The main cause of colic during the winter is from reduced water consumption. Snow will not provide enough water; one gallon (128 fluid ounces) of average moisture snow contains only 10 ounces of water. Also, eating snow will force your horse to burn precious calories to keep his body temperature steady.
Typically, horses need to drink 8 to 12 gallons per day; they will often not drink this much when the water is icy cold. Plan on heating your horse’s water to 50°F. And don’t forget the salt – it is necessary for electrolyte balance as well as to encourage your horse to drink. Either add table salt to each meal (one tablespoon, twice daily) or offer it free choice in a small bucket. A white salt block is helpful, though many horses avoid them. Mineralized or blue (from added iodine and cobalt) salt blocks are only appropriate if hay is the single feed source or if your horse is not receiving minerals from fortified feeds or supplements.
Educate yourself on other equine nutrition issues: www.GettyEquineNutrition.com