Regina Erber

Cortisol release, heart rate and heart rate variability in the horse and its rider: Different responses to training and performance

Although some information exists on the stress response of horses in equestrian sports, the horse-rider team is much less well understood. In this study, salivary cortisol concentrations, heart rate (HR) and heart rate variability (HRV), SDRR (standard deviation of beat-to-beat interval) and RMSSD (root mean square of successive beat-to-beat intervals) were analysed in horses and their riders (n=6 each) at a public performance and an identical rehearsal that was not open to the public. Cortisol concentrations increased in both horses and riders (P<0.001) but did not differ between performance and rehearsal. HR in horses and riders increased during the rehearsal and the public performance (P<0.001) but the increase in HR was more pronounced (P<0.01) in riders than in their horses during the public performance (from 91 ± 10 to 150 ± 15 beats/min) compared to the rehearsal (from 94 ± 10 to 118 ± 12 beats/min). The SDRR decreased significantly during the equestrian tasks in riders (P<0.001), but not in their horses. The RMSSD decreased in horses and riders (P<0.001) during rehearsal and performance, indicating a decrease in parasympathetic tone. The decrease in RMSSD in the riders was more pronounced (P<0.05) during the performance (from 32.6 ± 6.6 to 3.8 ± 0.3 ms) than during the rehearsal (from 27.5 ± 4.2 to 6.6 ± 0.6 ms). The study has shown that the presence of spectators caused more pronounced changes in cardiac activity in the riders than it did in their horses.

Physiological and behavioural responses of young horses to hot iron branding and microchip implantation.

Branding is the traditional and well-established method used to mark horses, but recently microchip transponders for implantation have become available. In this study, behaviour, physiological stress variables and skin temperature in foals were determined in response to hot-iron branding (n=7) and microchip implantation (n=7). Salivary cortisol concentrations increased in response to branding (1.8 ± 0.2 ng/mL) and microchip implantation (1.4 ± 0.1ng/mL), but cortisol release over time did not differ. In response to both manipulations there was a transient increase in heart rate (P<0.001) and heart rate variability (P<0.01). Branding and microchip implantation induced a comparable aversive behaviour (branding, score 3.86 ± 0.85; microchip, score 4.00 ± 0.82). Both techniques thus caused similar physiological and behavioural changes indicative of stress. Acutely, implantation of a microchip was as stressful as branding in foals. Branding caused a necrotising skin burn lasting at least 7 days. Moreover branding, but not microchip implantation (P<0.001), was accompanied by a generalized increase in skin temperature which was comparable to low degree post-burn hypermetabolism in humans.

Cortisol and progestin release, heart rate and heart rate variability in the pregnant and postpartum mare, fetus and newborn foal

The mechanisms leading to parturition in the horse in many aspects differ from those in other species. Pregnancy is maintained not by progesterone but by 5α-pregnanes and the progestin precursor pregnenolone originates from the fetus. As parturition approaches, the fetal adrenal switches from pregnenolone to cortisol synthesis but it is not known whether cortisol crosses the placenta. We hypothesized that in parallel to fetal cortisol release, cortisol in the maternal circulation increases before foaling and this increase can be determined in both saliva and plasma. In addition, maternal, fetal and neonatal heart rate and heart rate variability were measured. In 25 pregnant mares, saliva for cortisol analysis was collected 4 times daily from 15 days before to 5 days after foaling. In 13 mares, in addition, fetomaternal electrocardiogram (ECG) recordings were made and blood samples for progestin and cortisol analysis were collected once daily. Heart rate (HR) was recorded until 5 days after foaling. The heart rate variability (HRV) variables standard deviation of the beat-to-beat (RR) interval (SDRR) and root mean square of successive RR differences (RMSSD) were calculated. From Days 15 to 4 before parturition, progestin concentration increased (peak 267 ± 42 ng/mL) and decreased thereafter (P < 0.05, day of foaling 113 ± 18 ng/mL). A prepartum increase in maternal cortisol concentrations was evident in blood (P < 0.05) and saliva (P < 0.05) and paralleled the decrease in progestin concentrations. In mares, HR remained constant during the last days of pregnancy but decreased within one day after parturition (P < 0.05) while maternal HRV did not change. In the fetus and neonate, HR increased from before to after birth (P < 0.05) indicating increasing demands on the cardiovascular system with adaptation to extrauterine life.

Behavioral and physiological responses of young horses to different weaning protocols: a pilot study.

