Water is the most important nutrient a horse needs, and that is often overlooked, particularly in winter. Winter colic can be due to an impaction of dry digesta (partially digested food) within the gastrointestinal tract, which may be caused by dehydration from decreased water intake. Research has found that horses prefer water when it is 10°C, but as long as it is warmed above freezing, a little colder water shouldn’t deter drinking. The biggest factor in water temperature will likely be your facilities and management. A heated water bucket is sufficient inside, and these are economical and efficient.
Using buckets also provides a way for you to monitor your horse’s water intake daily (vs using automatic waterers). They do, however, require more work to fill daily, and this can be an arduous task in the winter!
Thirst is regulated by sodium in the blood (the Na of the NaCl/sodium chloride of salt), so you want to make sure your horse is getting plenty of salt. This can be given loose (about two tablespoons added to the feed), or fed via a salt block or commercially manufactured feeds with salt already added. Feeding another tablespoon per day in the winter to help stimulate drinking shouldn’t hurt (provided your horse does have access to unfrozen water and has healthy kidneys). You can also add water to his feed, mixing it with your commercial feed or some hay cubes – as long as it doesn’t freeze (a second heated bucket can help here).
Snow and ice are not suitable for horses to sustain their water requirements even when a horse is kept outside in the winter. A heated water hose helps facilitate refilling troughs, and while a heated water trough (>$300) or high quality water heater (>$100) may be expensive, they can save on labour and vet bills in the long run.