Despite different breeds, age and sex the three horses reacted similarly to the exercise challenge in the cold. The results illustrate how the cool environment first mobilized cool defence mechanisms and how the intensity, duration and frequency of exercise made the regulatory mechanisms to switch to heat dissipation.
The muscles demand increased blood flow during exercise, which is achieved by both a rise in heart rate and stroke volume. HR was measured and it increased during each exercise session with no difference between horses. It indicates that the intensity of exercise was similar, which made comparison between the responses of the horses to treatments meaningful.
In resting horses limb surface temperatures vary in direct proportion to changes in ambient temperature between 5 to 25°C [13
]. In cool conditions the autonomic nervous system directs blood away from the skin surface on extremities to internal organs and skin temperature falls. Peripheral vasoconstriction and closing of arterio-venous shunts are mediated by the sympathetic noradrenergic vasoconstrictor nerves and represents the first defense reaction during exposure to a cool environment [3
In the present study the first exercise session consisted of walking and trotting and skin temperature in the legs dropped in clipped horses. The reaction could have been initiated by a reflex vasoconstriction in the skin vessels at the onset of exercise [15
], but in this study the drop in skin temperature was observed after about 20 minutes of exercise. It appears that the effect was due to a combination of the low environmental temperature and the low intensity of exercise so that the muscles did not produce enough heat to raise core temperature. Therefore, heat dissipation mechanisms were not mobilized, and more blood could go to the muscles instead of to the skin for vasodilatation. However, as intensity of each exercise session increased the heat produced by the working muscles overcame the influence from the cool environment and skin temperature in the legs increased above basal level during recovery in all treatments.
There is some controversy whether a combination of exercise and environmental heat results in a competition between the exercising skeletal muscle and the skin compartments for the available blood flow [15
]. In the present study blood flow to the skin was markedly different on the neck and legs as judged from skin temperature. The low skin temperature in the legs could partly have been due to competition from the leg muscles which worked much more than neck muscles.
As the horses continued to exercise, the muscle work created more internal body heat, which elicited reflex neurogenic vasodilatation in the skin resulting in increased skin temperatures in legs as well as in the neck. In all treatments skin temperatures increased at the end of exercise, even in clipped horses, showing the efficient heat production of exercising muscles.
Unclipped horses had a wet hair coat after exercise, but no moisture could be observed in clipped horses. It is possible that sweat rapidly evaporated from the naked skin thereby efficiently preventing overheating due to exercise. It could explain the unchanged rectal temperature and lower respiratory frequency in clipped horses. However, sweat rate was not measured and it can therefore not be excluded that non-evaporative heat dissipation in the cool ambient temperature was sufficient to prevent an increased rectal temperature. Whichever the explanation, the horses seemed to benefit from clipping since they did not need to mobilize heat dissipating mechanisms.
The results show that the physiological thermoregulatory processes were sufficient to maintain basal body temperature in clipped horses during exercise in the cold and when the horse was stabled afterwards at a temperature near zero. Horses with winter coat showed small tendencies to peripheral vasoconstriction. Instead skin temperatures increased soon after onset of exercise and also rectal temperature increased. Similar results were found before and after clipping horses during exercise and recovery in a climatic chamber, although those experiments were performed at an ambient temperatures above zero [9
No ill effects were noticed in these horses during or after exercising in the cold, which was expected based on the thorough investigations of reactions of horses exercising at – 25°C [ 11].
The number of horses was few. However, they had been kept by the same management and routines for a long time. The horses did the exercises in the same order. Weather conditions could change during the day, but no dramatic changes occurred in the present study. It cannot be taken for granted that the thermistor was not affected by the wind outdoors.