Hey – here’s an FYI for you for that saying … “you can lead a horse to water but you can’t make him drink”
Your horse’s body gets the signal that it is thirsty when one of two things happens: either the blood volume drops, or the sodium concentration in the extracellular fluid increases – that is, it gets concentrated, or salty. Horses get the thirst signal more slowly than humans. This is because in humans, Na+ is less concentrated in sweat, so sodium becomes more concentrated in the blood, and the thirst signal goes out quickly. In horses, the sweat glands are very poor at conserving Na+ so even though the horse has lost a large amount of water and Na+ , the signal does not go out for the horse to drink until a serious drop in blood volume occurs due to dehydration. Consequently, even though your horse is clearly dehydrated, when you lead him to water you can’t make him drink! He’s not stubborn – His body is just not giving him an early enough warning signal.
Extract from TUFT School of Veterinary Medicine