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Feeling the heat?

June 26, 2018

Written by Andrea D. Ellis, PhD, MRSB 

The thermo-neutral zone of an animal (or human) is the environmental temperature range within which they do not need to spend a lot of additional energy in order to regulate body temperature. The thermo-neutral zone of the horse lies between -3°C and 18°C, although depending on breed and condition this may vary slightly. What this tells you, however, is that horses are used to cool climates and cope best in temperate zones of the world.

When temperature rises over 5°C above the upper limit horses start to slightly slow down to avoid over-heating, they become more active at night when it is cooler and seek shade when possible during the day. Our management may not allow this at all times.

If your horses are turned out in a paddock in the blazing sun, you should provide shade, either through planting or through ‘airy’ shelters. Horses may also suffer from sunburn on areas of the body with low hair cover, especially those with pink skin. Rugging up horses against insect bites will prevent some radiation and evaporation cooling, although some conduction of heat from skin to rug can occur. Ask yourself – does my horse really need to be rugged up to the ears in the summer? In the past horses coped without this.

If you cannot provide shelter or shade consider keeping your horses in, during the day and turn out at night. Beware – the most active time for insect bites is dusk and dawn, so perhaps by bringing horses in during that period, you will save them from worst ‘insect’ irritation.
Shelters and stables need to be designed for good ventilation. As hot air rises, there need to be openings on top of the building – ideally in the highest point and there needs to be openings at the bottom/lower down for fresh air to enter – it’s a chimney system. If your stable has no openings in the roof, then consider putting some in, as there is danger of stale, dusty and musty air. This will also improve air quality, even in the winter, as the thermo-neutral zone of the horse is quite low, they do much better in cooler stables. Cooler air also holds less moisture and this prevents airborne pathogens. One way of cooling a very hot stable down is to apply water to the floor – you may want to do this outside the stable, in the corridors. As the water evaporates it cools the air. In hot countries hosing down small yards creates small ‘cooler’ zones and this is often practiced at least twice a day. Of course your horse can also be cooled down through hosing or wet sponging.

Exercising horses in hot weather brings additional physiological stress with it. Apart from sweating profusely, breathing deeply and heavily when you stop exercising is a sign of heat stress. Horses require around 2 weeks of exercise in hot humid conditions before they start to acclimatise. High humidity prevents efficient water/sweat evaporation, so here over heating is more likely. Therefore, it is advised to exercise horses early in the morning, which is the coolest period of the day. Help horses to cool down after exercise by sponging down with water and a fan placed nearby during this process further helps to cool down overheated horses fast. If your horse starts tripping over poles or not picking up its feet during exercise, it may be time to stop and check for dehydration. Shorten duration and/or intensity of the exercise during extremely hot days and if you do have to travel your horse in trailers or horseboxes, ensure good ventilation and plan frequent ‘rehydration’ stops where water is offered.

Signs of heat stress:
⦁ increased respiratory rate
⦁ increased respiratory depth
⦁ low head
⦁ flared nostrils
⦁ apathy (refusal to eat or drink)

Dehydration must be avoided and continuous access to water is always advised. Check for dehydration by pinching the skin on the neck – if the skin does not return to flat immediately, this is called skin ‘tenting’ and is a good indication of dehydration. Horses in heavy work may benefit from an electrolyte supplement as a lot of electrolytes are lost with sweat – we advise consulting a nutritionist if you exercise a lot in hot conditions. If you are worried that your horse may be overheating, taking rectal temperature is one way of checking if they are coping with the thermostatic regulation. The temperature is normal at 37.5-28.5°C and should not go above 38.5°C. If it reaches 39°C, we suggest immediate cooling down with water and possibly a fan, offer ‘room’-temperature (not ice-cold) water and consider calling out the vet if heavy breathing occurs. If horses are in heat stress do not feed a lot of forage shortly after recovery, as the digestion of this will cause more heat, but start with a very sloppy sugar-beet feed or mash (even with water standing on top of the risen beetmash) as this is palatable and will help to re-hydrate horses.

To help avoid dehydration, making sure horses have access to fresh, clean water at all times is of course important. Do not allow ‘soaked’ hays or sugar beet pulp to ferment to avoid colic. Steaming hay is a good way to increase the water content and to help counterbalance dehydration. The amount of water that will be taken up will depend on the water content and maturity of the forage being steamed. Earing et al. (2013) found that steaming increased the water content of an alfalfa-orchard grass mixed hay by nearly 3 times the starting amount.


Bibliography
Geor, R., & Mccutcheon, L. (1998). Hydration effects on physiological strain of horses during exercise-heat stress. Journal of Applied Physiology (Bethesda, Md. : 1985), 84(6), 2042-51.
Geor, R.J., McCutcheon, L.J., Ecker, G.L. and Lindinger, M.I. 2000. Heat storage in horses during submaximal exercise before and after humid heat acclimation. Journal Apply Physiology, 89, 2283 – 2293
Earing, J.E., M.R. Hathaway, C.C. Sheaffer, B.P. Hetchler, L.D. Jacobson, J.C. Paulson, and K.L. Martinson. 2013. The effect of hay steaming on forage nutritive values and dry matter intake by horses. Journal of Animal Science. 91: 5813-5820.
Pritchard, J., Barr, A., & Whay, H. (2006). Validity of a behavioural measure of heat stress and a skin tent test for dehydration in working horses and donkeys. Equine Veterinary Journal, 38(5), 433-438






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