NAFTA's Effect on Mexican Agriculture:

Prior to passage of North American Free Trade Agreement (NAFTA) in 1994, only 2% of the corn consumed in Mexico was imported from the U.S. (Kraul). But once NAFTA came into effect, imports of cheap corn from the U.S. and Canada increased significantly (Enciso). U.S. corn is cheaper for a variety of reasons. U.S. and Canadian farmers grow corn on a much larger, more industrial scale than Mexico's 2.5 million growers, most of whom are indigenous and farm on less than five hectares of land. In Mexico, most production is seasonal; farmers use no irrigation and often grow two to six heirloom varieties suited for different growing conditions and markets. In addition, U.S. growers enjoy more government support, especially in the form of subsidies, than do Mexican growers. U.S. growers receive about 30% of the crop value while Mexican growers only receive about 9% (Kraul).

Similar accounts occurred in other aspects of Mexican agriculture. In short, because American farms are so generously subsidized, American corporations have the ability to sell their commodities at extremely low rates. Most importantly, these rates are so low that Mexican farmers cannot realistically compete with them.