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

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.

The Welfare of Horses Competing in Three-Barrel Race Events Is Shown to Be Not Inhibited by Short Intervals between Starts

“…Acute stress can lead to an increase in various blood biomarkers, which act as protectors against changes in homeostasis during exercise [7]. However, good physical conditioning can reduce the stress load and promote adaptive metabolic processes associated with exercise.…”

“…However, good physical conditioning can reduce the stress load and promote adaptive metabolic processes associated with exercise. This can also be reflected in the absence of variation in these biomarkers [7,18,39,40]. Studies have shown that cortisol, IL-6, and IL-1β remain stable during training, leading to improved performance in different equestrian sports, including classical and fieldwork on farms [4,12,20,39,41].…”

“…It is important to note that the temperature in the eye caruncle may be influenced by the regulatory mechanisms of the hypothalamus-pituitary-adrenal axis [22,25,47], which could indicate a loss of well-being and chronic stress if the temperature remains elevated for a prolonged period [7,11,25,30,44]. In contrast, a lack of, or a temporary change in, local temperature due to physical effort has been associated with good physical conditioning [25].…”

“…Findings also considered the level of experience of the horses and the riders and the pre-competition conditions of the horses (training, rest periods, diet, etc. ), including the numerous causes of poor performance and gut microbiota composition [1][2][3][4][5][6][7][8][9][10]. These different studies expand general knowledge about the impacts of physical exercises that occur in training and competitions and can be used in the evaluation of the welfare of horses used in different equestrian disciplines, whether classical (racing, jumping, etc.)…”

Hair Cortisol Concentrations in Feral Horses and the Influence of Physiological and Social Factors

“…While many candidate mammal species may serve as models to understand the relative influence of physiological and social factors on cortisol production, horses ( Equus ferus caballus ) present a superior model for research given our deep understanding of both equine biology and sociality. There is a growing body of work originating from domestically kept horses on equine HPA responses to various physiological and social stressors using HCC, including husbandry [ 38 , 39 , 40 , 41 ], injury and illness [ 40 , 42 , 43 ], strenuous exercise [ 41 , 44 ], novel experiences [ 45 ], and seasonality [ 26 , 38 ]. The reproductive state of domestically kept female horses has shown both negligible correlation to cortisol levels [ 46 ] and also peak releases of cortisol in late pregnancy [ 47 ] and prepartum [ 48 , 49 ].…”