Plasma biomarkers of urinary nitrogen excretion in beef cattle: repeatability and potential to detect differences...

Urinary nitrogen excretion (UNE) in cattle contributes to environmental pollution and economic losses. Direct UNE measurement is costly, labour-intensive, and unsuitable for extensive production systems. Although some UNE prediction equations exist, none address the ability to detect individual variability in beef cattle. This study aimed to identify plasma biomarkers reflecting both diet- and individual-induced variations in UNE, assess their temporal repeatability, and explore predictive equations based on plasma parameters. Sixteen Charolais bulls were fed either a Low- (116 g CP/kg DM) or high-protein (173 g CP/kg DM) diet and evaluated during two 15-day nitrogen (N) balance periods separated by 9 weeks. Blood samples collected on the final day of each period were analysed for 66 metabolites using targeted metabolomics, 4 biochemical parameters by spectrophotometry, and natural ¹⁵N abundance (Δ¹⁵N) by isotope-ratio mass spectrometry. High-CP diets increased N intake by 43-46% and more than doubled UNE (P < 0.001). Only nine of the 71 plasma parameters differed significantly between diets after false discovery rate (FDR) correction (FDR < 0.05). Among these, Δ¹⁵N, 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.