Rumen Fluid Metabolomics and Microbiome Profiling of Dairy Cows Fed Combinations of Prebiotics, Essential Oil Blend, and Onion Peel Using the RUSITEC System

Background and Objectives: Dairy products provide vital energy, high-quality protein, and micronutrients for over six billion people worldwide, with dairy cows contributing nearly 81% of global milk production. Sustainable strategies to enhance productivity are therefore critical. Feed additives such as essential oil blends (EOB), onion peel (OPE), and prebiotics including mannan oligosaccharides (MOS) and galacto-oligosaccharides (GOS) have been proposed to improve rumen fermentation, modulate microbial ecology, and mitigate greenhouse gas emissions. This study evaluated the combined effects of EOB, OPE, MOS, and GOS on rumen metabolism using the rumen simulation technique (RUSITEC). Materials and Methods: Rumen inoculum from three cannulated Holstein Friesian cows was incubated across 16 vessels (four treatments × four replicates) for nine days. Treatments included a control (CON; TMR only), GEO (TMR + GOS + EOB + OPE), MEO (TMR + MOS + EOB + OPE), and OLEO (TMR + a 1:1 mixture of GOS and MOS + EOB + OPE). Additives were included at 3 µL/g TMR for EOB and 30 mg/g TMR (3% w/w) for OPE, GOS, MOS, or OLG. Rumen effluents were collected for untargeted metabolomic profiling by liquid chromatography–mass spectrometry, identifying 661 metabolites. Results: Partial least squares-discriminant analysis revealed clear separation between CON and additive groups, confirming distinct metabolic shifts. GEO primarily enhanced tryptophan, tyrosine, and purine metabolism; MEO stimulated phosphonate and pyrimidine pathways and bile acid biosynthesis; OLEO promoted phosphonate, nicotinamide, and taurine metabolism. Microbial analysis showed enrichment of taxa such as Lachnospira, Succinivibrionaceae, Macellibacteroides, Lysinibacillus, and Christensenellaceae, indicating complementary effects on fermentation and microbial stability. Conclusions: These results demonstrate that dietary supplementation with GEO, MEO, or OLEO modulates rumen metabolism and microbial ecology without impairing fermentation, supporting improved nutrient utilization, antioxidant defenses, and metabolic resilience in dairy cows, with potential benefits for productivity and sustainability.