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

The research article investigates the potential of using plasma ACTH and cortisol levels in horses as physiological markers to determine their performance and training status. The study is carried out on horses subjected to various levels of training and the concentrations of these hormones are tested during different phases of a treadmill exercise test.

Study Design and Procedure

• There were 10 initially 3-year-old geldings of the Hannovarian breed used in the study. Fifty percent of the horses were involved in a training schedule while the others served as the control group.
• The experiment was divided into three phases. During phase 1, all horses were group-housed, with half of them undergoing a 20-week high-speed treadmill training. Phase 2 saw the groups switched, with one group undergoing the same training as in phase 1 while the control group remained in boxes. Phase 3 began with all horses being schooled for riding. Afterwards, all horses received regular schooling for dressage and jumping, and half continued with an additional endurance training for 24 weeks.
• Throughout all phases, horses were subjected to routine standardized treadmill exercise tests. They also had frequent blood samples taken from an indwelling jugular catheter to study ACTH and cortisol levels.

Key Findings

• Treadmill exercise resulted in increased levels of both ACTH and cortisol. Peak ACTH concentrations were seen at the end of exercise, while maximum cortisol levels were recorded 20 to 30 minutes later.
• No significant differences in ACTH levels were observed between trained horses and controls, except in one test. Similarly, there was no major impact of training on cortisol responses to treadmill exercises during Phase 1 of the tests.
• However, during Phase 2 and 3, higher cortisol responses were noted in control horses compared to trained horses. More pronounced differences in cortisol responses were observed when the treadmill inclination was at 5%, but not at 3%.
• Twenty-four weeks of training resulted in higher cortisol responses in controls than in trained horses after ACTH application. There was no overlap in net cortisol responses at 30 minutes between the trained and untrained horses.

Conclusion

• The study concludes that plasma cortisol responses to either treadmill exercise or ACTH injection may serve as a reliable physiological marker for performance evaluation.
• However, the above conclusion requires considerable difference in the training status and sufficient intensity of the exercise test conditions.