How delightful to have yet another spell of warm sunny weather during which to enjoy our horses. Such a hot dry summer has put extra demands on ourselves and our horses in terms of thermoregulation, however, as our bodies work to keep cool as we exercise.
Exercising muscles produce heat, which must be dissipated to avoid the horse or our body temperature increasing to a dangerous level. Sweat is produced for this purpose, and it transfers heat energy out of the muscles as it evaporates. Horses have a larger muscle mass in proportion to their total body weight compared to us, and therefore they need to sweat more to keep cool during exercise. If a horse could not sweat efficiently during exercise, his internal temperature would quickly rise to a dangerous level.
Sweating results in a loss of body water, which is described as dehydration. Dehydration has a detrimental effect on performance and a loss of just 2% of body water – about 10 kg from a 500 kg horse – will have an effect. Exercising horses can lose large amounts of body water in sweat, particularly in hot weather, and in hard work can lose 15 litres per hour. Dehydration also increases the risk of impactive colic.
Sweat contains body salts (electrolytes) and some protein (latherin, which helps spread the sweat) as well as water, and is more concentrated than body fluids, unlike for humans, whose sweat is less concentrated than body fluids. Horses do not dilute their sweat after hours of exercise unlike humans, so they can lose very large amounts of essential electrolytes during extended exercise.
The lost water and electrolytes need to be replaced if the horse is to remain healthy and able to perform well, and this is particularly important during UK summer months. An increase in environmental temperature from 13 to 21°C increases the horse’s water requirements by 15-20%. Working horses should have free access to clean fresh water, and should have electrolytes added to their daily diet. Electrolytes can be added to drinking water, but only to help rehydrate a dehydrated horse after exercise. You cannot feed enough electrolytes to hard working horses by mixing them into water, so you should mix them into the feed.
Water troughs in fields should be regularly thoroughly cleaned especially during summer. Water buckets in stables should be replaced daily to avoid tainting. Anything that limits a horse’s intake of water could affect his health and performance.
You can check your horse’s hydration level by taking a pinch of skin on his neck, letting it go and then timing how long it takes to return to normal. In fully hydrated horses, this is instant, and if it takes more than 1 second, this indicates a dehydration of 6% or greater. You can also check their ‘capillary refill’ time, by pressing your thumb onto their gums and watching the colour return. In a well hydrated horse, this will take about 1.5 to 2 seconds.
Make sure your working horses has access to fresh clean water and has enough electrolytes added to his diet to ensure optimal hydration after losses in sweat.