The average-sized horse needs a minimum of 6 gallons of water per day regardless of the time of year. For these Belgians, that requirement can more than double.

When temperatures get frigid, dehydration is probably the last thing on your mind. After all, that's really only a problem in the warm-weather times of year when your horse sweats heavily, right? Wrong! Your horse can become dehydrated at any time of the year—even in the winter—with some predictable health consequences.

Winter Water Needs
It’s true that sweating in warmer weather increases your horse’s water and electrolyte losses, but horses have baseline requirements for hydration that need to be met all year long.

Your horse loses water from his body in manure and urine, and even in the air he exhales. The very dry air that develops all around the country during the fall and winter months increases water loss from the respiratory tract and lungs.

On top of this, the very dry diets that horses eat in winter also increase their requirement for water. If you feed your horse extra hay during cold weather to help generate more internal heat, your horse will also need more water to process the extra fiber. Hay also contains much higher levels of potassium than your horse actually requires, so he’ll need increased water intake to make enough urine to excrete the extra potassium.

What this boils down to is that even in winter your horse needs a rock bottom bare minimum of 5 liters of water per 100 kg of body weight. So a 500 kg horse (1,100 pounds) needs 25 liters—or 6.6 gallons—of water per day. This is the minimum. Some studies have found minimum intakes of 6 to 6.6 liters/100 kg even in inactive horses. That amounts to 8.7 gallons of water for a 500 kg horse.

Now let’s put this in practical terms. The average stall bucket holds 5 gallons. You’ll need to refill your horse’s bucket at least once per day, and more often if ice forms during really cold weather. This can also help you to monitor your horse’s water intake to make sure he’s drinking enough.

Electrolyte Issues
Electrolyte losses don’t stop during winter either. As mentioned, potassium is present in hay in more than adequate amounts. In fact, about 5 pounds of grass hay will meet your horse’s potassium requirements in winter. Hay also comes close to meeting your horse’s chloride needs as well. But levels of chloride in hay vary widely, so you can’t count on it.

The most critical electrolyte for maintaining normal hydration is sodium. There’s so little sodium in your horse’s diet that it doesn’t even have to be counted!

You’ve probably heard before that blood is salty. Sodium and chloride, which together make up plain salt, are the two most abundant electrolytes in the body, particularly in blood. But without adequate sodium in the blood, the body will pull it out of the fluid in the tissues that surrounds the cell. This process is the beginning of dehydration.

Your horse’s urge to drink is also controlled by the level of sodium in his blood. If sodium drops, cells in the brain called osmoreceptors will first make the kidneys conserve sodium (which pulls water back into the body rather than eliminating it in urine), and the urge to drink will go down. When the horse eats salt and sodium rises, the urge to drink is triggered again.

As salt and water are pulled from the tissues into the blood, then dehydration starts. The brain’s osmoreceptors don’t "know" that the horse is getting dehydrated because the blood level of sodium is maintained. If this isn’t corrected by feeding salt, the tissues will stay dehydrated.

Make sure ice-free water is always available. In fact, horses prefer warm water—just under body temperature—to cold water. That’s a tall order in winter! Look into tank warmers, bucket or trough insulation, or mixing hot water into your buckets

Health Effects of Dehydration
The most obvious effect of ongoing dehydration—and an extremely common winter health problem in horses—is intestinal impaction. The thirsty body tissues will absorb more water from the bowel and at the same time will not secrete as much fluid and mucus into the bowel as they normally would. This leads to drying out of the intestinal contents, which eventually causes things to get clogged up. Impactions are extremely painful, often painful enough to mimic severe surgical colics. They can also take an extremely long time to resolve—sometimes up to a week—requiring daily vet visits, pain relieving drugs, and daily stomach tubing and water treatments.

When the horse is dehydrated, mucus production in the lungs also slows down and it becomes more difficult to move it out. Mucus is the first line of protection for the delicate cells of the respiratory tract. Since winter air tends to irritate them anyway, losing that good protective barrier makes things much worse. This sets the horse up for lung problems ranging from environmental irritants like dust to allergic reactions and even infection.

Dehydration even has a negative effect on appetite and, as mentioned, kidney function slows. The attendant lower saliva production also contributes to risk of choke.

Because dehydration can have such devastating systemic effects, it’s definitely advisable to avoid this at all costs. Here are some ways to do this.

Strategies for Avoiding Winter Dehydration
The first step to avoid dehydration is to make sure water is always available, and that it’s palatable to your horse. Horses prefer warmed water—just under body temperature—to cold water. That’s a tall order in winter, especially if you don’t have hot water in the barn or if the horses are out in a field. Insulation of buckets/troughs helps, and addition of a little salt (about 0.5 tsp/gallon) kills two birds with one stone by helping boost sodium intake and lowering the freezing point of water. If the horse is stalled and the barn does not have hot water, consider taking a gallon or two with you when you see the horse, to add to his bucket.

Studies have shown that horses drink more in the afternoons and evenings. They also drink more within two hours after eating hay, as well as while they’re eating hay (including dunking behavior). Feeding a high-protein meal may also trigger the desire to drink since high protein in the blood may also trigger osmoreceptors in the brain.

Feeding your horse a warm mash of either beet pulp, wheat bran, or both, can actually encourage him to drink.

Keeping your horse’s salt intake up by adding it to meals is excellent insurance against winter dehydration. I’d advise that you don’t use electrolyte replacement products to accomplish this, though. These contain potassium that the horse doesn’t need, which will make his kidneys waste water in getting rid of the extra potassium. Plain old salt works just fine, but don’t rely on free-choice intake alone. Licking (and possibly freezing your tongue to) a salt block in winter weather is no fun! A total of 1 ounce (2 tablespoons) of table salt should be divided between meals. It can be added to bucket meals, or mixed in water and sprayed on the hay. It doesn’t matter if sprayed-on salt freezes to the hay. In fact, that’s a good way to make sure the horse actually eats it. If adding salt to drinking water, assume the horse is drinking the amount needed.

Another excellent tactic is to feed warm mashes. Beet pulp holds up to four times its dry weight in water, so it’s a great choice. Wheat bran is the traditional mash and holds about twice its dry weight in water. Wheat bran can be difficult to ferment for horses that are not accustomed to it so don’t give the horse a large wheat bran meal out of the blue. It does have the advantage of being highly palatable though, especially when warm and aromatic. My dream mash for winter is 2/3 to ¾ beet pulp and 1/3 to ¼ wheat bran by dry weight, with salt added. Be sure you soak the mash first, otherwise it can expand in your horse's gut and cause colic.

If your barn doesn’t have hot water, invest in a submersible water heater. If you juggle the order in which you do chores, the water will have plenty of time to heat enough to be used to make a mash and take the chill off the water you put in your horse’s water bucket. A 20-minute soaking time for the mash will do it. Put your water to heating first thing, tackle stalls, make the mash, water and groom while it’s soaking, maybe with your horse starting on his hay, and serve up the mash last thing before you leave. Add extra heated water to your horse’s drinking water.

If you are adding salt to the drinking water to help prevent freezing, figure on about 6 gallons of water intake per day (minus what is in any mashes) and subtract the salt intake from salted water from the total daily salt requirement.