D-serine supplementation is associated with mucosal-prioritized rumen development and propionate-enriched fermentation with selective microbial shifts in pre-weaning Hu lambs.

This study evaluated whether dietary D-serine (D-Ser) could modulate rumen development, fermentation, microbiota, and metabolomic profiles in pre-weaning Hu lambs. Twenty healthy male lambs were assigned to a control group or a D-Ser group (n = 10/group); D-Ser was supplied at 2 g·kg-1 BW·d-1 from 7 to 48 days of age. Growth and starter intake were recorded, and rumen morphology, volatile fatty acids, 16S rRNA profiles, and untargeted LC-MS metabolomes were analyzed in slaughtered lambs (n = 6/group). D-Ser increased average daily starter intake during the 30-d starter-intake recording period by 25.96% (p = 0.036) and tended to advance first voluntary starter intake, whereas final body weight and average daily gain were numerically but not significantly higher. Rumen weight, rumen weight-to-body weight ratio, and rumen volume were significantly increased. Papillae length, width, density, and epithelial thickness were also enhanced, while muscle layer thickness was unchanged, indicating mucosa-prioritized morphological development. D-Ser increased total volatile fatty acids, acetate, propionate, and butyrate concentrations; propionate molar proportion rose from 21.41 to 25.11%, and the acetate-to-propionate ratio decreased from 2.90 to 2.44. Microbial diversity was not significantly altered, but Bacteroidota abundance increased, with enrichment of Shuttleworthia, Erysipelotrichaceae_UCG-006/UCG-009, Corynebacterium, and Sutterella. Metabolomics identified 248 differential metabolites enriched in amino acid, carbohydrate, and secondary bile acid pathways. The upregulation of L-N-carboxymethylserine and isodeoxycholic acid was consistent with possible microbial processing of D-Ser, but direct transformation was not experimentally verified. Overall, D-Ser improved starter intake and was associated with propionate-enriched fermentation and mucosal morphological development, whereas its growth-promoting effect and causal microbial mechanisms require further validation.