It can happen any time, but fall and winter are peak seasons for colic, especially impactions.
from The Merck Manual
There are many reasons for this, including change in diet when coming off pasture, lower water intake when switching off fresh pasture and in winter when water can be frozen or too cold, failure to supplement adequate salt and less exercise because of less riding, stall confinement or bad weather and ground conditions causing even turned out horses not to move much.
Reducing the risk comes from efforts to modify these risk factors. Make dietary changes slowly, including transition from grass to hay. Make sure the average size horse gets at least 1 oz of salt added to meals daily. If necessary, dissolve in water and spray over the hay. Water intake needs to be a bare minimum of 5 to 7 gallons/day. Heated water is best. Consider wet meals that contain beet pulp, which can hold 4 times its weight in water. Make an effort to ride or lunge as much as possible plus plenty of turnout.
Pay close attention to water intake and manure characteristics. A drop in water consumption is dangerous. Decrease in manure volume or size of fecal balls is a red flag for impaction developing, as is the appearance of a mucus coating on the manure.
If your horse does develop an impaction, take heart knowing that 90% of horses recover with medical treatment. The down side is that it can sometimes take a week or even a bit more to resolve an impaction.
Until impaction resolves, the horse will be depressed with periods of colic pain and have little appetite. If he does attempt to eat, pain will worsen. Intravenous phenylbutazone or flunixin meglumine are usually used for pain. A once daily visit from the vet to assess your horse, give pain medication, intravenous fluids if necessary and tubing with oral fluids, electrolytes and stool softeners or laxatives is the usual routine. As with eating, it is to be expected that pain will worsen temporarily after the horse is tubed with fluids. This is because it triggers the intestine to move but there is still a blockage.
There’s a tendency to get discouraged or give up when the problem lingers 2 or 3 days. Hang in there. Good mucus membrane color and rectal examination findings can help reassure you there is nothing more life-threatening going on. If there’s one good thing to come from riding out the course of an impaction it’s a strong determination to do everything you can to never have to go through it again.
Eleanor Kellon, VMD