Understanding the altered gastrointestinal microbiota is important to illuminate effects of maternal grazing (MG: maternally nursed and grazed) and barn feeding (BF: supplied milk replacer, starter feed, and alfalfa hay) on the performance and immune function of yak calves. Compared with the MG group, the significantly increased body weight, body height, body length, chest girth, and organ development of liver, spleen, and thymus were identified in the BF group, which were resulted from the significantly increased dry matter intake, increased concentrations of propionate, butyrate, isobutyrate, and valerate, increased ruminal pectinase, duodenal α-amylase, jejunal α-amylase and trypsin, and ileal trypsin, and promoted gastrointestinal epithelial development. Furthermore, genera of Sharpea, Sphingomonas, Atopobium, Syntrophococcus, Clostridium_XIVb, Acinetobacter, Oscillibacter, Dialister, Desulfovibrio, Bacteroides, Lachnospiracea_incertae_sedis, and Clostridium_sensu_stricto, which were involved in utilization of non-fibrous carbohydrate and further beneficial to improve the gastrointestinal digestion, development, and immune functions, were significantly increased in the BF group. Meanwhile, the significantly enhanced ruminal epithelial immune functions and intestinal immune functions based on enhanced ruminal immune related pathway, duodenal IL-1β, jejunal IL-1β, IL-2, TNF-α, and IFN-γ, and ileal IL-1β were identified in the BF group, which also may induced by the increased abundance of gastrointestinal microbiota. Overall, barn feeding significantly increased the diversity of species and abundance of microbes which used different carbohydrates and further benefit to the growth and immune function of yak calves.

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The present study aims to evaluate the effects of different early weaning paradigms, which supplied with extra alfalfa hay, or starter feeding, or both alfalfa hay and starter feeding, along with the milk replacer, on the gastrointestinal microbial community, growth, and immune performance of yak calves. Twenty 30-day-old male yak calves were randomly assigned to four groups, including the control (CON), alfalfa hay (A), starter feeding (S), and starter plus alfalfa hay (SA) groups. The gastrointestinal microbial colonization, the gastrointestinal development and function, and the growth and immune performance of all the yak calves were separately measured. Supplementation with alfalfa and starter feeding during the pre-weaning period significantly increased body weight, body height, body length, and chest girth. The significantly improved rumen fermentation and promoted intestinal digestion-absorption function in alfalfa and starter feeding groups, including the identified significantly increased concentrations of ruminal total volatile fatty acid (VFA); the significantly increased concentrations and proportions of acetate, butyrate, and isovalerate; the increased α-amylase activities in the duodenum, jejunum, and ileum; the increased papillae length and width of rumen epithelium and rumen wall thickness; and the increased villus height and crypt depth of the duodenum, jejunum, and ileum, could all contribute to promote the growth of calves. These significant improvements on rumen fermentation and intestinal digestion-absorption function could be further attributed to the increased proliferation of starch-decomposing, and cellulose-or hemicellulose-decomposing bacteria identified in the rumen, jejunum, and ileum. Furthermore, based on the expression of intestinal inflammatory cytokines and the rumen epithelial RNA sequencing results, alfalfa supplementation reduced the occurrence of ruminal and intestinal inflammation, whereas starter feeding supplementation was mainly beneficial to the differentiation of immune cells and the improved immune function. Meanwhile, the significantly altered relative abundances of genera in the SA group, including increased relative abundance