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

Equine behavioral traits are important criteria to assess the suitability of retired Thoroughbred racehorses for equestrian sports. 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. 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%. 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. Transition training of retired Thoroughbreds can assist in improving horses' suitability for equestrian sports, based on their successive responses to a novel startle test.