High-grain diet-induced ruminal acidosis triggers systemic inflammation and serum metabolic reprogramming in dairy cows

High-grain diets are widely used to meet the energy demands of high-producing dairy cows; however, excessive grain intake predisposes cows to ruminal acidosis and subsequent systemic inflammation, compromising health and productivity. This study aimed to characterize systemic inflammatory responses and associated serum metabolic alterations induced by high-grain feeding. Fourteen mid-lactation dairy cows fitted with permanent rumen fistulas were subjected to a gradual increase in dietary corn grain to induce ruminal acidosis. Blood samples were collected under normal and acidosis conditions; inflammatory biomarkers were quantified using enzyme-linked immunosorbent assay (ELISA), and serum metabolomic profiles were analyzed by gas chromatography-time of flight mass spectrometry <b>(</b>GC-TOF/MS). Rumen pH decreased significantly with increasing dietary corn grain (<i>p</i> < 0.05), confirming the successful induction of ruminal acidosis. Serum lipopolysaccharide (LPS) concentrations exhibited an inverted bell-shaped pattern during the induction process (<i>p</i> < 0.05), while concentrations of acute-phase proteins, including serum amyloid A (SAA), C-reactive protein (CRP), and haptoglobin (Hp), increased markedly (<i>p</i> < 0.05), indicating the development of systemic inflammation. Metabolomic analysis revealed a clear separation between normal and acidosis states, with 37 metabolites significantly different between them. Correlation analysis showed that multiple serum metabolites were closely associated with ruminal pH and inflammatory indicators, particularly SAA, CRP, and Hp. Lipid- and amino acid-related metabolites were positively correlated with inflammatory parameters, whereas several organic acids were negatively correlated, suggesting coordinated metabolic reprogramming during high-grain-induced inflammation. Biomarker analysis identified D-glycerol-1-phosphate and 4-hydroxypyridine as potential serum biomarkers that discriminate acidosis-associated metabolic alterations. In conclusion, a gradual increase in dietary corn grain induces progressive systemic inflammation and pronounced disturbances in serum metabolic homeostasis in lactating dairy cows. These findings highlight the tight linkage between rumen dysfunction and host metabolic regulation and provide potential blood-based biomarkers for early detection of inflammation associated with high-grain feeding.