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An analysis of Mexico’s response to the U.S. in the USMCA dispute

Read the article in Spanish here. Lea el artículo en español aquí.

Since Mexico imposed its restrictions on genetically modified (GM) corn in tortillas last February as precautionary measures to protect public health and corn biodiversity, the United States government has repeatedly justified its challenge to the policies under the countries' trade agreement with the claim that Mexico’s policies are not based on science. 

As U.S. Agriculture Secretary Tom Vilsack said last August when the U.S. case was filed under the formal dispute mechanism of the U.S.-Mexico-Canada Agreement (USMCA), “Mexico’s approach to biotechnology is not based on science and runs counter to decades’ worth of evidence demonstrating its safety and the rigorous, science-based regulatory review system that ensures it poses no harm to human health and the environment.”

Mexico has now filed its formal response to the U.S. in the trade dispute. Published March 5, Mexico shows that it has the latest independent science firmly on its side. 

As the Mexican government notes in its 200-page response, “Far from there being a consensus on the safety of GMOs, scientific evidence points to various negative effects on health, on native corn and on the environment, derived from the cultivation and consumption of GM corn.” (119 –quotes from the document are italicized and cited here by their numbered paragraphs since pagination is different in the English and Spanish versions). 

In the interest of offering a readers’ guide to this long and technical document, IATP highlights here some of the most important points. We include some quotes in the text and at the end from key academic, civil society and government leaders, who have been instrumental in the decades-long effort to stop GM corn and its companion herbicide glyphosate. 

Ten NGOs will submit their formal comments in eight invited submissions on March 15 in support of Mexico’s restrictions. Canada, as a third party supporting the U.S. complaint, will as well. The U.S. has until March 26 to rebut Mexico’s claims. It is now on the U.S. to respond concretely to the science presented by Mexico. That evidence includes:

Risks from direct consumption of GM corn: 13 pages of evidence that GM corn, particularly insect-resistant Bt varieties, poses potential health risks to humans through damage to the intestinal tract and other organs.

  • The section includes 66 academic references from peer-reviewed journals. (A partial list is included at the end of this article.)
  • The documented risks arise from: direct exposure through foods; epigenetic changes that can be passed to the next generation; increased antibiotic resistance; and reduced nutritional content.
  • Many studies highlight the increased risks in Mexico for a population that consumes 10 times the amount of corn as we do in the U.S. and does so in tortillas and other minimally processed forms that represent a very different scale of dietary exposure than we face in the U.S.
  • “Given the fundamental importance of corn as everyday staple food in Mexico, the population in Mexico is highly exposed and vulnerable to these risks due to the amount of corn grain consumed directly on a daily basis in the form of tortilla and other foods made with nixtamalized flour and dough.” (24)
  • “Although the United States has attempted to trivialize and dismiss this body of scientific evidence, Mexico's assessment indicates that the risks are real and of particular concern to human health in Mexico.” (382)

Risks from consuming glyphosate residues on GM corn: 16 pages of evidence, including 74 academic references, on the elevated risks to Mexican consumers from glyphosate residues on GM corn. Those residues have been documented to be present in Mexican tortillas already, even though it is illegal to grow GM corn in Mexico and the country produces nearly all its own white and native corn for tortillas. That evidence of traces of glyphosate, presumably from imported GM corn, is one of the main reasons for the GM corn restrictions.

