The longer length of time the animal spends lying down is NOT an indication of colic in and of itself, horses often will lay down for longer periods of time if they are old, or tired, or their feet or legs are sore, or they didn't sleep well the night before...etc
Look for a CHANGE in behavior.
Some people can look at a horse and read its face. Some cannot. One person ran up and told me my horse was colicking. My horse gets a certain expression on her face when she's rubbing her butt on the wall....some people can read their horse, some cannot. Too, some horses have stronger instinct to conceal their problems than others.
Horses often lie down briefly when they have colic and then get right back up('oh...well...that didn't seem to help much, I still hurt'). Getting up and down, rolling briefly - these are all possible colic symptoms.
And quite often, when the colic is getting really bad, the horse doesn't lie down at all. They often will just stand there, dull and unresponsive, not even seeming to notice things around them.
Another possibility is that the horse will repeatedly attempt to relieve the pain and sense of uncomfortable fullness by urinating or making manure, or both. You may see them stretch and try to pee over and over. Often no manure or urine will come out, but they keep stretching and trying.
Another sign the colic is getting very bad, is violent rolling - the horse doesn't just roll, he bashes his head, his hocks, which will show scrapes and injuries. Often in a severe colic the horse will alternate between violent rolling and standing dully, unaware of things around him. Becoming less active, ceasing to roll and simply standing, is NOT necessarily a sign 'he's getting better'.
What indicates one should investigate for colic, is unusual patterns of behavior. The horse is doing something he doesn't normally do.
It requires one to know what the horse's normal habits actually are.
At a time when the horse normally eats, it does not want to eat. It may nibble at grass, but not want its concentrates (corn, feed product, whatever). In general, not wanting to eat anything it normally eats eagerly, is a danger signal.
Another indication of colic is unusual sweat patterns. You often will see little tiny drops of sweat on the forehead, it almost looks as if someone sprayed it very lightly with a fine spray bottle. 'Cold' or patchy sweats are also possible.
The horse lays down at a time of day when it doesn't usually, or in a place where it doesn't usually lay down, such as right in a doorway, on a path, on a driveway, or the like.
The horse lies in a posture it doesn't usually lie in, such as with its head and neck turned, looking back toward its tail or stomach.
When worked, the horse is unusually dull or unresponsive. The rider may find himself frustrated that the horse is not reacting to his usual signals, he may even try to push the horse harder, and be surprised that the horse doesn't respond.
When called, the horse doesn't respond as it normally would. It doesn't notice people in its surroundings as much as it usually does.
Of course, if it's your horse, you count the heart rate, pulse, respiration, check the eye membrane and gum color (in a healthy horse, they are a rather pale pink - but not bright pink or red, dark brick red colored, white or blue-ish), and call the vet. The vet may advise you to give the horse some of that oral banamine you have on hand....
Horses often colic in summer due to a lack of water - the dry feed causes a blockage. In winter, horses often colic due to inactivity. They have an instinct to hold still and drink less water when it's cold - winter pasture doesn't always help to keep things moving, and winter hay is often more fibrous and less helpful. My vets always seem to get worked to death doing farm calls for colic, in the winter.
Hand walking, exercising, providing warm water in a bucket, adding extra moist fiber to the diet (such as soaked beet pulp), providing frequent small meals, can help.
