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Mexico’s closing argument in its ongoing dispute with the United States over its restrictions on genetically modified (GM) corn and glyphosate residues in tortillas was published in translation June 19. The government argues persuasively in the 264-page document that it has the right to take such precautionary measures under the trade agreement, that the measures have had minimal impacts on U.S. corn exporters and that its restrictions are indeed based on peer-reviewed science documenting the risks of consuming GM corn with glyphosate residues. These risks are particularly elevated for Mexicans, who consume more than 10 times the corn consumed in the U.S. and do so in minimally processed preparations, such as tortillas. 

Mexico refutes the U.S. rebuttal, which failed to acknowledge or rebut that evidence, relying instead on outdated studies that do not take Mexican consumption patterns into account and are often corrupted by conflicts of interest with biotechnology companies. As Mexico states in its accessible four-page introduction:

“Mexico has demonstrated throughout this controversy that there are legitimate concerns related to risks to human health and the diversity of native maize derived from the consumption of GM maize and has presented the scientific basis for these concerns, which will be addressed in detail throughout this paper. Mexico is protecting its population, which basically subsists on corn, as it is legally obliged to do so. The United States superficially analyzes and criticizes the evidence and risk assessment prepared by Mexico, but in its criticisms, it does not present arguments backed by science to support its position, but simply disqualifies with adjectives.” (paragraph 4)

“It is truly astonishing that the United States makes a plethora of superficial, false, contradictory claims and, above all, that it fails to present the technical-scientific evidence on which its claims are based - most likely because it does not exist.” (paragraph 13)

A readers’ guide

Following is a “readers’ guide” to the document in which I focus primarily on Mexico’s defense of the scientific foundations of its policies. Quotes are from the Mexican rebuttal itself, with paragraph numbers cited in parentheses for easy reference between the Spanish and English versions of the document.

The four-page introduction to the 264-page document presents a clear summary of Mexico’s main arguments, and the detailed table of contents offers a guide to the factual and legal fallacies in the U.S. arguments. All are well-documented, with 460 references in addition to an extensive set of case-law references.

The rebuttal is supported by two appendices that refute in detail the critiques offered by the U.S. as “scientific annexes” to its own rebuttal, which were widely criticized for being incomplete and lacking scientific evidence.

The rebuttal also is supported by written testimonies from four outside experts invited to contribute:

  • Dr. Michael Antoniou, on toxicology 
  • Dra. Ana Laura Wegier Briuolo, on maize biodiversity and gene flow 
  • Dr. Eckart Boege Schmidt, on the importance of maize in Indigenous communities 
  • Dra. Dulce Espinosa De la Mora, on the cultural importance of maize in Mexico

(I have not yet reviewed their written testimonies, though some are quoted in the document.)

The factual arguments are straightforward. “Mexico explains that the United States has ignored the relevant information in the evidence presented by Mexico regarding the risks to human health, biodiversity, and the diversity of corn. Mexico correctly characterizes Decree 2023, and explains that, in fact, U.S. imports have not been affected by Decree 2023.” (16)

Mexico argues that its decree is not merely a Sanitary and Phytosanitary measure but has broader goals, including protecting the diversity of its native maize. “Mexico is protecting this gene pool along with its cultural heritage, including traditional Mexican gastronomy, which is recognized by UNESCO as a world heritage site, and safeguarding the indigenous and peasant communities of Mexico, who maintain the gene pool in a complex system of constant domestication that faces risks posed by GM maize.” (7)

Mexico again points out that the U.S. incorrectly characterizes the measures in the decree as trade restrictions. “The United States continues to erroneously refer to certain provisions of Executive Order 2023 as the ‘Tortilla Corn Ban’ and the ‘Substitution Instruction.’ In the interest of non-repetition, Mexico refers to its explanation in its Initial Submission, and merely notes that there is no ban at all, but rather an End-Use Limitation on corn; and that no action has been taken to implement the Gradual Substitution Instruction.” (9)

