Winter weather can be responsible for stresses that can compromise your horse's health. The good news is that you can prevent some of that stress through proper feeding management. Most horses have some "down time" in winter, where adverse weather will not permit much riding or showing activity. It is during these times that it is easy to neglect your horse's nutrition program.
A reduction in your horse's activity level usually means a reduced need for calories, and requirements for grain or concentrate feeding can be lowered. In cold weather the horse's caloric requirements will be greater as the horse attempts to generate enough heat to maintain normal body temperature. But caloric needs are only slightly increased, 10% to 20% over maintenance needs for all but the most extreme conditions.
Old-time horsemen thought if they fed corn during the winter months, it would generate more body heat and help alleviate cold weather stress, because it's a "heating" feed. In reality, the horse's body generates more heat from the fermentation process in the hindgut as a result of eating forage (hay and pasture). So feeding more hay and less grain will allow the horse to more easily maintain its body temperature. Substitute two pounds of hay for every pound of grain you decrease in the horse's daily ration.
With the onset of cold weather there is also a greater incidence of impaction colic in horses. This is mainly due to the horse becoming dehydrated because he will consume less water due to cooler temperatures (no sweating), less water availability (frozen ponds, cold water, etc.), and a diet of hay (10% water content) instead of pasture (80% water content). When horses drink cold water during the winter, their bodies must expend additional calories to warm their tissues back up from the heat loss that is incurred, so they instinctively drink less. Warming water or using insulated or heated buckets that keep water temperature above freezing will allow the horse to consume more water. Research has shown that horses drink the most water when the temperature is between 45 and 65º F. Optimum water consumption will keep the fiber in the horse's lower digestive system more hydrated, allowing it to be broken down more quickly by intestinal bacteria and to be more flexible, and less likely to "ball up" and cause a blockage in the large intestine.
Monitoring your horse's body condition can be difficult in the winter if you do not clip your horse's hair coat. Trying to see if your horse is a little "ribby" is hard to do when he has a winter coat. Instead use a weight tape or weigh him on a scale, if you have one available, and check how he is maintaining his weight every 30 to 60 days. Then you can adjust his feeding program and get him back on the right track before warmer weather comes. It is no fun to clip your horse in the spring and realize he is a "bag of bones" and you will need to bring him back up to proper weight before beginning your riding activity.
Winter is also the time when the barn may be "closed up" in an effort to make the environment warmer and less drafty for the horse. Good ventilation is more important than providing a little more warmth, so be sure to provide good air flow in your barn even in winter time. Due to decreased ventilation, it is also important to be careful about hay feeding. Research has shown that horses fed hay in hay nets placed above their heads will have an increased incidence of respiratory problems. Feed hay off the ground or position the hay net below the horse's head so that continuous drainage of the respiratory tract can occur.