Stomach ulcers are caused not only by stress, but by diet. On pasture, horses speed around 70% of their time grazing, and their stomachs secrete acid continuously.
Confined to a stable with specific feeding times, there is nothing to neutralise the acid that will damage the stomach lining. Instead of continuous access to fibrous food, high protein intermittent feeding is thought to maximise performance, compromising the health of the horse.
A study of race horses at Randwick in NSW found that 89% had stomach ulcers. Many had deep, bleeding ulcers within 8 weeks of starting race preparation.
A US study found that of 42 horses that had all raced within the previous two months, 100% had stomach ulcers. Another study by the same researchers showed that untrained yearlings had very few mild lesions. This indicates that the stress of racing increases ulceration and while a horse continues to race, their ulcers tend to get worse.