The most common cause of winter colics is large colon impaction. Many
cases result from reduced water intake, usually because the water is
unpalatably cold or frozen. Here are some tips to help keep your equine
companion drinking during the winter  weather:
o    Provide palatable water free-choice at all times.
o    Make a red salt block available to encourage drinking
o    The average 1000 pound horse will drink approximately 12 gallons of
water daily during the winter months

Information provided courtesy of Harry Werner, VMD