Aloe juice is good for allowing ulcers to heal, but it will not prevent them if the horse's proper grazing needs aren't met. Horses are designed to graze at least 22 hours a day, they constantly produce stomach acid, unlike us or dogs who only produce stomach acid when we eat. A horse's full stomach empties in two hours, after that ulcers start to form if there's nothing/not much in the stomach. After 7 hours it is impossible for them to not form. A misconception is that ulcers will always show outward symptoms or always show when a horse is scoped, and neither is the case. This is why there are slow feed nets, one of the benefits is allowing horses to "graze" like they need to. This is among other benefits such as a stress/boredom reliever and stabilizing weight on easy and hard keepers.
If you suspect your horse has ulcers then adding the aloe juice will be good, but you will still have to fix what's causing them in the first place. What is your horse being fed, how much, how many times a day, and what is your horse's stall/turnout schedule?
Sully ~Sullivan's Fly Supply~ [17.1 TB] RIP 2/24/14
Rio ~Camperio~ [18.0 Oldenburg]