Effects of protein restriction on performance, ruminal fermentation and microbial community in Holstein bulls fed high-concentrate diets

The aim of this work was to study the impact of a crude protein (CP) restriction on performance, ruminal fermentation and microbial community composition in fattening Holstein bulls (from 118 to 331 d of age and from 165 to 522 kg body weight [BW]) fed high-concentrate diets. Twenty animals were assigned to two dietary treatments: concentrate CP was formulated either based on the levels used commercially (CTR: 140 g CP/kg dry matter [DM]) or reducing them (LP: 120 g CP/kg DM). Concentrate was supplemented with barley straw and both were supplied ad libitum. Animal BW and concentrate intake were automatically recorded on a daily basis. Feces, urine and ruminal fluid were sampled twice, during the growing period (160 d of age and 225 kg BW) and during the finishing period (280 d of age and 444 kg BW), for digestibility, ruminal fermentation and microbial population characterization. No differences in BW or cumulated concentrate intake were found between treatments, despite the fact that average daily gain was lower in LP group at the beginning of the growing period (P < 0.001). Crude protein limitation did not penalize dry matter (P = 0.654) or organic matter (P = 0.526) apparent digestibility, but it did affect CP apparent digestibility during the finishing period (P = 0.042). Nitrogen (N) excretion was greater in CTR animals (P = 0.017). Regardless of treatment (P = 0.511), ruminal ammonia-N concentration was low (4.36 +/- 1.01 mg/L). Even though 135 OTUs (out of 489) were shared between treatments and periods (gathering 98.7 % of analyzed sequences), ruminal microbial community composition was different between periods (P = 0.003) and also between diets in either growing (P < 0.001) or finishing (P = 0.046) bulls. Bacteroidetes, Firmicutes and Actinobacteria were the three dominant phyla and Prevotella ruminicola was the most abundant species. Ruminal microbial biodiversity was low but increased with age (P = 0.002 for Shannon index and P = 0.035 for Simpson index), as well as, ruminal microbial heterogeneity. Crude protein limitation increased functional interdependency among microbial genera, so LP-fed bulls were found to have a more complex microbiota community structure than CTR-fed bulls. No relevant correlations between microbial genera and ruminal fermentation parameters were detected.