In this study, effects of weaning on behavioral and physiological stress parameters in young horses (foals) were determined. Foals were weaned either simultaneously without the presence of adult horses (group A, n = 6), or in the presence of two adult females familiar but unrelated to the foals (group B, n = 5), or weaned consecutively by removing two mother horses per day (group C, n = 6). Behavior, locomotion, salivary cortisol concentration, beat-to-beat (RR) interval, heart rate variability (HRV) and weight were determined. Group A foals lost weight for 2 days (mean ± SEM) − 8.3 ± 1.6 kg, p < 0.05. Weaning was followed by increased vocalization which was least pronounced in foals of group B (p < 0.05). Locomotion was most pronounced on weaning day in foals of group A and lowest in group B (p < 0.05). Weaning increased salivary cortisol concentration on the day of weaning in groups A and B and for 2 days in group C (p < 0.05). The RR interval decreased most pronouncedly in group A foals (p < 0.05). There were no consistent changes in HRV. Based on cortisol release and behavior, weaning is associated with stress but this was least pronounced in foals weaned in the presence of two familiar but unrelated adult female horses.

Effects of season, age, sex, and housing on salivary cortisol concentrations in horses

Analysis of salivary cortisol is increasingly used to assess stress responses in horses. Because spontaneous or experimentally induced increases in cortisol concentrations are often relatively small for stress studies, proper controls are needed. This requires an understanding of the factors affecting salivary cortisol over longer times. In this study, we have analyzed salivary cortisol concentration for 6 mo in horses (n = 94) differing in age, sex, reproductive state, and housing. Salivary cortisol followed a diurnal rhythm with the highest concentrations in the morning and a decrease throughout the day (P < 0.001). This rhythm was disrupted in individual groups on individual days; however, alterations remained within the range of diurnal changes. Comparison between months showed highest cortisol concentrations in December (P < 0.001). Cortisol concentrations increased in breeding stallions during the breeding season (P < 0.001). No differences in salivary cortisol concentrations between nonpregnant mares with and without a corpus luteum existed. In stallions, mean daily salivary cortisol and plasma testosterone concentrations were weakly correlated (r = 0.251, P < 0.01). No differences in salivary cortisol between female and male young horses and no consistent differences between horses of different age existed. Group housing and individual stabling did not affect salivary cortisol. In conclusion, salivary cortisol concentrations in horses follow a diurnal rhythm and are increased in active breeding sires. Time of the day and reproductive state of the horses are thus important for experiments that include analysis of cortisol in saliva.

Parturition in horses is dominated by parasympathetic activity of the autonomous nervous system.

External and internal stressors prolong parturition in different species. At parturition, sympathoadrenal activation should be avoided because an increased sympathetic tone may cause uterine atonia via β2-receptors. We hypothesized that at physiological parturition, horses are under parasympathetic dominance, and stress-response mechanisms are not activated during delivery of the foal. To evaluate stress responses, heart rate, heart rate variability, catecholamines, and cortisol were analyzed in mares (n = 17) throughout foaling. Heart rate decreased from 2 hours before (51 ± 1 beats/minute) to 2 hours after delivery (41 ± 2 beats/minute; P < 0.05). Heart rate variability variables, standard deviation of the beat-to-beat interval, and root mean square of successive beat-to-beat differences, changed over time (P < 0.05) with the highest values within 15 minutes after delivery. The number of mares with atrioventricular blocks and the number of atrioventricular blocks per mare increased over time (P < 0.01) and were significantly elevated from 15 minutes before to 45 minutes after birth of the foal. Salivary cortisol concentrations increased to a maximum at 30 minutes after delivery (25.0 ± 3.4 ng/mL; P < 0.01). Plasma epinephrine and norepinephrine concentrations showed significant fluctuations from rupture of the allantochorion to expulsion of the fetal membranes (P < 0.01) but were not markedly elevated at any time. In conclusion, mares give birth under high parasympathetic tone. Cortisol release during and after foaling is most likely part of the endocrine pathways regulating parturition and not a labor-associated stress response.

Effects of the level of experience of horses and their riders on cortisol release, heart rate and heart-rate variability during a jumping course

Equestrian sports require the co-operation of two species, horses and humans, but it is unknown to what extent stress responses in the rider affect the horse. In this study, the stress response of experienced and less-experienced horses and riders at showjumping was analysed. Sixteen sport horses were divided into two groups (n = 8 each) by experience and were ridden by highly experienced professionals (n = 8) and less-experienced riders (n = 8). Riders jumped a course of obstacles with an experienced and a less-experienced horse and horses took part with an experienced and less-experienced rider. Salivary cortisol, heart rate and heart-rate variability (HRV) variables, standard deviation of RR interval (SDRR) and root mean square of successive RR differences (RMSSD) were analysed. Cortisol and heart rate increased and HRV decreased in all riders and horses. In less-experienced riders, cortisol release was higher on a lessexperienced versus an experienced horse but the horses’ cortisol release was not affected by experience of their riders. Heart rate did not differ between groups of horses and was not affected by experience of the rider but was higher in less-experienced versus experienced riders. The HRV decreased in horses and riders and SDRR was lower in less-experienced versus experienced riders. Thus, lower experience of riders appears not to affect physiological stress parameters in their horses during a showjumping course.

