Metabolomic shifts in beef steers rotationally grazing toxic endophyte-infected tall fescue under fall conditions

Background: . While the vascular system is a major EA target, their biogenic amine-like properties can trigger wider physiological effects. This study used untargeted metabolomics and targeted volatile fatty acid (VFA) analysis to characterize EA-induced metabolic disruption in steers under rotational tall fescue grazing. Methods: 18 steers grazed toxic (E+), novel (NE), or endophyte-free (E-) fescue pastures. After 14 days, groups switched diets (toxic to nontoxic and vice versa). Urine, saliva, plasma, rumen fluid (RF), and feces were collected. Untargeted high-resolution metabolomics (HRM) was performed on liquid matrices, and gas chromatography-mass spectrometry quantified VFAs in RF and feces. Results: Total and individual VFA increased in RF during E+ exposure and returned to baseline after removal. Discriminative analyses showed that E+ steers had a distinct metabolome, while previously exposed steers in period 2 resembled those never exposed. Pathway analysis revealed downward shifts in beta-oxidation, fatty acid and arachidonic acid metabolism in E+, whereas aromatic amino acids (e.g., tyrosine, tryptophan), branched-chain amino acids, vitamin B6, and carbohydrate (e.g., gluconeogenesis) pathways shifted upwards. Upward-shifted pathways were mainly amino acid-related (45%), and downward-shifted mostly lipid-related (60%). Several metabolites, including tyramine, methyltyramine, methoxytyramine, and dopamine, were discriminatory for E+. HRM detected clavine alkaloids, and lysergic acid derivatives in all matrices except plasma, rising and returning to baseline within 2 days of E+ exposure/removal. Conclusion: Grazing E+ disrupts metabolism in steers, shifting energy use from lipids toward amino acids and carbohydrates. The main detected EAs were clavine-type alkaloids and simple lysergic acid amides, not ergopeptines, suggesting extensive biotransformation. The EA dynamics and the similar metabolic profile of previously and never-exposed steers in period 2, indicate minimal or no lasting effects metabolomic after exposure ends. The upward shift in metabolite abundance associated with aromatic amino acid pathways (e.g., tyrosine and tryptophan), vitamin B6 metabolism (a key cofactor for aromatic amino acid decarboxylases), and their downstream products, biogenic and trace amines, suggests a coordinated metabolic shift that may contribute to and/or amplify the FT pathophysiology.