(1) Background:The objective of this study was to explorethe dynamic changes of gut microbiota in Simmental calves before weaning, and compared microbial composition and functionality differences between diarrhea and health calves; (2) Methods: Fourteen neonatal Simmental calves were divided into a health group (n = 8) and diarrhea group (n = 6). The rectal stool samples were collected from each calf on days 1, 3, 5, 7, 9, 12, 15, 18, 22, 26, 30, 35, and 40. The high-throughput sequencing analysis of the 16S rRNA gene V1–V9 region was conducted to explore the changes in gut microbiota over time in both groups and assess the influence of diarrhea on microbiota structure and function; (3) Results: E.coli, B.fragilis, and B.vulgatus were the top three bacterial species of pre-weaning Simmental calves.Meanwhile, the major functions of fecalmicrobiota included the “Metabolic pathways”,“Biosynthesis of secondary metabolites”,“Biosynthesis of antibiotics”, “Microbial metabolism in diverse environments”, and “Biosynthesis of amino acids”. For calves in health group, PCoA revealed that the bacterial profiles generated from days 1, 3, 5, 7, and 9 separated from days 15, 18, 22, 26, 30, 35, and 40. While the microbial profiles of calves on day 12 crossed with both clusters, suggesting that the dynamic changes of microbial structures with the increasing of ages. When compared the difference of the relative abundance of bacteria between calves in health and diarrhea groups, the results showed that the relative abundance of the beneficial L.johnsonii, F.prausnitzii, and Limosilactobacillus in the health group were significantly higher than those in the diarrhea group (p

The objective of this study was to explore the dynamic changes in the gut microbiota of Simmental calves before weaning and to compare the microbial composition and functionality between healthy calves and those with diarrhea. Fourteen neonatal Simmental calves were divided into a healthy group (n = 8) and a diarrhea group (n = 6). Rectal stool samples were collected from each calf on days 1, 3, 5, 7, 9, 12, 15, 18, 22, 26, 30, 35, and 40. High-throughput sequencing of the 16S rRNA gene V1–V9 region was conducted to examine changes in the gut microbiota over time in both groups and to assess the influence of diarrhea on microbiota structure and function. Escherichia coli, Bacteroides fragilis, and B. vulgatus were the top three bacterial species in preweaning Simmental calves. Meanwhile, the major functions of the fecal microbiota included “metabolic pathways”, “biosynthesis of secondary metabolites”, “biosynthesis of antibiotics”, “microbial metabolism in diverse environments”, and “biosynthesis of amino acids”. For calves in the healthy group, PCoA revealed that the bacterial profiles on days 1, 3, 5, 7, and 9 differed from those on days 15, 18, 22, 26, 30, 35, and 40. The profiles on day 12 clustered with both groups, indicating that microbial structure changes increased with age. When comparing the relative abundance of bacteria between healthy and diarrheic calves, the beneficial Lactobacillus johnsonii, Faecalibacterium prausnitzii, and Limosilactobacillus were significantly more abundant in the healthy group than those in the diarrhea group (p < 0.05). This study provides fundamental insights into the gut microbiota composition of Simmental calves before weaning, potentially facilitating early interventions for calf diarrhea and probiotic development.