This same process occurs on lush spring or early autumn regrowth pastures containing high sugar content to cause grass founder . This type of problem can be avoided by limiting raw grain intake at each meal, by feeding 3-4 meals daily not exceeding 2kg grain per meal mixed with an equal volume of chaff. You mentioned extruded feed. It also seals the grain particles to reduce their oxidation during storage. The limited starch digesting enzyme capacity of the small intestine is then able to more efficiently attack the bonds between the long chain sugars, greatly improving digestion of the starch.
Cooked corn becomes a cool energy feed rather than heating feed for a horse.
Cracking grains does not significantly increase digestibility in the small intestine, but opening raw grain hastens the oxidation of fats and vitamins in the grain. Basically, their digestive processes and efficiencies are relative to their size and digestive tract length and volume. Some horses have a more efficient metabolic function than others, requiring less feed. However, a nervy horse may expend more energy and need more feed than a quieter horse. The same balances of hard feed to hay should be maintained to ensure efficient digestive function, but for the same amount of work, smaller horses may need a higher energy density because their appetite and capacity for bulk may be limited for an equal amount of energy expenditure.