Different milk replacers alter growth performance and rumen bacterial diversity of dairy bull calves

The diets of calves predominantly comprise milk and the start of solid feed intake triggers a critical process in terms of the activity of the rumen microbiota and ruminal fermentation. However, little is known about alterations of the rumen bacterial diversity in dairy bull calves induced by milk replacers and other starters, such as solid pellets or hay. Thus, we analyzed the effects of partially replacing milk with pellets and Chinese wildrye in the diet on the growth performance and rumen bacterial composition in dairy bull calves. Thirty-two newborn Holstein bull calves were randomly allocated to four dietary treatments: group MFC was fed milk replacer, pellet diet, and Chinese wildrye; group MFR was fed milk replacer and Chinese wildrye; group MCO was fed milk replacer and pellet diet; and group MIL was fed milk replacer only. The diets were provided for 1 week before the trial and for 7 weeks in the experiment. At the end of the experiment, three calves from each treatment were slaughtered to determine the growth performance and slaughter traits, and the rumen bacteria composition was analyzed by 16S rRNA gene sequencing. The body weight, withers height, body length, and heart girth were higher (P < 0.05) in the MCO and MFC groups compared with the MIL and MFR group. In addition, the fresh weights of the liver, spleen, lung, kidney, rumen, reticulum, and abomasum were higher (P < 0.05) in the MCO and MFC groups. The sequencing results showed that at both the phylum and genus levels, the MIL and MFR groups clustered together, whereas the MCO and MFC groups clustered together. Bacteroidetes and Proteobacteria were more abundant in the MIL and MFR groups, whereas Firmicutes were more abundant in the MFC and MCO groups. The abundances of genera such as Dialister, Megasphaera, Acidaminococcus, and Mitsuokella were higher (P < 0.05) in the MCO and MFC groups. Our results showed that the growth performance of calves could be increased by feeding milk replacer with pellet diet, which may have been associated with changes in the diversity or abundances of rumen bacteria, especially bacteria from the phylum Firmicutes.