Influence of semen collection on salivary cortisol release, heart rate, and heart rate variability in stallions

The aim of this study was to evaluate the stress response of stallions (n = 16) aged 3-13 years with previous sexual experience to semen collection by determination of heart rate, heart rate variability, and cortisol in saliva. Recordings were done on two consecutive days. The time intervals from leaving the box until arrival in the collection barn and from first exposure to the teaser mare until ejaculation as well as the number of mounts until ejaculation were neither affected by collection day nor by age, sexual experience (i.e., the number of breeding seasons the stallion experienced), or sexual workload of the stallion (i.e., the mean number of semen collections per week). Heart rate was continuously determined from 30 minutes before to 30 minutes after ejaculation and significantly increased in response to the semen collection procedure (P < 0.001). Changes in heart rate were significantly influenced by sexual experience (P < 0.01) and sexual workload (P < 0.05) but not by the age of the stallions. Day of semen collection did not have any effects. The heart rate variability variable root mean square of successive RR differences was not affected by semen collection procedures. Cortisol concentration in saliva was determined from 60 minutes before to 120 minutes after ejaculation and did not change significantly. The results indicate that semen collection is perceived as not more than a modest temporary stressor in sexually experienced and well-trained stallions.

Physiological and behavioral stress parameters in calves in response to partial scrotal resection, orchidectomy, and Burdizzo castration

Establishing artificial cryptorchids by partial scrotal resection without removing the testicles is a technique for castration of bull calves that recently has gained new interest. In contrast to orchidectomy and Burdizzo castration, the stress response of calves to shortening of the scrotum is unknown. In this study, partial scrotal resection in bull calves was compared with orchidectomy, Burdizzo castration, and controls without intervention (n=10 per group, ages 56 ± 3 d). Procedures were performed under xylazine sedation and local anesthesia. We hypothesized that partial scrotal resection is least stressful. Salivary cortisol, heart rate, heart rate variability, behavior, and locomotion were analyzed. Cortisol concentration peaked 60 min after start of the procedures. Cortisol release was at least in part xylazine induced and none of the experimental procedures released additional cortisol. Heart rate increased in calves of all groups with initial handling, but immediately after xylazine sedation decreased to 30% below initial values and was not modified by surgical procedures. The heart rate variability variables standard deviation of beat-to-beat interval and root mean square of successive beat-to-beat differences increased when calves were placed on the surgery table but effects were similar in calves submitted to surgeries and control calves. Locomotion increased, whereas lying time decreased in response to all surgeries. Locomotion increase was most pronounced after orchidectomy. Plasma fibrinogen concentrations increased after orchidectomy only. With adequate pain medication, orchidectomy, Burdizzo castration, and partial scrotal resection do not differ with regard to acute stress and, by inference, pain. Partial scrotal resection when carried out under xylazine sedation and local anesthesia thus is an acceptable castration technique in bull calves.

Increased cortisol release and transport stress do not influence semen quality and testosterone release in pony stallions

The use of breeding stallions for equestrian competitions requires that fertility is not negatively affected by competition or transport to the competition site. In this study, effects of cortisol release induced by road transport (600 km), adrenocorticotropic hormone (ACTH) administration (3 × 0.5 mg synthetic ACTH) and placebo treatment on semen quality and testosterone release were investigated in Shetland stallions (N = 13) using a crossover design. Saliva for cortisol and blood for testosterone analysis were collected for 10 weeks after treatments. Semen was collected daily for 5 days directly after treatments and twice weekly for another 9 weeks. Total sperm count, sperm morphology, motility, and membrane integrity were analyzed. We hypothesized that elevated cortisol decreases testosterone concentration and semen quality. Cortisol concentrations increased in response to transport and ACTH (P < 0.001) but not control treatments (peak concentration, transport: 7.6 ± 2.4, ACTH: 13.7 ± 1.5, control: 3.8 ± 0.9 ng/mL). No treatment effects on testosterone existed. Total sperm count decreased with daily semen collections in week 1 (P < 0.01) but did not differ between the treatments. The percentage of motile, progressively motile, membrane-intact, and morphologically defective spermatozoa did not change over time from Days 2 to 6, and there existed no differences between the treatments. In conclusion, road transport evoked a stress response which was mimicked by ACTH treatment. Both treatments had no effect on testosterone release and semen quality. Testicular function in stallions is apparently well protected against transiently elevated cortisol concentrations, and stallions can be transported over longer distances without negatively affecting their fertility.