Effects of Partial Replacement of Palm Oil with Schizochytrium Oil on Rumen Microbiota, Ruminal Volatile Fatty Acid Concentrations, Rumen Epithelial Transcriptome, and Serum Metabolome in Hu Lambs Fed a Medium-Fiber Diet

Schizochytrium oil (SO) is rich in docosahexaenoic acid (DHA), but its ruminal effects in sheep remain unclear. Twenty Hu lambs were fed CK or SO diets for 84 d. The CK diet contained 4% palm oil, 31.43% neutral detergent fiber (NDF), and 5.58% ether extract (EE). The SO diet contained 3% palm oil and 1% SO, with similar NDF (31.76%) and EE (5.45%). Blood samples were collected one day before slaughter, before the morning feed replenishment, whereas rumen contents and epithelial tissue were collected after slaughter following a 16-h fast. Rumen microbiota, volatile fatty acids (VFAs), epithelial morphology (n = 10/group), RNA-seq (n = 4/group), and serum metabolomics (n = 5/group) were analyzed. The results showed that the rumen microbial community structure did not differ between the two groups (PERMANOVA, R2 = 0.101, p > 0.05), but SO selectively altered the relative abundance of seven genera (FDR < 0.05), including higher Fibrobacter and lower Methanosphaera. These changes were accompanied by higher acetate, propionate, butyrate, valerate, and total VFA concentrations and a lower acetate/propionate ratio (p < 0.05). Histological analysis showed that rumen papilla length and total epithelial thickness were higher in the SO group than in the CK group (2286.49 vs. 1764.30 μm and 114.52 vs. 80.07 μm, respectively; p < 0.05). Rumen epithelial RNA-seq showed enrichment of pathways related to VFA metabolism, energy metabolism, and fatty acid metabolism in the SO group (FDR < 0.01). Serum metabolomics showed higher levels of DHA and several DHA-containing lipids in the SO group (FDR < 0.05). Overall, partial replacement of palm oil with 1% SO selectively altered selected rumen genera and increased ruminal propionate concentration, accompanied by changes in rumen epithelial morphology and metabolism and increased serum DHA-related lipids. These findings support the potential of SO as a dietary DHA source for Hu lambs and provide evidence for its effects on the rumen and serum DHA-related lipid profile.