Horses, if left to themselves, will spend the whole day grazing. Grass is an inefficient food. If you want to use the horses for work or riding, they need better food. If you have ever owned a horse you know they eat a lot, hence the term "eats like a horse". It takes hay and grain to feed a horse, which takes farm land out of "human food production". If you are raising large numbers of horses to equip armies or export as a trade good, you are going to need A LOT of horse food. I think they got this right in Col2.