Mexico points out that it complied with the U.S.-Mexico-Canada Agreement (USMCA) mandate to use the “least trade-distorting measure” available to achieve a desired policy. “The measures have not affected trade between the two countries at all, since, among other things, in 2024 there has been an increase in imports of white maize from the United States.” (10) Mexico documents this in a short section of the rebuttal (pp. 182-214), noting that:

  • The drop in U.S. white corn exports to Mexico in 2023 was not because of the decree but because U.S. lost market share to South Africa when Mexico eliminated tariffs to fight food inflation.
  • U.S. white corn exports to Mexico increased 62% in the first four months of 2024 compared to 2023 after tariffs on South African white corn were reinstated. (211)
  • Total U.S. corn exports to Mexico continue to increase, despite the decree, undermining the U.S. argument that the so-called Substitution Instruction for GM feed corn is having a chilling effect on U.S. exports. (214)

The legal arguments are more extensive. “Mexico refutes the alleged incompatibilities of Decree 2023 with the USMCA, and explains that, in any case, the measures would be exempted by the exceptions included in the Treaty itself.” (17)

Mexico points out that the U.S. declined to respond in any way to the eight written submissions by non-governmental organizations, including IATP, despite being given the opportunity to do so. “In addition, Mexico notes that, on May 3, 2024, the date on which comments to the NGO Written Opinions were due, the United States decided not to respond to the NGO Written Opinions by not submitting any documents. Perhaps this is because, for the most part, the arguments made by the NGOs are forceful and emphasize that ‘there are significant concerns based on scientific facts and 'lessons learned' in an industry that has been at best very lightly regulated.’” (11)

U.S. ignores the evidence presented by Mexico on the risks to human health, biodiversity and native maize

“Instead of refuting the evidence presented by Mexico, the United States merely characterizes it as a ‘[s]harp turn away from legitimate science,’ based on isolated statements, unsubstantiated disqualifications, and irrelevant evidence. The truth is that the United States fails to refute that Mexico clearly identified the risks associated with GM corn and glyphosate.” (20)

Mexico cites U.S. NGO submissions, especially Friends of the Earth (FOE), on the inadequacy of U.S. approvals for Mexican consumption levels. “In any case, the safety assessments made by regulatory authorities in the United States and other countries are irrelevant to the extent that they do not take into account the level of protection that Mexico has identified, nor do they consider the high consumption of corn in the Mexican diet and traditional agricultural practices in Mexico.” (27)

Citing FOE’s submission, the lack of any safety evaluation of the rising levels of stacked GM traits in GM maize varieties means that U.S. assurances of safety are unfounded. “The evidence presented by the United States, based on records from the EPA or other regulatory authorities, is not helpful in demonstrating the absence of risks regarding adverse health effects from the consumption of GM maize.” (31)

Mexico reiterates the raft of academic studies it presented that show risks to human health and the environment from GM maize and glyphosate residues, none of which were refuted by the U.S. rebuttal, even in its supposed scientific annexes. In addition to a point-by-point refutation of the dismissive characterizations presented in the U.S. annexes, in a separate appendix to the document, Mexico provides a 24-page section on the risks to human health in 14 pages, with 10 devoted to risks to native maize varieties.

“The factual section of the U.S. Submission describes the evidence presented by Mexico, in terms of health risks, as ambiguous and imprecise, as if mere adjectives were sufficient to dismiss the scientific results presented by Mexico. This type of unsubstantiated assertion fails to demonstrate the lack of scientific method in any of the cited articles, much less to dismiss the conclusions presented by Mexico.” (46)

