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cohort·equine medicine, veterinary medicine, sports medicine, behavioral medicine·PMC10079128

Variation in salivary cortisol responses in yearling Thoroughbred racehorses during their first year of training

PLOS ONE · 9 authors, 5 centres

AI SUMMARY

FIDELITY 100%

POPULATIONYearling Thoroughbred racehorses (n=96, 46 female, 50 male) entering training at a single yard

INTERVENTIONExposure to three novel training events: first time being driven with long reins, first time being backed by a jockey, and first time ridden on the gallops

COMPARISONPre-event vs. post-event salivary cortisol concentrations; resting samples at nursery yard, within 3 days of entering main yard, and after 2–3 weeks in main yard

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TL;DR

This study measured salivary cortisol in 96 yearling Thoroughbred racehorses during their first year of training and found that cortisol levels significantly increased after three novel training events: first time being driven with long reins, first time being backed by a jockey, and first time on the gallops. The wide range of post-event cortisol concentrations (e.g., 2.71–36.18 nmol/L after first backing) indicates substantial individual variation in the acute stress response. These findings suggest salivary cortisol could serve as an objective biomarker for identifying horses that may need tailored management to improve welfare during early training.

Full summary

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**Background:** Thoroughbred racehorses undergo intense training from a young age, requiring rapid adjustment to novel environments. The hypothalamic-pituitary-adrenal (HPA) axis releases cortisol in response to stress, and chronic over-activation can have deleterious health and behavioural effects. Identifying horses with more sensitive temperaments via objective measures could enable proactive management. Salivary cortisol is a less-invasive proxy for serum cortisol and has been used to evaluate stress responses in horses.

**Methods:** The study cohort comprised n=96 yearling Thoroughbreds (46 female, 50 male) born in 2017, progeny of 18 sires, all housed and trained at a single yard. Saliva samples were collected using a dental sponge and stored in 1.5ml tubes with a plastic straw segment to prevent reabsorption. Samples were refrigerated within 30 minutes, centrifuged at 1500 × g for 10 minutes, and cortisol concentration measured using the Salimetrics™ Salivary ELISA Kit. Resting samples were collected between 14:00–15:30 at three timepoints: at the nursery yard before moving to the main yard (RN, n=66), within three days of arrival at the main yard (RY_T1, n=67), and after 2–3 weeks in the main yard (RY_T2, n=50). A timecourse experiment sampled n=5 female horses every two hours from 08:00–16:00. Pre- and post-event samples were collected for three novel training events: first time being driven with long reins (FD, n=6 paired samples), first time being backed by a jockey (FR, n=34 paired samples), and first time ridden on the gallops (FG, n=10 paired samples). Post-event samples were taken ~30 minutes after the event. Statistical analyses used ANOVA and paired t-tests in RStudio (version 3.6.0), with P<0.05 considered significant.

**Key Results:** The inter-assay CV was 0.96%; mean intra-assay CV was 0.83% (SD±0.69). Across all samples, cortisol concentrations ranged from 1.03–36.18 nmol/L. In the timecourse experiment, mean cortisol at 08:00 (T1) was 3.33 nmol/L (SD±0.63), significantly higher than later timepoints (Paired t-test, P<0.05). No significant difference was found among the later four timepoints (10:00–16:00). For resting samples, there was no significant difference in mean cortisol across RN (mean 2.69 nmol/L, SD±1.08), RY_T1 (mean 2.30 nmol/L, SD±0.92), and RY_T2 (mean 2.36 nmol/L, SD±1.19) (ANOVA, F=2.98, df=2, P>0.05). For the training events: first driving showed a significant increase from pre (mean 3.09 nmol/L, SD±1.42) to post (mean 9.73 nmol/L, SD±5.66) (paired t-test, t=-4.493, df=5, P<0.005). First backing showed a significant increase from pre (mean 3.17 nmol/L, SD±1.48) to post (mean 11.09 nmol/L, SD±7.21) (paired t-test, t=-10.48, df=33, P=4.928×10⁻¹²). First gallops showed a significant increase from pre (mean 2.67 nmol/L, SD±0.56) to post (mean 12.69 nmol/L, SD±5.77) (paired t-test, t=-11.22, df=9, P=1.364×10⁻⁶). Post-event ranges were wide: driving 3.88–19.62 nmol/L, backing 2.71–36.18 nmol/L, gallops 4.40–24.33 nmol/L.

