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

Abstract 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