All horses must have access to clean drinking water 24 hours a day. Horses should always be provided with more water than they need so that there is no risk of them not getting enough to drink.
How much water does my horse actually need?
An average 500kg (approximately 15hh) horse drinks around 30-50 litres a day. This amount may be higher in hot weather (because the horse will sweat more and use up water reserves in the body) and if working very hard (again the horse will sweat more). A mare with a foal needs more because the milk that she is making to feed the foal requires water. Horses that are grazing on rapidly growing grass will usually drink less because the grass that they are eating has a high water content compared to more mature grass and hay which is very dry.
How clean does this water need to be?
Clean uncontaminated water is a must for horses. If horses are forced to drink water that is contaminated with dirt, algae or manure/urine they can become sick. In addition to having a delicate digestive system horses are unable to vomit (a valve on the top of the stomach prevents vomiting). Once a horse has ingested food or water it has to pass right through the system – no matter how bad that food or water is (unlike a dog for example which can vomit and therefore quickly get rid of bad food or water).
Why does a horse sometimes refuse to drink?
Horses have a very good sense of smell and taste and will refuse to drink, even to the point of dehydration, if their water supply is polluted, stagnant or sometimes even if the water supply changes suddenly – irrespective of whether the water is clean or not. Observe horses that are new to a property to check that they are drinking enough. When you take your horse out for the day (to a show or for a trail ride for example) be aware that your horse may not drink enough as the water may smell different to the water they are used to, even if it is very clean. Try to take some water from home with you so that your horse has access to familiar water for the day. One way of getting horses to accept unfamiliar water is to flavour the water at home (you can use a little molasses for this) for a few days before travelling and then flavour the new source of water, gradually reducing the flavour until the horse has accepted the new water source.
Why does a horse need so much water?
There are two main reasons why horses need so much water. Horses have a digestive system that requires lots of clean fresh water in order to function properly. Their naturally high fibre diet (grass, hay etc.) requires high levels of water to help keep the fibre moving through the digestive system. If horses have restricted access to water or have only poor quality water and do not drink enough they are at risk of impaction colic (where fibre blocks the digestive system). Colic (in its various forms) can be a very serious condition in horses. Impaction colic needs immediate veterinary attention.
Horses are one of the few animals (including humans) that rely on sweating to cool themselves down. This requires lots of water so when horses are working hard it is especially important that they do not have their access to water restricted or the horse will become dehydrated. It is an outdated myth that you should withhold water from horses after work. If a horse has just completed very fast work (i.e. finished a race) they should be allowed to drink, then be walked for a few minutes, then be allowed to drink again. This procedure should be carried on until the horse has drunk its fill and the heart rate has returned to normal. The sport of endurance racing has proved that to withhold water from horses is dangerous. Horses taking part in endurance races or long trail rides should be allowed to drink throughout the day.
For more information please see: https://www.equiculture.net/responsiblehorsecare