Maize straw is an agricultural by-product that has the potential to be used as an alternative feed for ruminants. In order to utilise this crop residue rationally, their nutritional value should be improved to promote their use as silage. This study aimed to evaluate in vitro fermentability and digestibility of maize by-product silage in ruminant feed rations based on Indigofera zollingeriana as a substitute for Napier grass (NG). A completely randomised design with 3 treatments and 8 replications was applied. The proportion of forage and concentrate in rations was 45:55% on a dry matter (DM) basis. The experimental treatments included a ration containing 35% NG (R0), a ration containing 17.5% NG and 17.5% maize straw silage (MSS) (R1), and lastly a ration containing 35% MSS (R2). The results showed that increasing the levels of maize straw silage in rations reduced in vitro digestibility including DM and organic digestibility, ammonia (NH₃), total and proportional volatile fatty acid (VFA) contents, gas production, and the total protozoan and bacterial populations (P < 0.01). However, the numerical rumen pH values (6.45–6.71), VFA levels (69.08–89.23 mM), NH₃ concentrations (10.50–12.88 mM), and gas production (185.46–193.33 ml/g DM) were within the normal range for ruminant requirements in terms of gas production and in vitro feed fermentation in the rumen. There was no significant difference found in methane production (P > 0.05). Considering rumen fermentation products and pH ruminal condition, it can be concluded that maize straw silage up to 50% can be used as a feed substitute in ruminant rations.