The objective of the present study was to investigate the hypothesis that 9,10-anthraquinone (AQ) in combination with fumaric acid (FMA) may provide complementary effects to inhibit methanogens and enhance rumen\'s capacity for better utilisation of FMA towards propionate production. Three levels of AQ and four levels of FMA were tested in a 3 × 4 factorial design using in vitro gas production technique. AQ reduced the total gas and methane production significantly. The combination of 4 ppm AQ with FMA had additive effect on concentration of propionate. Supplementation of AQ alone resulted in hydrogen accumulation (p < 0.001), whereas presence of FMA (up to 6.5 mM) along with AQ declined hydrogen concentration (p < 0.001). The level of 4 ppm AQ did not affect in vitro digestibility, however, a reduction of organic matter digestibility was caused by 8 ppm AQ (p < 0.001), which was partially compensated by the addition of FMA (p = 0.06). The optimum FMA level depended on the AQ concentration. At 4 ppm AQ, a FMA level of 3.5 mM had best possible effect on partitioning factor and microbial biomass production (p < 0.001), though, at 8 ppm AQ the higher level of FMA (6.5 mM) responded better. Overall, FMA in combination with AQ provided an alternative hydrogen sink and might be introduced as a novel strategy for mitigation of enteric methane emission. Nevertheless, the result should be proved by in vivo experiments.

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