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Summary The objective of the study was to examine whether age, racing time record or v 4 of standardbred horses had an effect on the changes of cortisol concentration in plasma during exercise. For this purpose standardbred horses of two race barns were submitted to a standardized exercise test (SET). Speed and duration of each run of a SET were such that the blood lactate concentration of each horse exceeded 4 mmol/ l after the fourth run or later only. The blood lactate concentration measured after each run was plotted against running speed to derive v 4 from the blood lactate-running speed relation. The linear relation between speed and plasma cortisol concentration in all horses during SET was low but significant (in race barn A: r 2 = 0.16, p < 0.001; in race barn B: r 2 = 0.18, p < 0.001). When the relation between speed and plasma cortisol concentration during SET was examined for each horse individually it was found that in all two-year-olds the linear coefficient of determination was above 0.50 (r2 = 0.75 ± 0.18), whilst for the older horses it was often below 0.50 (r 2 = 0.50 ± 0.34). Age did not have a significant effect on mean plasma cortisol concentration in horses after warm-up. But after all runs of SET mean plasma cortisol concentration of two-year-old horses was lower than in horses older than 2 years. The highest linear coefficient of determination between the racing time record and plasma cortisol concentration was calculated after the last run of SET for horses in one of the race barns (r 2 = 0.57). All other coefficients of determination were below 0.40. The highest linear coefficient of determination between v 4 and plasma cortisol concentration was determined after the last run of horses older than 2 years (r 2 = 0.63). In conclusion, age affects the development of cortisol concentration during exercise. Cortisol measurements may provide useful information for describing racing time record and v 4 of standardbred racehorses.

Effect of age, time record and v4 on plasma cortisol concentration in standardbred racehorses during exercise