Here's some good evidence about how NAFTA undermines farmers, not only  domestically, but also in the countries where we " free trade"
This article from The Nation focuses on pigs, who eat corn. 

For nearly two decades, Smithfield has used NAFTA and the forces it unleashed to become the world’s largest packer and processor of hogs and pork. But the conditions in Veracruz that helped Smithfield make high profits plunged thousands of rural residents into poverty. Tens of thousands left Mexico, many eventually helping Smithfield’s bottom line once again by working for low wages on its US meatpacking lines. “The free trade agreement was the cause of our problems,” Ceja says.

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