Fibre degradation in the rumen needs the concerted action of numerous microbial species. The removal of molecular hydrogen (H2) by methanogenesis is postulated to eliminate the inhibitory effect of H2 on the microbial degradation of feed material. Different fibre structures in forages can alter fibre digestibility and influence fibrolytic microorganisms in the rumen, which may be associated with ruminal dissolved H2 (dH2) concentration. Napier grass (NG) silage and corn stover (CS) silage were compared as forage sources in the present study. In the first experiment, both forages were incubated to determine fibre degradation, total gas, methane (CH4) and hydrogen (H2) accumulation through 48-h in vitro batch cultures. Corn stover silage had greater (P < 0.05) 48-h DM, NDF and ADF degradation, and total gas and CH4 volumes, and lower (P < 0.05) 48-h H2 volume than NG silage in in vitro incubations. In the second experiment, 24 growing beef bulls were allocated to 12 blocks according to body weight, and each animal within a block was fed a diet including 55% (DM basis) NG or CS silage. Bulls fed the CS diet had greater (P < 0.05) DM intake (DMI), average daily gain (ADG), total-tract digestibility of OM and NDF, ruminal dissolved methane (dCH4) concentration, and gene copies of protozoa, methanogens, Ruminococcus albus and Ruminococcus flavefaciens, and had lower (P < 0.05) ruminal dH2 concentration, and molar proportions of valerate and isovalerate, in comparison with those fed the NG diet. There was a negative correlation between dH2 concentration and NDF digestibility in bulls fed the CS diet (R2 = 0.48, P = 0.02), and a lack of relationship between dH2 concentration and NDF digestibility with the NG diet (R2 < 0.01, P = 0.90; interaction dH2 × diet P = 0.006). In summary, the fibre of CS silage was more easily degraded by rumen microorganisms than that of NG silage. Increased dCH4 concentration with the CS diet presumably led to the decreased ruminal dH2 concentration, which may be helpful for fibre degradation and growth of fibrolytic microorganisms in the rumen.