I found this trivia in a historical strategy game (whose name shall not be disclosed) and I want to know its credibility.
It is stated that warhorses — say, destriers or other types of European warhorse used by knights or heavy cavalry — can't graze naturally on grass fields because they got used to eating their exclusive type of hay — I don't know the name of this type of horse food. Hence, the cavalry or the supply units also had to bring this horse food, too.
So, is this generally true? If so, what are specific examples of this happening?