Amino acid (AA) metabolism and the adaptation of muscles to increased physical performance are interdependent. This field study examined changes in plasma amino acid (PAA) concentrations and two metabolites (ammonia and urea) before and after cross-country (CC) competitions throughout an entire eventing season and their associations with performance outcomes in eventing horses. Twenty horses from nine different Warmblood breeds were evaluated between two and seven times across 14 international eventing competitions over a 23-week period. A total of 55 rides were sampled. Blood samples were collected at four time points: before the start (TP0), 10 min after completion of the CC ride (TP1), 30 min post-CC (TP2), and the next morning (TP3). A mixed ANOVA revealed significant effects (p < 0.05) of calendar week on PAA concentrations for asparagine, ornithine, and proline. Correlations between CC scores and PAA concentrations were observed for 4 of the 25 measured parameters. TP0 leucine concentrations and TP2 histidine concentrations correlated negatively with CC penalty score, whereas TP1 proline concentrations and TP3 alanine concentrations correlated positively with CC penalty score. These results indicate that higher plasma leucine concentrations may contribute to improved performance in eventing horses. Additionally, alanine and proline show potential as performance-related diagnostic biomarkers.
