Summary To investigate whether horses were able to acclimate to conditions of high temperature and humidity, 5 horses of different breeds were trained for 80 min on 15 consecutive days on a treadmill at 30°C and 80% RH. Training consisted of a combination of long duration low‐intensity exercise, medium duration medium intensity exercise and short duration high intensity exercise. Between training sessions the horses were maintained at 11 ± 3°C and 74 ± 2% RH. Before (PRE‐ACC) and after acclimation (POST‐ACC) the horses undertook a simulated Competition Exercise Test (CET), designed to represent the Speed and Endurance Test of a 3‐day event, at 30°C/80% RH. Maximal oxygen uptake (VO2PEAK) was not changed following acclimation (PRE‐ACC 141 ± 8 ml/min/kg bwt vs. POST‐ACC 145 ± 9 ml/min/kg bwt [STPD], P>0.05). Following acclimation, 4 of the 5 horses were able to complete a significantly greater amount of Phase D in the CET (PRE‐ACC 6.3 ± 0.3 min vs. POST‐ACC 7.3 ± 0.3 min, P<0.05; target time = 8 min). Resting body temperatures (pulmonary artery [TPA], rectal [TREC] and tail‐skin [TTSK] temperatures) were all significantly lower following acclimation. During exercise, metabolic heat production (M) and heat dissipation (HD), for the same exercise duration, were both significantly lower following acclimation (P<0.05), although heat storage (HS) was significantly higher (P<0.05). The higher heat storage following acclimation was associated with a lower TTSK for a given TPA and a decreased total fluid loss (% bodyweight, P<0.05). Plasma volume was not changed following acclimation. The relationship of sweating rate (SR) to TPA or TTSK on either the neck or the gluteal region was not significantly altered by acclimation, although the onset of sweating occurred at a lower TPA or TTSK following acclimation (P<0.05). The horses in the present study showed a number of physiological adaptations to a period of 15 days of exposure to high heat and humidity consistent with a humid heat acclimation response. These changes were mostly similar to those reported to occur in man and other species and were consistent with thermal acclimation and an increased thermotolerance, leading to an improved exercise tolerance. It is concluded that a 15 day period of acclimation is beneficial for horses from cooler and or drier climates, that have to compete in hot humid conditions and that this may redress, to some extent, the decrement in exercise tolerance seen in nonacclimated horses and reduce the risk of heat related disorders, such as heat exhaustion.

Summary Whereas the efficacy of cold water cooling of horses has been demonstrated by several studies, the dynamics of temperature changes within and between compartments (primarily muscle, blood [core], skin and deep core [rectal]) have not been investigated. Changes in body temperature associated with cold water cooling were investigated in the hyperthermic horse. Muscle (TMU), pulmonary artery (TPA), rectal (TREC), tail‐skin (TTSK) and coat surface (TCOAT) temperatures, were monitored continuously in 5 Thoroughbred horses during and after exercise in hot humid (30°C and 80% RH) conditions on a treadmill. Horses were cooled in the hot humid environment with cold water (∼6°C) for 6 30 s periods. Between each 30 s cooling period the horses stood for 30 s. A total of 180 1 of cold water was applied. Horses were monitored for a further 4 min following the final cooling period. From the end of exercise to the end of the final cooling (6.5 min), mean (± s.e.) rates of decrease for TTSK and TPA were similar (0.8 ± 0.1 and 0.8 ± 0.1°C/min, respectively). The effects on TMU and TREC were less marked, with average rates of 0.2 ± 0.1 and 0.0 ± 0.1°C/min, respectively. During the first 4 min of cooling, TPA fell during the 30 s period of water application and rose during each 30 s period of standing. When TPA fell below ∼36.5°C, these variations were suppressed and TPA rose steadily, despite continued applications; TREC and TMU continued to fall, although less rapidly than before. These observations are consistent with the onset of skin vasoconstriction at low TPA. The mechanism is mediated through a cooling of circulating blood volume providing a greater capacity for heat transfer between muscle and circulation. Intermittent application of cold water (∼6°C) improves heat removal without apparent deleterious effects and is well tolerated. Even when hypothermia develops (based on TPA), muscle and rectal temperatures continue to fall.