**Clinical Implications:** The significant cortisol increases after all three novel training events confirm these milestones are acutely stressful for yearling Thoroughbreds. The wide individual variation in post-event cortisol, particularly after first backing (range 2.71–36.18 nmol/L), suggests that some horses mount a substantially greater HPA axis response than others. This variation is not explained by the new training yard environment itself, as resting cortisol did not differ across locations. The findings support using salivary cortisol as an objective phenotype for stress reactivity, which could enable trainers to identify horses requiring more gradual or tailored introduction to training. Such proactive management may reduce chronic stress, improve welfare, and potentially decrease the >6% of Thoroughbreds that discontinue racing due to 'unsuitable temperament/behaviour'. Limitations include convenience sampling, variation in timing of training events, and the timecourse experiment using only female horses. Nonetheless, the uniform husbandry and single-yard design are strengths that reduce confounding variables.

PICO

PPOPULATION

Yearling Thoroughbred racehorses (n=96, 46 female, 50 male) entering training at a single yard

IINTERVENTION

Exposure to three novel training events: first time being driven with long reins, first time being backed by a jockey, and first time ridden on the gallops

OOUTCOME

Salivary cortisol concentration measured by ELISA

STUDY TYPE

cohort

SPECIALTY

equine medicine

SUMMARISED BY

AI pipeline

FIDELITY CHECK

100% · A

ORIGINAL PAPER

PMC10079128

Variation in salivary cortisol responses in yearling Thoroughbred racehorses during their first year of training

PLOS ONE

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ABSTRACT

Abstract Thoroughbred horses are bred for competitive racing and undergo intense training regimes. The maintenance of physical soundness and desirable behavioural characteristics are critical to the longevity of a racing career. Horses intended for Flat racing generally enter training as yearlings and undergo introductory training prior to exercise conditioning for racing. This period requires rapid adjustment to a novel environment. As a prey animal, a horse’s ‘fight-or-flight’ response is highly adapted, in which a well-understood component of this response, the hypothalamic-pituitary-axis, is activated in response to a stress stimulus, releasing cortisol. In the Thoroughbred, a significant difference in salivary cortisol concentrations between pre- and post-first time ridden (i.e., first backing) by a jockey have previously been identified. Here, to test the hypothesis that salivary cortisol concentrations may be used to objectively detect individual variations in the acute physiological stress response we investigate individual variation in cortisol response to training milestones. Saliva samples were collected from a cohort of n = 96 yearling Flat racehorses, at the same training yard, across three timepoints at rest: before entering the training yard ( n = 66), within three days of entry to the training yard ( n = 67) and following 2–3 weeks in the training yard ( n = 50). Salivary cortisol concentration was measured using an ELISA. There was no significant difference in cortisol concentration (ANOVA, P > 0.05) across the samples collected at timepoints at rest. Samples were also collected before and 30 minutes after exposure to three novel training events: first time long-reined ( n = 6), first time backed by a jockey ( n = 34), and first time ridden on the gallops ( n = 10). Mean salivary cortisol concentration after all three novel training events was significantly higher than prior to the training event (Paired t-test, P <0.005). The ranges of post-event salivary cortisol concentration across all timepoints suggest individual variation in the measured stress response, reflecting individual differences in stress response to the early training environment. This measure may be used as an objective assessment of the stress response of Thoroughbred racehorses during training.

CITATION EXPORT

Amy R Holtby, Beatrice A McGivney, John A Browne et al.. Variation in salivary cortisol responses in yearling Thoroughbred racehorses during their first year of training. PLOS ONE. (2023). https://doi.org/10.1371/journal.pone.0284102

AUTHORS

9 · 5 CENTRES

Amy R Holtby, Beatrice A McGivney, John A Browne, Lisa M Katz, Keith J Murphy, Emmeline W Hill, Chris Rogers, Chris Rogers, Chris Rogers

1UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin, Ireland
2Plusvital Ltd., The Highline, Dun Laoghaire, Co. Dublin, Ireland
3UCD School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland
4UCD Neurotherapeutics Research Group, Conway Institute, UCD School of Biomolecular and Biomedical Science, University College Dublin, Belfield, Dublin, Ireland
5Massey University, NEW ZEALAND