Nutrient utilization efficiency, ruminal fermentation and microbial community in Holstein bulls fed concentrate-based diets with different forage source

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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 corn-based 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 promoted an increase in the overall network complexity, basically in terms of node degree and betweenness centrality. Some lactate-degrader genera were found to be central nodes in the microbial network when quality forage was provided, highlighting their probable implication in ruminal acidosis prevention.