The health of your horse can depend very much on how that corn is prepared. Considering the high starch levels in corn, around 75%, for the health of your horse you want that to be entirely, if not mostly, digested before it reaches the hind gut. It is important to note that the foregut has a limited capacity for digesting sugars and starches. Excess sugars and starches are passed into the hindgut (cecum and large intestine), where they can cause gastric ulcers, digestive upset or laminitis. The hind gut being designed to digest fibre, a process of fermentation in the cecum, and having the relevant bacteria and microbes to do so - because just like humans horses can not digest fibre.
When undigested starch and sugars reaches the hindgut, the microbial fermentation process in the cecum produces a higher level of lactic acid. This creates a more acidic environment in the hindgut (lowering the pH), resulting in the state known as hindgut acidosis.
Digestibility of cereal starch varies from 20% to 90% in the foregut depending on the process used. Physical processes have a lesser effect than thermal and hydrothermal ones. Physical processes increase significantly the prececal digestibility of cornstarch but have a moderate impact on other cereals. Starch digestibility is increased by thermal and hydrothermal processes whatever the botanical origin. Further investigations are needed to identify the process which allows the highest prececal digestibility and decreases the hindgut fermentability of starch.
Ref; KER, Science Direct, Meyer, The Horse.