Continuous Monitoring of the Thermoregulatory Response in Endurance Horses and Trotter Horses During Field Exercise: Baselining for Future Hot Weather Studies

Abstract

Establishing proper policies regarding the recognition and prevention of equine heat stress becomes increasingly important, especially in the face of global warming. To assist this, a detailed view of the variability of equine thermoregulation during field exercise and recovery is essential. 13 endurance horses and 12 trotter horses were equipped with continuous monitoring devices [gastrointestinal (GI) pill, heartrate (HR) monitor, and global positioning system] and monitored under cool weather conditions during four endurance rides over a total of 80 km (40 km loops) and intense trotter track-based exercise over 1,540 m. Recordings included GI temperature (T _c ), speed, HR and pre- and post-exercise blood values. A temperature time profile curve of T _c was constructed, and a net area under the curve was calculated using the trapezoidal method. Metabolic heat production and oxygen cost of transport were also calculated in endurance horses. Maximum T _c was compared using an independent samples t-test. Endurance horses (mean speed 14.1 ± 1.7 km h^-1) reached mean maximum T _c (39.0 ± 0.4°C; 2 × 40 km in 8 horses) during exercise at 75% of completion of T _c exercise and T _c returned to baseline within 60 min into recovery. However, the mean T _c was still 38.8 ± 0.4°C at a HR of 60 bpm which currently governs "fit to continue" competition decisions. Trotters (40.0 ± 2.9 km h^-1) reached a comparable mean max T _c (38.8 ± 0.5°C; 12 horses) always during recovery. In 30% of trotters, T _c was still >39°C at the end of recovery (40 ± 32 min). The study shows that horses are individuals and thermoregulation monitoring should reflect this, no matter what type of exercise is performed. Caution is advised when using HR cut-off values to monitor thermal welfare in horses since we have demonstrated how T _c can peak quite some time after finishing exercise. These findings have implications for training and management of performance horses to safeguard equine welfare and to maximize performance.

Keywords: endurance; exercise; gastrointestinal pill; hyperthermia; metabolic heat (H); recovery; thermoregulation; trotters.

FIGURE 1

Flow chart of study design describing two types of exercise and distance; km, kilometers; m, meters; n, number of horses; min, minutes; endurance exercise; moderate trotter warm-up exercise; and fast trotter exercise.

FIGURE 2

Monitoring equipment: A modified Equivital belt for use on horses with the external receiver device (Sensor Electronics Module) located inside of the belt at of the ventral part of the thorax to monitor the Tc by GI pill, while the Garmin GPS watch and the Polar heartrate electrodes were attached at the upper part of the belt.

FIGURE 3

Box and whisker plots demonstrating median (solid line), interquartile range (identified by rectangular area), and minimum to maximum (max) ranges of data (whiskers or error bars) of boxplots of T_c (°C) in horses (H; horse number) in endurance (H1–H8) and trotter (H14–H25) exercise (A). Scatterplots with T_c (°C; left y-axis) over time [duration of exercise in hours (h) and minutes (min)] (x-axis) in trotter horses (H14–25) and its mean (thick black line) ± SD (dotted black line, calculated when n > 3 horses) during high intensity trotter exercise over time (B).

FIGURE 4

Scatterplots of T_c (°C; left y-axis, black line) and cumulated net AUC (T_c°C × min; right y-axis, red line) over time (hours, h) during T_c exercise periods (identified as gray blocks) and recovery in endurance horses H1–8; H1 data loss at end of leg 1 and H1, H2 without T_c recording during the first recovery (identified as spotted blocks); H4, H6 continued to 100 km total.

FIGURE 5

Scatterplots of T_c (°C; left y-axis, black line) and cumulated net AUC (T_c, °C × min; right y-axis, red line) over time (hours, h) during exercise and recovery in trotter horses H14–25, warm-up exercise (moderate intensity, identified as blue blocks), and trotter exercise (high intensity, identified as light-red blocks).

FIGURE 6

Boxplot of the dynamic thermal response presented by net AUC T_c (°C × min) during T_c exercise period of endurance and trotter exercises (presented with mean net AUC on top bar). The 1^st 40 km of endurance exercise was not significantly different to the 2^nd 40 km. Net AUC T_c of exercise and recovery from full speed trotter exercise over 1,540 m was minor during exercise while T_c increased post-exercise as represented by positive net AUC T_c.

FIGURE 7

Bar graph of blood parameters: lactate and pH on the y-axis presented as mmol L^–1 and the number, respectively, and the PCV (Hct) on the right y-axis presented as %. The type of exercise is presented on the x-axis as pre-, and post-ride in endurance and trotter exercises. The mean value is presented on top of the bar while sample size at the bottom in column.