Horses are hindgut digesters and what this means is that most of the food they eat gets processed in the hindgut. They require a large amount of fiber which is converted to energy during this digestion process.
Horses fed high starch/sugar diets such as grains of all types, senior feeds, processed feeds, etc. are at high risk of developing hind gut acidosis. The starch quickly ferments in the hind gut which turns to acid therefore creating an acidic condition in the gut. This disrupts the microflora of the intestinal tract and has been linked to laminitis and founder.
This has become a widespread problem with the modern domesticated horse. They are confined and fed large starch meals twice a day instead of having access to grass throughout the day. The symptoms can manifest in many different ways, such as cribbing, weaving, diarrhea, poor appetite, poor performance, hot and excitable behavior.
One way to check to see if this is the problem is to feed a tablespoon of baking soda mixed with 1 ounce of coconut oil in a syringe. If there is improvement, chances are your horse's gut was too acid. The baking soda mixture works well but it doesn't heal the gut. To do this, it is important to make the correct dietary changes by feeding only grass hays and supplementing with a good vitamin B supplement such as Ex-stress and following the Succeed Digestive conditioning protocol. This is highly effective in healing the hind gut.
Also read my post on ulcers (dec 2010 post).