Gastric ulcers affect up tp 37% of leisure horses, 63% of performance horse and up to 93% of racehorses.  Foals are more at risk with up to 50% of foals developing gastric ulcers.    Horses kept in stables are more at risk than horses which live out. It is caused by an excess of stomach acid which is "splashed" around when they exercise.
Horses fed high forage diet of hay/grass are a lot less likely to develop ulcers than horses on a high grain diet, also horses which aren't able to eat for several hours on end regularly, leads to a build up of stomach acid.  Recent studies show than horses who crib usually have ulcers.

Risk Factors.
Horses kept in stables
Horses on a grain diet
Run down/ill horses
Horses on bute/other medication
Foals
Horses stressed by competition/stabling/travelling ect
Horses left for more than 6 hours without food/forage
Breeding mares

Reducing the risk

Take the grain our of your horses diet, most horses don't need it at all.
Turn your house out more or let him live out.
Try not to overload him with travelling/competition and stressful situations
Feed more forage/hay
Make sure he isn't left without hay/grass for more than 6 hours.


Signs of Ulcers
Poor appitite, picking at food
Blood in the droppings (hard to see)
Poor body condition, have trouble keeping weight on
Scruffy coat
Attitude changes,bting,kicking when they didn't used to
Decrease in performance when exercised
Mild/moderate colic which reoccurs
Foals which arent suckling as much as they should/foals lay on their back/poor looking foals with loose stools
Teeth grinding and excess saliva
Cribbing/windsucking
Bleeding from the nose


What to do if you suspect Gastric Ulcers
Feed more hay/forage and reduce his grain to nothing and phone the vet.  Tell the vet your suspect Ulcers and he will do some blood tests to check the red blood count.  They may try a few weeks of anti ulcer medication to see if it works.  An improvment indicates they have gastric ulcers.  The vet may also "scope" your horse which involved putting a small camera into the horses stomach to check.  The vet may prescribe medication for a while, then prescribe some more for periods of risk/competition to lessen the risk of ulcers developing.