Horses can develop ulcers for all kinds of reasons, and one of the biggest reasons I see horses develop ulcers is from the emotional stress of having the wrong career.
Researchers rule out nervous stress as a cause for ulcers, but I disagree. My experience has been that I see more ulcers in horses who do not like their jobs or are challenged beyond their capabilities.
Often horses are not over-faced physically but mentally, especially where showing or competing is involved. On the opposite side, boredom can create stress in a type A horse who likes to compete.
In either case, stress often results in horse ulcers. For instance, the nervous horse who feels overly challenged by showing often develops ulcers in the duodenum (the upper part of the small intestine). This occurs because when a horse is anxious in this way, it causes a vagal response, which dumps partially digested or undigested food into the duodenum, along with hydrochloric acid. The acid, which is not properly buffered by bile and pancreatic enzymes, irritates the duodenal lining while the undigested food simply rots, causing further irritation. All of this irritation eventually leads to ulcers.
Horse ulcers can also develop in the upper stomach, lower stomach, and colon-all for different reasons. Luckily, there are holistic treatments for each type of ulcer. To learn more about horse ulcers, their causes, and their cures, check out these resources: