Plasma biomarkers of urinary nitrogen excretion in beef cattle: repeatability and potential to detect differences across diets and between-individuals

N, urea, phenylacetylglycine and p-cresol sulphate were the strongest discriminators of dietary CP level, all being linked to ruminal protein degradation pathway and likely reflecting the higher supply of rumen-degradable protein from the high-protein diet. At the individual level, homocysteine, cysteine, and creatinine were most consistently and significantly correlated (FDR < 0.05) with UNE (g/d), likely reflecting host metabolism related to methionine-cysteine pathways and muscle turnover. Plasma creatinine and histidine showed moderate to high repeatability across time and were associated with individual variations in UNE. Exploratory predictive models effectively captured individual differences in UNE, but predictor consistency varied with dietary protein level and fattening period, likely reflecting changes in the balance between protein supply and time-specific protein requirements in growing beef cattle. Overall, our findings revealed promising candidate biomarkers of UNE in beef cattle, though they were not the same when assessing diet-induced (rumen origin) vs individual-induced (host metabolism) variations in UNE.