Plasma cortisol concentration during standardized exercise in Standardbred racehorses within a racing season

Performance diagnosis of Standardbred racehorses needs to be improved. It was examined whether parameters derived of the blood plasma cortisol concentration (CORT) during a standardized exercise test (SET) used to determine the endurance parameter v(4) (velocity run under defined conditions inducing 4 mmol/L blood lactate concentration) provided additional information for performance diagnosis of two-, three-, and four-year-old Standardbred horses during a racing season. The CORT parameters were CORTstart (CORT after warm-up but before starting), CORTmax (maximal CORT) and CORTmax- min (difference between the CORTmax and CORTmin). The v(4) of the horses was related to all CORT derived parameters and the effect of testing in April, June and August on the CORT derived parameters was examined for all age-groups. The date of testing did not influence CORT derived parameters in any age group (P > 0.05 all). CORTmax-min of the four-year-old horses was the only CORT derived parameter showing a significant relationship with v(4) (P = 0.048; r(2) = -0.27). None of the CORT derived parameters was different among age-groups. In conclusion, further studies need to be done to clarify whether the CORT parameters defined are useful for performance or poor performance diagnosis.