By Ashley Killough, CNN.

For Iowa's economy, corn is everything.

So it's no surprise that the Republican governor and other powerful voices in state agribusiness are vocal backers of the Renewable Fuel Standard, a federal rule that requires gasoline to be comprised of up to 10% in renewable fuels.

One of those is ethanol -- with corn often a key ingredient.

"Don't mess with the RFS," Gov. Terry Branstad said Saturday, offering a not so subtle warning as he kicked off a daylong agriculture summit that featured a string of likely Republican presidential candidates.