Plasma cortisol and ACTH concentrations in the warmblood horse in responseto a standardized treadmill exercise test as physiological markers for evaluation of training status

Reliable physiological markers for performance evaluation in sport horses a re missing. To determine the diagnostic value of plasma ACTH and cortisol m easurements in the warmblood horse, 10 initially 3-yr-old geldings of the H annovarian breed were either exposed to a training schedule or served as co ntrols. During experimental Phase 1, horses were group-housed, and half of the horses were trained for 20 wk on a high-speed treadmill. During Phase 2 , groups were switched and one group was trained for 10 wk as during Phase I, whereas the control group was confined to boxes. During Phase 3 horses w ere initially schooled far riding. Thereafter, all horses were regularly sc hooled for dressage and jumping, and half of the horses received an additio nal endurance training for 24 wk. During all phases horses were exposed at regular intervals to various standardized treadmill exercise tests. During and after the tests frequent blood samples were taken from an indwelling ju gular catheter for determination of ACTH and cortisol. Treadmill exercise i ncreased both hormones. Maximum ACTH concentrations were recorded at the en d of exercise, and maximum cortisol levels were recorded 20 to 30 min later . Except for one test there were no differences in ACTH levels between trai ned horses and controls. There was no significant effect of training on the cortisol response (net increase) to treadmill exercise in any of the tests during Phase 1. During Phase 2 higher cortisol responses were recorded in controls than in trained horses (P less than or equal to .05) after 10 wk o f training (controls confined to boxes). During Phase 3 plasma cortisol res ponses were also higher in controls than in trained horses (P less than or equal to .05 after 6, 18, and 24, P less than or equal to .07 after 12 wk o f training) when the inclination of the treadmill was 5%, but not at 3%. Th ere was no overlap in net cortisol responses at 30 min between trained and untrained horses. An ACTH application after 24 wk of braining resulted in h igher cortisol responses in controls than in trained horses (P less than or equal to .05), without any overlap between the groups at 30 min after ACTH . Plasma cortisol responses to either treadmill exercise or ACTH injection may be a reliable physiological marker for performance evaluation. Prerequi sites are sufficient differences in training status and sufficient intensit y of exercise test conditions.