Summary The aim of the present study was to determine the effect of different environmental conditions on physiological response to exercise. Four winter acclimatised, nonheat acclimated horses of different breeds were exercised at 20°C/40%RH (CD), 30°C/40%RH (HD) and 30°C/80%RH (HH). The exercise test was designed to represent the structure and intensity of a One star Speed and Endurance test (competition exercise test [CET]). All 4 horses were able to complete the full CET (60 min + 30 min active recovery) in CD and HD, but only one horse completed the CET in HH. Two horses were stopped because of pronounced general fatigue and one because of a right atrial temperature (TRA) of 43°C. Oxygen uptake on each phase was not different between CD and HD, but was higher during Phases B, C and D in HH. Mean peak TRA at the end of Phase D was 40.3 ± 0.2, 41.6 ± 0.4 and 42 ± 0.3°C for CD, HD and HH, respectively. Corresponding, mean peak rectal temperatures (TREC) following Phase D were 39.5 ± 0.1, 40.6 ± 0.1 and 41.5 ± 0.1°C for CD, HD and HH, respectively. Mean time to peak TREC was 9.3 ± 1.1 (CD), 7.3 ± 1.8 (HD) and 10.8 ± 2.3 (HH) min and was not significantly different between conditions (P>0.05). Heat dissipation amounted to 83 ± 1, 73 ± 2 and 70 ± 1% of heat production in CD, HD and HH, respectively. Weight loss was significantly correlated with both body surface area (CD r = 0.85; HD r = 0.87; HH r = 0.81) and bodyweight (CD r = 0.97; HD r = 0.93; HH r = 0.94). The greatest weight loss recorded was 4.6% bodyweight in one horse in HD. The mean increase in exercise intensity over the whole CET (in terms of V̇O2) of HD and HH compared with CD was 5 ± 3 and 14 ± 3% higher, respectively. The exercise induced hyperthermia and the reduced capacity for heat dissipation produced partial compensatory responses in minute ventilation (V̇E), particularly during Phase C, when the horses were trotting. In HD, the increase in V̇E was achieved mainly through an increase in frequency, whilst in HH it was achieved through an increase in tidal volume (VT). The horses demonstrated a high degree of tolerance to environmental heat load, suggesting a high thermoregulatory capacity. However, for unacclimatised animals exercising in severely hot and humid conditions, performance may be limited.

Summary The present study was undertaken to investigate physiological, metabolic, haematological and biochemical changes in horses competing in the Speed and Endurance test of a Concours Complet International (CCI) **** 3‐day‐event held under FEI rules. A total of 28 horses competing in the Burghley Horse Trials Speed and Endurance test were selected to be monitored: 11 horses in 1993 and 17 horses in 1994. Of the 28 horses selected, 17 completed the Speed and Endurance test and went on to complete the showjumping test. Mean ± s.d. shade temperature and relative humidity, black globe temperature and wind speed were 13 ± 1 and 20 ± 2°C, 54 ± 3 and 55 ± 10%, 17 ± 2 and 29 ± 4°C and 2.7 ± 0.7 and 1.2 ± 0.3 m/s, for 1993 and 1994, respectively. Mean heart rate during Phases A, B and D was not significantly different between years, but mean heart rate during Phases C and X was significantly higher in 1994. Mean (± s.d.) heart rate on Phases B and D for all horses in both 1993 and 1994 was 198 ± 8 and 188 ± 11 beats/min, respectively. Mean heart rate during Phase D showed a poor correlation with mean speed (r=0.412). Total mean (± s.d.) weight loss from the start of Phase A to the end of Phase D was 15.5 ± 6.1 kg in 1993 and 16.5 ± 5 kg in 1994 and did not differ significantly between years. Following 14–18 h completion of Phase D, mean bodyweight was not significantly different from that at the start of Phase A in either year. Mean rectal temperature at the end of Phase D was 41 ± 0.6°C and 41.1 ± 0.6°C in 1993 and 1994, respectively (P>0.05). Both the lowest (39.7°C) and highest (41.8°C) rectal temperatures were recorded at the end of Phase D in 1994. Plasma lactate concentrations at the end of Phase D were 8.5–38.5 mmol/1. The highest lactate concentration also coincided with the highest plasma glucose concentration (11.4 mmol/1) as well as the joint fastest time in either year, although overall lactate showed only weak correlations with mean speed on Phase D (r=0.12, 1993; r=0.58, 1994). While the Speed and Endurance test at CCI**** level run in a temperate climate presents a considerable challenge to the fitness and ability of the horses competing, the metabolic and physiological changes are not extreme. The majority of horses that finish the test appear to undergo a rapid and considerable degree of recovery and are able to present sound at the final inspection, take part in the showjumping test and complete the competition.

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