"If Iowa wants to sell you food or if you want to eat?"
There is a reason agriculture is at the bottom of the economic food chain. I can choose to buy my food elsewhere, it will be just more expensive. Iowa being unable to sell its food, will have basically nothing.
This is a common argument against urban america "You urbanites need our food!". The reality is, no they don't, but the rural areas surely need the technology, transportation, trade and manufacturing. The real world examples of this in action is Hong Kong and Singapore, neither of which can produce food on any scale, yet have no issues with feeding their populaces.
There is a reason the US and most of the developed world has steadily urbanized. In more recent history, see the mass migrations from eastern China to the coasts.