Ensiling grass with microbial inoculants is a promising strategy to enhance forage quality, animal performance, and environmental sustainability. This study evaluated the effects of a multi-strain inoculant (Lactobacillus plantarum, L. buchneri, Propionibacterium acidipropionici, and P. thoeni) on silage fermentation, nutrient digestibility, milk production, methane emissions, and rumen microbiota in dairy cows. In a 2 × 2 crossover design, 24 lactating Polish Holstein–Friesians were fed total mixed rations differing only in grass silage treated with or without inoculant. Inoculated silage had lower pH (4.56 vs. 5.06; p = 0.02) and higher crude protein (129 vs. 111 g/kgDM; p < 0.05). Cows fed inoculated silage showed higher ruminal propionate (28.3 vs. 26.3 mM; p = 0.03), reduced ammonia (7.61 vs. 8.67 mM; p = 0.02), and fewer protozoa (1.21 vs. 1.66 × 10⁵/mL; p = 0.03). Nutrient digestibility improved (p < 0.05), while methane emissions declined both per cow (368 vs. 397 g/d; p = 0.01) and per kgDMI (15.1 vs. 16.5; p = 0.01). Milk yield increased (p = 0.04), and the fatty acid profile improved. Our study revealed that cows fed inoculated silage had higher nutrient digestibility, lower methane emissions, and microbial shifts in the rumen detected by 16S rRNA sequencing (p < 0.05).