Behavioral and cardiac responses to a model startle test to assess retired Thoroughbred racehorses for equestrians.

Importance: Equine behavioral traits are important criteria to assess the suitability of retired Thoroughbred racehorses for equestrian sports.

Objective: This study aims to develop a practical procedure to assess retired Thoroughbreds for their suitability for equestrian sports or leisure purposes and to assess if horses identified as having reactive temperaments in response to a standardized startle test, will have improved test results following a period of transition training.

Methods: Behavioral reactivity data and cardiac autonomic responses to a startle test (comprising of a plastic bag applied to the neck area) were collected from twelve retired Thoroughbred racehorses (6 males and 6 females; average age: 4.33 ± 0.89 years old; average body weight 465.17 ± 25.33 kg), before and after the three-month transition training for equestrian sports. A priori power calculation indicated a requirement of 12 individuals for a power of 80%.

Results: Following the training, there was a significant reduction in the behavioral score (BS), peak heart rate (HR), and response duration (RD) of all horses compared to their pre-training scores. However, 25% of horses (3/12) retained a BS of 2 or above post-training, exhibiting significantly higher peak HR and longer RD compared to those with a BS below 2. Post-training, the females demonstrated a significant increase in the root mean square of the successive differences between normal heartbeats compared to their pre-training levels.

Conclusions and relevance: Transition training of retired Thoroughbreds can assist in improving horses' suitability for equestrian sports, based on their successive responses to a novel startle test.