Although less common than in foals, manure eating (coprophagy) in adult horses is still common. It is thought to most likely result from boredom and hunger. There are theories that it can be a consequence of lack of adequate forage or fiber, or other dietary imbalances or deficiencies. But none of this is proven.

Horses that are very hungry will eat their own manure and that of herd-mates. There is actually significant nutritional value in manure that can help nourish an otherwise starving horse.

The behavior usually occurs in bored adult horses on dry lots. Theoretically, horses engaged in this behavior ingest more parasites, which can lead to other problems. However, mild coprophagy alone, with no other signs of illness or disease, probably does not cause significant problems.