This study determined potential microbial and metabolic biomarkers of feed efficiency in Angus heifers. Seventeen ruminal cannulated Angus heifers underwent a 70-day feed efficiency trial. Residual feed intake was used to determine high and low feed efficient heifers. On day 70 of the trial, rumen content and blood were collected and used for microbial and metabolomic analyses, respectively. Bacterial populations were examined by targeting the V4 region of the 16S rRNA gene and analyzed using QIIME and SAS. Rumen fluid and serum metabolites were analyzed using MetaboAnalyst. No microbial taxa differed after false discovery rate correction, but seven did differ (p ≤ 0.05) prior to correction, including Lachnospiraceae (Other), Desulfobulbaceae, Neisseriaceae, Shuttleworthia, Corynebacterium, p-75-a5, and L7A-E11. No differences were observed in alpha diversity metrics. Beta diversity utilizing unweighted UniFrac distances analyzed via PERMANOVA was significant (p = 0.03). Several metabolites in rumen fluid metabolites were correlated with bacteria that differed by feed efficiency phenotype. The metabolites correlated with bacteria were primarily involved in nutrient signaling and microbial crude protein availability. These data suggest variation in the availability of nutrients, primarily amino acids, as well as a relationship among microbiota, metabolome, and host feed efficiency phenotypes in heifers.

Fescue toxicosis impacts beef cattle production via reductions in weight gain and muscle development. Isoflavone supplementation has displayed potential for mitigating these effects. The objective of the current study was to evaluate isoflavone supplementation with fescue seed consumption on rumen and serum metabolomes. Angus steers (n = 36) were allocated randomly in a 2 × 2 factorial arrangement of treatments including endophyte-infected (E+) or endophyte-free (E−) tall fescue seed, with (P+) or without (P−) isoflavones. Steers were provided a basal diet with fescue seed for 21 days, while isoflavones were orally administered daily. Following the trial, blood and rumen fluid were collected for metabolite analysis. Metabolites were extracted and then analyzed by UPLC-MS. The MAVEN program was implemented to identify metabolites for MetaboAnalyst 4.0 and SAS 9.4 statistical analysis. Seven differentially abundant metabolites were identified in serum by isoflavone treatment, and eleven metabolites in the rumen due to seed type (p < 0.05). Pathways affected by treatments were related to amino acid and nucleic acid metabolism in both rumen fluid and serum (p < 0.05). Therefore, metabolism was altered by fescue seed in the rumen; however, isoflavones altered metabolism systemically to potentially mitigate detrimental effects of seed and improve animal performance.