The present study was conducted to evaluate the effect of a live culture of Aspergillus oryzae (A; CCT4359) and fibrolytic enzyme (E; Fibrozyme Alltech Inc.) on fibre digestibility by a gas production bioassay and in vitro degradation of maize silage and sugarcane silage. A completely randomized design trial was performed to evaluate: A doses (0, 20, 60 and 100 mg/l), E doses (0, 160, 320 and 480 mg/l) and roughage source (R; maize and sugarcane silage) in a 4 × 4 × 2 factorial arrangement. The inclusion of increasing doses of A and E increased dry matter and neutral detergent fibre in vitro digestibility linearly, but for E this effect occurred only in maize silage. There was a linear increase in the potential for gas production at the highest dose of A only in sugarcane silage, with no effect on lag time (L). Increasing doses of E increased the volume of gases produced linearly, and a trend of linear reduction of L, regardless of the roughage. There was a linear reduction in ammonia-nitrogen concentration in response to increasing doses of A and E, and an increase in acetic acid concentration at the highest dose of A, regardless of roughage. The additives had no synergistic effect on gas production and digestibility, but were efficient in altering the fermentative pattern, demonstrating the potential to increase fibre degradation.