The effect of fumarate used as a feed additive on the reduction of methanog
enesis in the rumen was evaluated by in vitro experiments. The addition of
fumarate to the culture of mixed ruminal microorganisms that were fermentin
g hay powder and concentrate reduced methane production. Most fumarate was
metabolized to propionate, and a slight increase was noted in other volatil
e fatty acids. Fumarate was utilized by mixed bacteria but not by mixed pro
tozoa. Fibrobacter succinogenes, Selenomonas ruminantium ssp. ruminantium,
Selenomonas ruminantium ssp. lactilytica, Veillonella parvula, and Wollinel
la succinogenes oxidized H-2 by using fumarate as a final electron acceptor
, suggesting that these bacteria compete with methanogens for H-2, which is
the main substrate for methanogenesis in the rumen. However, the affinity
of these bacteria to H-2 was lower than their affinity to methanogens. Thes
e fumarate-utilizing bacteria metabolized malate to products that were simi
lar to those from fumarate, suggesting the possession of fumarate dehydrata
se. Fibrobacter succinogenes, V. parvula, and W. succinogenes utilized form
ate, another substrate for methanogenesis, as an electron donor for fumarat
e reduction. The affinity of these bacteria to formate was higher than the
affinity methanogens have for formate. When methanogens were cocultured wit
h an equal cellular amount of each of the fumarate-utilizing bacteria, meth
ane production was markedly decreased, not only from formate, but also from
H-2. These results suggest that the addition of fumarate to ruminant feed
reduces methanogenesis and enhances propionate production in the rumen.