The objective was to examine individual and synergistic effects of recombinant bacterial expansin proteins and an exogenous cellulolytic and xylanolytic enzyme (EFE) on preingestive hydrolysis, in vitro ruminal fermentation and digestion, and sugar profile of whole-plant corn silage. In Experiment 1 (E-1), the EFE (0, 2.33 mg/g) and expansin (0, 304, 616, 888 μg/g) were incubated with dried ground corn silage (0.50 g; 1 mm) in buffered rumen fluid in quadruplicate for 24 h at 39°C in 3 independent runs. Gas production was measured after 0, 3, 6, 12 and 24 h. In Experiment 2 (E-2), a similar approach was used to examine simulated preingestive effects by incubating the corn silage with deionized water containing 0.02% sodium azide in quadruplicate for 24 h at 25°C in 2 independent runs. Treatments were arranged in a 2 × 4 factorial. Data were analyzed using the NMLE package of R for a randomized block design and run was the blocking factor. In E-1, EFE alone increased gas production at 3 h and 6 h compared with the control (15.3 vs 17.3 and 16.1 vs 17.8 mL/ g DM, P < 0.01) and adding the highest dose of expansin tended to increase gas production (22.2 vs 21.6 mL/ g DM P = 0.09). Adding EFE alone increased NDF digestibility (34.0 vs 32.9% P = 0.04) but expansin did not (P = 0.45). Total CH₄ production and VFA profile were not affected by EFE (P > 0.05) or expansin (P > 0.05) addition. In E-2, EFE application increased (P < 0.05) DM, NDF and ADF disappearance as well as (P < 0.01) concentrations of cellobiose, glucose, xylose, and arabinose. Whereas adding expansins alone increased only arabinose concentration (2.1 vs 1.4 mg/ ml P < 0.01). In conclusion, EFE application alone increased fiber digestibility and sugar release and the highest expansin dose increased gas production, however no synergistic effects of applying both additives on rumen fermentation parameters or fiber digestibility were detected.