Here's something I copied for ya:
horses sometimes eat manure when they need certain bacteria called probiotics (meaning "for life") that aid in digestion. Because horses eat mostly grass and roughage, they rely on these microorganisms to ferment and break down their food. Without probiotics, horses cannot properly digest their food.
Probiotics do not occur in large enough numbers for a horse to acquire naturally. A newborn foal is born without any type of digestive bacteria but soon receives some from contact with his mother or other horses. These bacteria settle in the horse's digestive tract and multiply. When a horse is under stress, bacteria populations can suffer which can lead to improper digestion, and in extreme cases, colic in your horse. Administering antibiotics can also kill off these good bacteria, which is why some veterinarians recommend giving probiotics along with antibiotics.
Bacteria grow accustomed to an individual horse's diet and different horses will have differing probiotic populations. A sudden change of diet can shock the intestinal bacteria and cause discomfort or colic in a horse.
Because many horses today are fed refined grains and grasses, instead of a natural diet, horses may suffer from microbial imbalance.
Probiotics can be purchased at a feed store and given as a supplement. They are supposed to be eaten and take up residence in your horse's digestive tract. While there is little scientific evidence that they significantly improve digestion, they have no side effects, so some horse owners do include them in their feed regiment.
Eating manure may also be a sign of vitamin or mineral deficiency. Always provide mineral and salt blocks, and contact your veterinarian if this strange behavior persists.
