Ruminal acidosis can penalize cattle performance and modify ruminal microbiota composition; in that sense, providing quality forage may be a useful tool to cope with such dysfunction. This assay aimed to control animals' performance and assess ruminal microbiota shifts and interactions when fattening Holstein bulls are fed corn-based concentrate and different quality forages. Thirty animals (from 119 to 332 d of age and from 164 to 511 kg body weight [BW]) were fed cornbased concentrate and were allotted to three experimental treatments or forage sources: barley straw, considered as control (Hordeum vulgare, CTR), oats haylage (Avena sativa, OATS) or vetch haylage (Vicia sativa, VETCH). Both concentrate and forage were supplied ad libitum in a free choice system and animals had free access to drinking water. Bulls' BW and concentrate intake were automatically recorded on a daily basis. Feces and ruminal fluid were sampled twice, during the growing period (158 d of age and 220 kg BW) and during the finishing period (280 d of age and 434 kg BW), for digestibility, ruminal fermentation and microbial population characterization. No differences in concentrate intake or BW could be detected between CTR and OATS-fed animals; however, VETCH-fed bulls had lower concentrate intake (P < 0.047) and slaughter BW (P = 0.034) than CTR. The use of oats haylage as forage source did not affect nutrient apparent digestibility rates but vetch haylage did penalize dry matter (P = 0.048) and crude protein (P < 0.001) digestibility in finishing animals. Differences in neither ruminal volatile fatty acids concentration nor pH were detected, but acetate-to-propionate ratio increased with the incorporation of vetch haylage in diet (P < 0.041). Ammonia-nitrogen concentration in ruminal fluid was low (20.63 ± 2.55 mg/L) but it significantly improved when oats (P = 0.001, only in finishing) and vetch (P = 0.001) haylage were provided. Core microbial community gathered 75 % of analyzed sequences; however, ruminal microbial community was different between CTR and OATS (P < 0.001) or VETCH (P < 0.001) bulls. Shannon and Simpson diversity indexes were improved by quality forage feeding, mainly during the growing period (P < 0.081 in OATS and P< 0.004 in VETCH). Microbial network analysis revealed that the use of oats or vetch haylage