I found this article hope it helps:
If your adult horse is eating his own manure, or the manure of stable mates, then there's cause for concern. Your horse may be suffering from dietary imbalances, boredom and/or lack of exercise.
Dietary Imbalances. A diet that's too low in protein, fiber or is simply unbalanced is thought to contribute to coprophagia. In this case, your horse is seeking nutrients in the manure that are missing from his diet. Another word for this behavior is pica, which includes the consumption of dirt and other foreign objects. The best way to correct this problem is to review your horse's current diet with your veterinarian and make the recommended changes. Your veterinarian may recommend a different brand of food, a vitamin supplement or a different type or volume of hay.
Boredom. If your horse isn't turned out (exercised) on a regular basis, he'll probably begin to develop unsavory habits, one of which may be coprophagia. Again, this is a behavior that can be changed. Instituting a regular turnout schedule and/or exercise program – along with providing some means of distraction in the stall – can alleviate your horse's boredom and, in turn, bring an end to this unpleasant activity. Distractions include toys (balls), a bigger window, or closer proximity to a friend.
Other Causes. In the older literature, there is discussion about coprophagia linked to various cravings, including trace minerals, salts, and fiber. Again, if this becomes a problem, especially when it involves multiple animals, think about having the diet evaluated by your veterinarian and possibly tested. While coprophagia is an unpleasant subject, we should remember that it's a normal part of the maturation process in your foal. However, with regards to your adult horse, coprophagia indicates that there may be a managerial problem on your farm or homestead, such as inadequate nutrition and insufficient exercise. Of course, your veterinarian is one of the best resources to help identify and correct such problems.
I had a foal that went through a poop eating phase I guess its normal for babys. she quit after a few bites :P
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France