Sport horses need to fulfill high physical and psychological requirements during training and competition. These as well as certain conditions of modern husbandry may affect their wellbeing. Here we aimed to (1) evaluate effects of demographic and management factors as well as personality traits on stress reactivity of sport horses, (2) investigate if elite sport horses have elevated stress levels compared to amateur sport horses, and (3) assess whether different equestrian disciplines differentially influence equine adrenal cortex responsiveness. For this purpose, we visited 149 healthy elite (n = 94) and amateur (n = 54) sport horses in Switzerland and performed an adrenocorticotropic hormone (ACTH) stimulation test. Additionally, a person who was familiar with the horse completed a questionnaire about demographic and management factors and their horses’ personality traits. Linear models were calculated to assess associations between the questionnaire data and salivary cortisol 60 (T60) and 90 (T90) minutes after ACTH stimulation. While the model at T90 was not significant, post-stimulatory cortisol after 60 minutes appears most informative in line with a previous study and was significantly affected by the breed and by three management factors: “number of riders”, “hours spent outside” and “group housing” (adjusted r2 = 15%, p<0.001). Thoroughbred and Warmblood horses displayed an increased adrenal response compared to Franches-Montagnes horses. Horses with several riders had a less pronounced reaction than horses with one rider, and horses that spent more time outside had a decreased response compared to horses that were stabled most of the time. Horses living in groups showed higher post-stimulatory cortisol values than horses that were housed singly. However, no significant associations of cortisol responsiveness with personality traits were found, and neither the use as elite or as amateur sport horses nor the discipline had an effect on the cortisol response. This suggests that optimizing husbandry conditions may be more important for improving equine welfare than changing their use.

18Sport horses need to fulfill high physical and psychological requirements during training 19 and competition. These as well as certain conditions of modern husbandry may affect their 20 wellbeing. Here we aimed to (1) evaluate effects of demographic and management factors 21 as well as personality traits on stress reactivity of sport horses, (2) investigate if elite sport 22 horses have elevated stress levels compared to amateur sport horses, and (3) assess whether 23 different equestrian disciplines differentially influence horses' adrenal cortex 24 responsiveness. For this purpose, we visited 149 healthy elite (n=94) and amateur (n=54) 42 their use. 43 44 50 competition purposes and training [4, 5]. However, modern keeping conditions often bear 51 little resemblance to the horse's natural habitat and social structure and in many cases 52 allow only limited natural foraging behavior [1]. Sport horses, especially, are now mostly 53 housed in single stalls [6] and their time on pasture and the amount of roughage fed is 54 often restricted [7]. In equestrian sports, horses often complete high intensity trainings with 55 different trainers and riders, go to national and international horse shows, and are thus 56 often required to live in different stables and to travel long distances in trailers and 57 airplanes. As a consequence, modern sport horses are significantly challenged [8]. The 58 high requirements that they are expected to fulfill, as well as the conditions of modern 59 husbandry, may lead to significant stress, both acute and chronic. 60Stress is the response of the body to a demand (stressor) placed upon it [9]. Whenever a 61 stimulus is perceived as a stressor, it results in a biological response in an attempt to cope 62 with the situation [10]. This biological reaction consists of the behavioral response, the 63 sympathetic-adrenal medulla (SAM) axis response and the hypothalamic-pituitary-adrenal 64 cortex (HPA) axis response, which are often activated concurrently and interact during 65 stressful situations [11]. Studies investigating acute or chronic stress in horses have 66 assessed either one or several of these pathways [12-14]. SAM axis parameters, such as 67 evaluation of heart rate or heart rate variability and endocrinological parameters such as 68 cortisol levels are typically used as measures of acute stress [10]. HPA axis stimulation 69 tests such as adrenocorticotropic hormone (ACTH) or corticotropin releasing factor (CRF) 70 challenge tests are described as potential tools for the assessment of long-lasting effects of 71 stress in horses [10]. Stimulated cortisol may give an impression of an animal's overall 72 adrenal capacity, with both a depression [15-17] or an exaggerated response to stimulation 73 [18-21] reflecting a long-term stress-related alteration of the HPA axis.74Several management factors as well as personality traits have been investigated in relation 75 to stress responsivity in horses. For instance, a lack of social contact was linked to changes 4 76 in stress...