Japanese researchers evaluated the effect of fumarate as a feed additive on the reduction of methanogenesis. The addition of fumarate to the culture of mixed ruminal microorganisms that were fermenting hay powder and concentrate in vitro reduced methane production. Some bacteria oxidized H2 by using fumarate as a final electron acceptor, suggesting that they compete with methanogen for H2, which is the main substrate for methanogenesis in the rumen. However, the affinity of these bacteria for H2 was lower than the affinity of methanogen. Some utilized fumarate, as an electron donor for fumarate reduction. The affinity of these bacteria to fumarate was higher than the affinity of methanogen for fumarate. When methanogen were cocultured with an equal cellular amount of each of the fumarate utilizing bacteria, methane production was markedly decreased, not only from fumarate, but also from H2. These results suggest the addition of fumarate to ruminant feed reduces methanogenesis.