Temporal shift of faecal microbiome in weaning calves supplemented with  Saccharomyces cerevisiae
The transition from birth to weaning represents a critical period for the development of the gastrointestinal microbiome in dairy calves. This study aimed to characterize the temporal dynamics of the fecal microbiota and to evaluate the effects of dietary supplementation with inactive Saccharomyces cerevisiae (Group Y) in comparison to control group until weaning. Ten Italian Simmental calves were allotted to each group and fecal samples were collected at three time points . Microbiota was assessed using 16S rRNA gene sequencing, and functional potential was inferred using PICRUSt2. Alpha diversity metrics increased significantly over time , and beta diversity analyses identified time as the primary driver of microbial variation , whereas yeast supplementation exerted modest but significant effects on community structure. A total of 7 and 16 taxa were differentially abundant in the T2 vs T1 and T3 vs T1 comparisons, respectively, with taxa such as Oscillospiraceae, Lachnospiraceae, and Faecousia contributing to temporal shifts. Functional predictions produced 130 and 145 significantly enriched pathways in the T2 vs T1 and T3 vs T1 contrasts, respectively. From T2 to T3, pathways related to carbohydrate, nucleotide, and energy metabolism showed a predominance of down-regulation, whereas amino acid, lipid, and cell structural pathways increased. Correlation analyses found associations between specific taxa (e.g., Oscillospirales and Lachnospirales) and key metabolic functions. Overall, microbiome development in dairy calves was primarily driven by age, with yeast supplementation exerting a secondary modulatory role. These changes suggest a model of microbiome maturation characterized by increasing specialization and ecological stability from birth to weaning.