  • Mexico's submission cites the growing mountain of evidence that direct exposure to glyphosate causes cancer, as jury after jury has found in damage cases against Monsanto and its owner Bayer.
  • Multiple academic references show risks from low-level exposures from residues on consumed food. Again, Mexico's far higher levels of corn consumption multiply the risk and make U.S. and most international standards of acceptable residue levels moot, justifying Mexico's precautionary approach.
  • “Mexico considers that the ingestion of residual glyphosate and other contaminants present through the direct consumption of GM corn grain represents a serious food safety risk in Mexico.” (174)

Lax U.S. regulatory processes that fail to ensure safety for Mexico: Mexico questions the U.S. presentation of the science claiming safety: “The United States, far from proving that the measures identified are not based on science, presents information lacking scientific rigor, is outdated, or with conflicts of interest.” (230-235)

  • Many of the sources cited by the U.S. — Mexico highlights 33 — are not from peer-reviewed or academic sources.
  • Others are outdated, showing research that is dangerously out of date for such a relatively recent technology on which new science is emerging all the time. (Sixteen of the studies cited by the U.S. are more than 10 years old.)
  • The U.S. repeatedly cites broad surveys by science agencies, but most are more than 10 years old. 
  • Even those are selectively cited, such as the U.S. claim that the National Academy of Science in a 2016 survey confirmed GM safety. In fact, NAS evidence of safety comes from comparisons of U.S. (with GM) and United Kingdom (without GM) consumption, but none with a profile such as Mexico’s. 
    • As researchers noted in the American Academy of Pediatrics: “There were no long-term, published epidemiologic studies directly assessing the potential health impact of genetically engineered food and associated herbicide exposure, so conclusions about health were largely made in the absence of available data.”
  • Many of the studies cited by the U.S. show conflicts of interest, indicating funding by biotech companies or researchers associated with biotech interests.
  • U.S. regulatory standards are weak since they do not require animal studies or other safety assessments before a new GM variety is approved.
  • The oft-cited “4,000+ studies that show GM safety” are rife with these errors. Most come from companies’ own tests when they sought approval from U.S. regulators to commercialize new GM varieties. Few are peer-reviewed. Fewer are based on long-term animal feeding trials.
  • Mexico highlights that the U.S. cannot produce a single academic study that shows that the long-term consumption of large quantities of minimally processed GM corn treated with glyphosate is safe to eat. That is why Mexico took the precautionary measures it did. As Mexico’s Undersecretary of Agriculture Victor Suárez told Reuters
    • “To this day we have not seen any scientific studies that have been presented by the U.S. and the companies on the safety of continued consumption over years. So there is no scientific basis for the U.S. and the companies to claim that their corn is safe.”

Mexico’s careful risk assessment in accordance with USMCA guidelines: Mexico presents ample evidence that it has indeed done the risk assessment required by the trade agreement and that the agreement gives Mexico the right to determine the levels of protection it deems necessary, then interpret the available science in light of that commitment.

  • In that context, the relatively lax U.S. regulatory standards offer little useful evidence for a country as highly dependent on corn consumption as Mexico. The absence of long-term studies with high levels of consumption forced Mexico to conduct its own risk assessment based on the available science.
  • Mexico lists the documentation of that risk in a 31-page 2020 publication by the national science agency, which has been available for the U.S. government and the media to review. Mexico has constantly updated that evidence base in a publicly available databank that further informed its updated 2023 decree.
  • “The Risk Assessment evaluated the potential adverse effects on the health of Mexicans from the presence of contaminants, specifically glyphosate and GM proteins residues in foods made from GM corn commonly consumed by Mexicans.” (403)

Risks to native corn varieties from GM corn: Mexico also presents extensive evidence of the risks to native corn varieties from cross-pollination by GM corn, including a comprehensive study by NAFTA's own environmental commission. Such contamination can undermine the genetic integrity of Mexico's native corn varieties, which it argues is a unique and endangered natural resource valuable not just to Mexico but also to the world for future plant-breeding.

  • Mexico cites 13 distinct national laws and international treaties that obligate it to protect native corn.
  • That obligation includes a special exception in the current trade treaty that allows countries to take actions that guarantee cultural and Indigenous rights, of which native corn is considered an integral part.

The use of “least trade-distorting” measures: Mexico shows that its measures are carefully chosen to minimize the impact on international trade, satisfying the USMCA obligation that a government use the least trade-distorting measures available to achieve a legitimate policy goal.

