Sweating is a way for animals to lose heat: due to the evaporation of fluids on the skin the body cools down. Compared to humans horses sweat in an inefficient way. The way horses sweat can result in an accumulation of bodily heat that adds up the entire duration the horse is worked. The horse can sweat 6-20L per hour, so dehydration can easily occur.
Heat stress and electrolytes
When humans sweat, they expel water first, after which electrolytes (salts) follow.
The result is that the concentration of salts in the body rises, due to the decline in fluids, making the environment of the body hypertonic (too much salt in the body). This in turn makes us thirsty, making us want to have a drink. Drinking water then ensures the fluid balance of the body will recover its isotonic state, due to the fluids being absorbed through passive osmotic diuresis.
When horses sweat they expel electrolytes/salts first, followed by water. The result is that the concentration of salts in the body lowers down, resulting in an hypotonic environment (more water, less salts in proportion). Horses can lose 6-20L of water per hour, but because of the hypotonic state of the body they do not get thirsty. Therefore it is difficult to make horses drink when they are dehydrated.
There is also no osmotic diuresis when horses do drink just water, as the body thinks it is already containing enough water. When drinking pure water the body will then start expelling the water through the kidneys with electrolytes, making the dehydration state of the body worse. So to rehydrate our horses we need to give them salty water.
Take home message: supplying electrolytes dissolved in water is essential to rehydrate horses. Supplying electrolytes in feed is possible as well, however they will take much longer to take effect as the feedstuff will need to be digested first. In dehydrated horses you would rather have a fast effect of rehydration
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