Ulcers may have psychological (stress/work/growth) or dietary causes.
Cereal intake should be restricted since high starch content promotes the onset of hyperacidity in the stomach due to the lowered pH.
Various sources of energy are available: starch + sugar/cellulose/vegetable fat.
Consumption of adequate quantities (5 to 7 kg) of meadow hay (long fibre to encourage chewing) each day will help limit the development of ulcers.
ROYAL HORSE Course/Race and Mature (elderly horses) feeds contain clays and sodium bicarbonate to help prevent ulcers.
Contact your veterinary surgeon.