  • Mexico's restriction on GM corn use in tortillas affects a tiny share of U.S. corn exports to Mexico — perhaps 1% — because some 97% goes to animal feed and industrial uses. Mexico is largely self-sufficient in white and native corn for tortillas. 
  • Affected U.S. farmers are not prevented from exporting to Mexico. In fact, they can earn premium prices if they switch to non-GM white corn.
  • The restrictions apply to all GM corn from any source, including from within Mexico, so they in no way discriminate against the U.S.
  • The measures do not involve trade restrictions of any kind, just a restriction on the use of GM corn in tortillas.
  • The U.S. challenge of Mexico's so-called “substitution instruction,” to gradually replace GM corn in animal feed and industrial uses, is misguided and at best premature. At this point it imposes no specific actions nor indicates any trade restrictions nor any date by which any measures will be enacted. To prove the point, Mexico notes that since the decree was announced in February 2023, U.S. corn exports to Mexico have increased significantly, not decreased.
  • The U.S. in its complaint fails to identify a “less trade-distorting alternative” policy than Mexico’s minimal restrictions, as required by USMCA. As Mexico points out, the U.S. suggests that “because the Tortilla Corn Ban does not achieve any [Appropriate Level Of Protection], a reasonably available, less trade-restrictive alternative would be to withdraw it altogether.” (450)

Experts have praised Mexico’s response as a strong science-based justification for its policies: 

"If we win, we will challenge an entire model of production. It would be a huge achievement, setting an international standard. If our maize is defeated in its center of origin, we would see the same in other centers of origin for other crops. The biotech companies would be emboldened," said Monserrat Téllez, Seeds of Life (Mexico).

“Mexico presents ample scientific evidence on the risks to human health and the environment of consuming GM corn with residues of the herbicide glyphosate. The government of Mexico has every right to determine the appropriate level of protection to protect human health,” according to Fernando Bejarano, Ph.D., Red de Acción sobre Plaguicidas y Alternativas en México (RAPAM)/Pesticide Action Network in Mexico. 

"Mexicans are the largest consumers of corn, especially through tortillas. We have the right to prohibit the use of transgenic corn in the preparation of tortillas not only because of the presence of transgenic corn but also the higher concentrations of glyphosate residues. In addition, our country is the center of origin and diversity of corn, the basis of our culinary culture, which has been declared a world heritage site. We have the right to protect this food and this cultural asset,” stated Alejandro Calvillo, Poder del Consumidor.

“We welcome this vigorous defense of Mexico’s programs to transform its food system. The science they present backs up longstanding civil society campaigns for healthy foods and biodiverse agricultural systems. There’s a lot here that could contribute to more substantive debates on our food and agriculture system in the U.S., as well,” observed Karen Hansen-Kuhn, IATP’s director of trade and international strategies.

Ever since Mexico first announced its intentions to limit GM corn and glyphosate in its tortilla chain, the U.S. government has asserted that Mexico’s policies are not based on science. Mexico’s comprehensive response refutes that claim, presenting hundreds of academic studies that show cause for concern about human health and the threat to native corn diversity. The onus is now on the U.S. government to respond by March 26 with science, offering its evaluation of the dozens of studies Mexico cites that show cause for concern.

Ten U.S. and Mexican NGOs will submit their formal comments in eight invited submissions on the dispute March 15, due for publication April 4. IATP maintains a resource page on the GM corn conflict.

 Partial list of academic journals cited by Mexico on the science:


Env Health Perspectives 

Journal of American Science 

Food & Nutrition Sciences 

British J of Nutrition 


Scholarly J of Agric 

Science J of Applied Toxicology 

Food & Agric Immunology 

International Immunopharmocology 

Agroecology & Sust Food Systems 

Int J of Biological Science 

J of Organic Systems 

Environmental Science Europe 

Frontiers in Plant Science 

Food & Chemical Toxicology 

African Journal of Biotechnology

Read the article in Spanish here. Lea el artículo en español aquí.


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