In the winter, when the water can be very cold, some horses drink less. This is a problem because water is one of the most important items a horse needs to be healthy. Horses need quite a bit of water to wash all the food they ingest through the intestinal tract. If horses don't drink enough, they can be subject to a really bad colic or intestinal impaction.
However, every horse is an individual. Young horses, before they have their full set of teeth, are very tender in the mouth. So are many older horses and horses with any kind of dental problem, such as a sore tooth. A horse with sensitive teeth, mouth, or gums might refuse to drink cold water because it's painful. But for many normal, adult horses, cold water tends to be less of a problem. While some older horses are very reluctant to drink cold water at the beginning of every winter, as they get a little farther into the winter, they adjust to colder water.
What's important is that owners watch their horses, no matter how young or old the animals are, to make sure all are drinking enough. The usual way is to check for dehydration. Watch the manure for a covering of mucus; this covering indicates an inadequate water intake. Also, pinch the skin on the shoulder into a little tent between your fingers. The tent should bounce back immediately. A tent that lasts a couple of seconds lasts too long, indicating dehydration. People should do this with their horses during normal times so they have a baseline to know that horse's normal reaction.
There are several ways of encouraging a horse to drink. We found one way to get these younger or older horses through that transition when the weather starts to get cold is to make them thirsty by feeding salt. (You can top-dress salt on some feed right before offering water.)
It's always good to at least take the chill off the water you offer. How warm the water needs to be depends on the individual. Some horses will drink water that's 32.5* Farenheit!
Horses are very suspicious about any change, so if they're used to the water getting cool as the weather cools off and you warm the water up, it's going to be different. That might make them suspicious. So don't warm it up too much.
Any livestock supply house has a variety of devices you can put in the water trough or in individual drinking buckets to warm the water or take the chill off. These devices can include an electric bucket or tank heaters, propane heaters, and so on.
For pastured horses, or for troughs where there's no electrical service, the most important thing in keeping water from freezing is to insulate your water tank. You can get a lot of heat from the ground if the ground is not completely frozen or if the tank is set into the ground. If the tank is suspended in the air, it will freeze quicker, just like a highway freezes faster on a bridge than on the roadway. Even in snowpack country, at the interface of the snowdrift with the ground, the temperature is right at 32* F. Above the snow, temperatures are going to be equal to the ambient air temperature. So, to help keep water warmer and prevent freezing, set the trough or tank low into the ground.
In situations where you have no means mechanically to take the chill off the water or keep it from freezing, you might only be able to offer warm water during certain times of the day. This probably means carrying a bucket of warm water out to your horse. How often you make the trip depends how much water the horse will take in before the water chills or freezes.
Studies show that an idle horse requires a minimum of 10 to 12 gallons of water a day. But each horse is an individual--some horses might need more; others, less. So, again, you have to watch for signs of dehydration. Usually you should offer your horses water at least twice a day, but if a horse doesn't drink enough before the water freezes, then twice a day won't be enough. Still, horses seem to figure it all out pretty quickly. They're creatures of habit and will get into the routine of twice daily watering. If you give them a little bit of grain with salt on it before watering, then they're more likely to take on water right after that.
One thing that you need to keep in mind--as I've already mentioned--is that horses are very suspicious of change. If you use the same bucket, sometimes the water will assume a taste or taint. If they're used to it, they'll do fine. But if you switch buckets and the water tastes strange or different, they might not want to drink as much. Keep your buckets and watering equipment familiar to your horses, especially as you head into and through the winter months.