By Mica Rosenberg

MEXICO CITY, Jan 31 (Reuters) - Thousands of Mexican farmers, some riding tractors and herding cows, flooded the capital on Thursday to demand government protection against cheap U.S. imports.

Trade barriers under the North American Free Trade Agreement, or NAFTA, were lifted in January, opening Mexico for the first time to tariff-free U.S. exports of traditional food like corn and beans.

Mexican farmers complain the government of President Felipe Calderon is not doing enough to protect them against highly subsidized U.S. goods.

Protesters are demanding Mexico renegotiate the treaty with the United States to maintain protections for corn and beans.

Long lines of slow-moving tractors choked highways from rural areas toward Mexico City for a march toward the main Zocalo square in the city center.

"The free trade agreement is like an open wound for the Mexican countryside," said Victor Suarez, who heads a small farmers' group. "You can give the patient medical attention but if you don't stop the hemorrhage first the patient will die."

Since NAFTA took effect in 1994, corn tariffs have gradually been phased out and imports of U.S. yellow corn to Mexico, mostly used in animal feed, have soared. They now account for close to 35 percent of Mexican consumption.

Mexican farmers fear zero trade barriers will encourage highly mechanized U.S. farms to start producing white corn, which has been Mexico's main crop since the Aztec times and is a staple food.

Opposition legislators who support the rural sector have called for the resignation of Agriculture Minister Alberto Cardenas for failing to do enough to support farmers.

In an effort to dampen criticism, Cardenas announced on Wednesday an expansion of cash supports to meat and egg producers to buy corn for animal feed, since international prices for the grain have skyrocketed in recent months.

Cardenas said the negative effects of the trade deal for corn and wheat growers will be offset by high international prices on increasing U.S. demand for ethanol.

"High prices are helping us bring thousands of Mexican farmers out of poverty. We have support programs for all the agricultural sectors in place," Cardenas said. (Editing by Bill Trott)

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