Mexico is ready to hit the US where it hurts most: corn.

Mexico is today one of the main buyers of American corn in the world. And Mexican Senator Armando Ríos Piter, who heads a congressional foreign relations committee, says he will introduce a bill this week under which Mexico will buy corn from Brazil and Argentina instead of the United States.

It is one of the first signs of possible concrete action by Mexico in response to President Trump’s threats against the country.

“I’m going to send an invoice for the corn we’re buying in the Midwest and… switch to Brazil or Argentina,” Ríos Piter, 43, told CNN’s Leyla Santiago at a conference Sunday. Protest against Trump in Mexico City.

And he added: It is a “good way to tell them that this hostile relationship has consequences, I hope it changes.”

American corn is used in much of the country’s diet. In Mexico City, from fine dining restaurants to street taco stands, corn-based favorites like tacos can be found everywhere.

Related: Daughter of a Mexican farmer: NAFTA destroyed us

The United States is also the largest producer and exporter of corn in the world. Shipments of American corn to Mexico have catapulted since NAFTA, a free trade agreement signed between Mexico, the United States and Canada.

U.S. farmers shipped $2.4 billion in corn to Mexico in 2015, the most recent year for which data is available. In 1995, a year after NAFTA became law, corn exports to Mexico amounted to just $391 million.

Experts say such a bill would be very costly for American farmers.

“If we do see a trade war in which Mexico starts buying from Brazil… we will see that it will affect the corn market and spread to the rest of the agricultural economy,” says Darin Newsom, senior analyst at DTN. an agricultural management company.

Ríos Piter’s bill is another sign of Mexico’s willingness to respond to Trump’s threats. Trump wants to make Mexico pay for a border wall and has threatened to impose taxes on Mexican imports ranging from 20% to 35%.

Trump also wants to renegotiate NAFTA. He blames it for a flood of manufacturing jobs in Mexico. A non-partisan congress investigation The report found that was not true.

Related: Mexico redoubles its commitment to Trump’s ‘contingency plan’

Still, Trump says he wants a better trade deal for American workers, although he has not said what a better deal would look like.

All sides signaled two weeks ago that negotiations would begin in May after a 90-day consultation period.

But Trump says that if negotiations don’t result in the deal he wants, he threatens to withdraw from NAFTA.

These types of harsh speeches are not well received by Mexican leaders like Ríos Piter. You are not alone. Mexico’s Economy Secretary Ildefonso Guajardo said in January that Mexico would respond “immediately” to any Trump tariffs.

“It is very clear that we have to be prepared to be able to immediately neutralize the impact of a measure of that nature,” Guajardo said Jan. 13 on a Mexican news program.

–Shasta Darlington contributed reporting to this story.

CNNMoney (Mexico City) First published February 13, 2017: 12:06 pm ET