the methanogens. They grow better because they make more ATP per mol of glucose that they use. This is a consequence of interspecies hydrogen transfer.
Nevertheless, as we have seen previously another outcome of the presence of methanogens is to reduce the availability of succinate for propionate production in species such as Veillonella and Selenomonas. This is also a direct outcome of interspecies hydrogen transfer.
Since propionate is gluconeogenic in the host liver, glucose production from protein is "spared" to an extent depending on propionate production. Some improvement in food conversion efficiency therefore results from selective inhibition of methanogenesis.
Too large a reduction in the numbers of methanogens, however, adversely affects the growth of some syntrophic cellulase producing micro-organisms, and therefore of all the other heterotrophic species that rely on their cellulase activity.
Understanding this material requires a good working knowledge of the
various syntrophic relations among the rumen microorganisms that have been
dealt with in detail in the sections above.
Suppression of Methanogen Growth
As noted above reduction in the growth of methanogens in the reticulo-rumen has been found to be an effective means of improving the feed conversion efficiency of adult ruminants. The effect is dependent on the age and plane of nutrition of the animals being studied and is demonstrable most readily in growing steers under controlled feeding regimes.
Two antibiotics that have been used for this purpose are Monensin and Avoparcin.
The above diagram shows the structure of the polyether ionophore Monensin
as a sodium salt with a central pore containing the sodium ion.
Monensin is a polyether ionophore that increases the efficiency of feed utilization by increasing the proportion of propionate produced in the reticulo-rumen at the expense of acetate and butyrate.
Monensin works as an antibiotic to disrupt microbial activity by altering the internal ionic state of bacteria as a result of facilitating the entry of sodium into the cell. It is a selective antibiotic but surprisingly its target is probably not the methanogens themselves. Rather it reduces the growth of some hydrogen producing organisms and as a result methane production decreases by some 20-40%.
The dosage of monensin has to be finely adjusted to get a beneficial effect since some of the monensin-susceptible microorganisms produce succinate which is a nutrient for propionate producers. Too high a dose of monensin will therefore cause a reduction in the growth of propionate producing bacteria.
Avoparcin also increases the proportion of propionate available to the
host. It is a polypeptide antibiotic and it acts by inhibiting cell wall
glycopeptide biosynthesis. Methanogens are most susceptible to its actions.