Assessing retired Thoroughbred racehorses for other equestrians uses

The discussion at the bottom of the summary has practical information for experienced horsemen training retired racehorses for other equestrian uses.
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Behavioral and cardiac responses to a model startle test to assess retired Thoroughbred racehorses for equestrians

J Vet Sci. 2024 Nov;25(6):e84. doi: 10.4142/jvs.24177.
Authors
Seung-Ho Ryu 1 , Kyung-Eun Lee 2 , Eliot Forbes 3 , Sung-Jun An 4 , Joon Gyu Kim 1 , Hang Lee 5 , Byung Sun Kim 6
Affiliations

1 Department of Equine Science, Cheju Halla University, Jeju 63092, Korea.
2 Department of Life and Environmental Sciences, The University of Sydney, New South Wales 2050, Australia.
3 Racing Integrity Board, Private Bag 17902, Greenlane, Auckland 1546, New Zealand.
4 Department of Intelligent System Engineering, Cheju Halla University, Jeju 63092, Korea.
5 Center for Animal Welfare Research, College of Veterinary Medicine, Seoul National University, Seoul 08826, Korea.
6 Department of Equine Science, Cheju Halla University, Jeju 63092, Korea. bskim0929@naver.com.

PMID: 39608778
DOI: 10.4142/jvs.24177

Abstract

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.

Keywords: Heart rate; behavioral score; heart rate variability; horse; response duration; startle test.

© 2024 The Korean Society of Veterinary Science.
Conflict of interest statement
The authors declare no conflicts of interest.

DISCUSSION (excerpted from this open access article)
This study investigated the response of retired Thoroughbreds to a three-month period of transition training and assessed if an anecdotal threshold for behavior rating could be useful for screening horses better suited to equestrian sports. Horses typically react to unknown circumstances or possible threats by fleeing or avoidance, however, over time they become accustomed to their surroundings and non-threatening stimuli [20]. All horses demonstrated significant reduction in reactivity after the three-month transition training. The BS and peak HR were significantly decreased and the RD was significantly shortened, after the training in all horses. These results are likely due to the calmer nature of basic equestrian training, which involved tasks such as standing still for mounting and dismounting without additional assistance of other people, etc. [21, 22].

Nevertheless, 25% of the horses received a BS of 2 or higher after the training, posing a potential hazard for inexperienced riders and handlers according to the NSW Code of Practice. Furthermore, a significant difference in BS was observed between horses with a post-training BS of 2 and above and those with a post-training BS below 2, both before and after the training. Previous studies showed similar results with horses demonstrating more sensitive behavioral responses, with higher HRs before training and lower HRs after training, when subjected to the startle test [5, 23]. In the present study, horses with post-training BS of 2 and above did not exhibit a significantly diminished fear response to challenging objects or situations following the training, with a marked difference when compared to horses with BS of below 2, future research is warranted to explore whether extended or more intensive transition training could enhance horses’ post-training responses of BS 2 and above to match those of horses with BS of below 2. Guidelines could be established for assessing horses before transition training, and horses scoring 2 or higher could undergo tailored training protocols designed to enhance post-training outcomes.

Measurement of HRV is a non-invasive technique that can be used to investigate the function of the autonomic nervous system. HRV measures have been used in animal research, to analyze changes in sympathovagal balance related to psychological and environmental stressors or individual characteristics such as temperament [24]. In horses, HRV is used to assess the limbic system and sympathetic nervous system’s response to harmful events like pain, fear, stress, or anxiety. These events trigger physiological changes such as tachycardia, tachypnea, hypertension, and hyperthermia, which are linked to increased HRV. These modifications are often accompanied by altered emotional states, poor rest, and even aggression, situations observed in the present study, which in the long term would favor that animals may show deficiencies in their ability to cope with changes in the environment due to alterations in the functioning of their immune, nervous and endocrinological systems [25, 26, 27, 28, 29, 30]. Though a high correlation, reliability, and agreement between HRV measured by Polar and electrocardiogram has been reported [31], the usefulness of HRV data in horses has nevertheless come under scrutiny. HRV data may be less reliable when horses are in motion or moving [24, 32] or the measurement duration is short. It is generally recommended to use HRV data collected for at least 5 min [33, 34, 35]. In this study, the mean duration to reach peak HR was 30.58 sec, and any additional stimulus was discontinued once peak HR was attained.

Females showed considerably increased mean rMSSD and SDNN after training compared to before training. Even though females are considered to be more sensitive than males [36], this study confirms that both sexes can be desensitized following transition training. The rMSSD and SDNN values showed significant variability among the horses, and there were inconsistent correlations observed with other test outcomes. A previous equine study found that rMSSD values measured when horses were more static (compared to this experiment) were more reliable [37]. The movement of the horses when responding to the startle test in this study, likely impacted the usefulness of rMSSD as an indicator. Similar to rMSSD, SDNN values also exhibited significant differences between the sexes before the training. This finding aligns with previous research that has identified a positive correlation between rMSSD and SDNN [23]. However, this study did not find a significant difference in the LF/HF ratio, whereas a significant increase in the LF/HF ratio was detected after the startle test with horses in another study [12]. This discrepancy may be attributed to the short duration of measurement in this study.

Significant correlations between BS and cardiac variables during novel object tests in horses have been reported [38, 39, 40]. The present study aligns with previous research and indicates that the correlation between BS and peak HR will be a useful observation, to assess the suitability of retired Thoroughbreds for equestrian sports. This study was conducted on a relatively small number of animals and future studies could be strengthened by enlarging the number of test animals. A practical challenge to study design was the limited availability of sufficient numbers of professional equestrian riders for the three-month transition training.

Based on the observations in the present study, it is suggested that retired Thoroughbreds from racing can be evaluated for behavioral traits suitable for equestrian sports prior, to commencing any transition training. The importance of transition training cannot be underestimated in preparing horses for careers after racing. The startle model employed in the current study can be easily reproduced and provides a practical evaluation of horses’ responses that can assist in planning the transition training. It is important to note that this study was conducted using experienced riders and handlers. The NSW Government’s Code of Practice identifies horse handling risks to new or inexperienced riders. While it may be useful to utilize inexperienced staff in experimental design this was not feasible due to requirements of the Occupational Safety and Health Act in Korea (Article 140), which state that in the case of dangerous and skill-intensive work, a person who lacks the necessary experience or skills should not be allowed to do the work. Additional behavioral measures, such as spontaneous and forced approach tests to analyze horse-human interactions, as well as tracking the time each horse takes to reach specific training milestones, would provide deeper insights into horses’ reactivity and their relationships with humans. This approach would likely lead to a more thorough evaluation of safety and suitability. Future studies will be necessary to develop additional transition training techniques with additional behavioral measures that may enhance the responses of reactive horses after the three-month training employed in this study.

• This topic was modified 2 days ago by Robert Oglesby DVM.
• This topic was modified 2 days ago by Robert Oglesby DVM.