Colic can happen at any time, but during cold weather the incidence of painful gastrointestinal impactions typically rises sharply. Mild dehydration is a normal underlying cause.
Source: Equus, January 2016.
To protect horses from impaction colic this winter, ensure they always have access to fresh water. It’s a pretty good bet that ice will dissuade a horse from drinking, so do your best to keep the water from freezing—not only in a solid sheet of ice on top of the trough but also as free-floating chunks of ice.