Supporters of GMO foods, whose DNA is altered to be resistant to pests, say they are a way to boost world food supplies. But farmers in Mexico’s rural south, where corn has been grown for thousands of years, worry GM corn will cross-pollinate with native species and alter their genetic content.

Under the new rules, the farmers who want to plant GMO crops must register with the agriculture ministry and environmental authorities to request a permit.

GMO corn seeds will not be allowed into certain parts of the country that are determined to be “centers of origin” for genetically unique corn strains found only in Mexico.

Bio-tech food producer Monsanto Co welcomed the decision in a statement, although the company noted that “the passage of these rules does not mean that permission will automatically be granted” to plant GMO crops.

Some farmers decried the decision. “This is a step in the government’s intention to bow to pressure from Monsanto to allow the contamination of Mexico’s native corn,” said Victor Suarez, who leads a group of small farmers opposing GMO crops.

Corn was first planted in Mexico as some 9,000 years ago and the country is now home to more than 10,000 varieties. The grain was adopted by Spanish conquistadors in the early 1500s and eventually spread to the rest of the world.

On Jan 1 Mexico, the United States and Canada lifted all corn tariffs under the 1994 North American Free Trade Agreement. Mexico now imports between 8 million and 9 million tonnes of U.S. yellow corn a year, close to 35 percent of local consumption. More than 70 percent of U.S. corn is genetically modified.

With U.S. corn prices hitting record highs near $6 a bushel on increased demand for corn-based ethanol, corn farmers in the north say GMOs will help Mexicans cut down on expensive U.S. imports by producing more at home.