my gf's father is a farmer and has a corn field i hunt over. They have the ability to cut the corn basically whenever they want. Yes, it needs to dry out to a certain point, but they can harvest before prime moisture content due to having "blower" fans within the silos that could dry to corn even more if they wanted to harvest early. Optimally they would like to let it stay in the field as long as possibly, but due to other weather and natural issues, they cant always due that.
In essense, it really does depend on the farmer.