“Mexico submitted more than a hundred scientific articles providing evidence of the health risks associated with the consumption of GM corn. Most of these articles were commented on superficially by the United States.” (47) Mexico footnotes a subset of the academic evidence the U.S. ignored in its rebuttal: Association of Lifetime Exposure to Glyphosate and Aminomethylphosphonic Acid (AMPA) with Liver Inflammation and Metabolic Syndrome at Young Adulthood: Findings from the CHAMACOS Study. Environ Health Perspect MEX-195; Gunier Gadotti, C., Oliveira, J., Bender, J., Lima, M., Taques, G., Percio, S., Romano, M., Romano, R. 2023. Prepubertal to adulthood exposure to low doses of glyphosate-based herbicide increases the expression of the Havcr1 (Kim1) biomarker and causes mild kidney alterations. Toxicology and Applied Pharmacology MEX-200; Kiliçgün, H., C. Gürsul, M. Sunar & G. Gökşen. (2013). The Comparative Effects of Genetically Modified Maize and Conventional Maize on Rats. J Clin Anal Med MEX-130; Then, C. y Bauer-Panskus, A., “Possible health impacts of Bt toxins and residues from spraying with complementary herbicides in genetically engineered soybeans and risk assessment as performed by the European Food Safety Authority EFSA,” 2017 MEX-287. 

Mexico cites study after study that the U.S. failed to refute or claimed to refute with outdated evidence. The U.S. claims to refute one recent study of the potential harms from GM Bt corn to organs and tissues by citing U.S. research that is 22 years old, highlighting Mexico’s criticism that much of the U.S. science is out of date. (55)

Mexico challenges the U.S. allegation that its critique of ultra-processed foods is not relevant to the GM maize issue, citing the FOE comments in detail. (61) The U.S. also questions the relevance of animal feeding studies in evaluating risks to human health despite the documented value of such research in the absence of feeding trials with human subjects. (66)

“Mexico considers that the experimental studies conducted in animal models that reveal health effects from the consumption of GM maize allow us to know the potential and possible risks to human health; we reaffirm that Mexico cannot expose its population to these risks, considering the Mexican diet and its high levels of consumption of maize in unprocessed forms.” (68)

“There are animal studies, fed with GM maize, that have evidenced: potential effects on male fertility;81 immunological alterations;82 renal and hepatic toxicity;83 affectations to the digestive system, liver and pancreas84 and biochemical alterations of the blood.”85 Similarly, "Bt toxin protein circulating in the blood of pregnant and non-pregnant women and the blood supply to fetuses" has been found.”86 (69)

Similarly, Mexico shows that the U.S. continues to ignore the risks of glyphosate residues on GM maize. “The United States does not refute the evidence presented by Mexico on the risks of GM maize exposed to this highly hazardous herbicide nor the evidence provided by Mexico to demonstrate the risk to human health from the consumption of GM maize and glyphosate.” (70)

Mexico cites one metastudy, ignored by the U.S., which “compiles 1,108 scientifically rigorous studies that demonstrate the negative effects of glyphosate on health and the environment. Each of these studies evaluates the genotoxic potential of glyphosate and details the diseases or health problems resulting from glyphosate.” (76)

Mexico cites testimony from expert in toxicology and molecular genetics, Dr. Michael Antoniou, "[t]he body of epidemiological literature on the effects of Roundup [Glyphosate] exposure on humans is vast.... Dr. Michael Antoniou explains various risks to human health arising from the accumulation of residues of various pesticides can have adverse health effects, even when each individual pesticide is present at a level considered ‘safe’ by regulatory agencies.” (78,79)

Mexico reiterates why standard risk assessments from the U.S. and international agencies are inadequate for Mexican consumers. “In particular, the CODEX Maximum Residue Limits (MRLs) that identify glyphosate residue limits for corn are not appropriate or relevant to Mexico's unique circumstances. As explained above, the very high levels of consumption of whole corn grain in Mexico, coupled with the fact that corn grain is primarily consumed in unprocessed form as masa nixtamalizada or tortillas, means that global or regional averages for daily consumption - which are based on consumption patterns in other countries - are not applicable to Mexico.” (351)

Mexico goes on to make a long, detailed defense of its GM corn policies as fully legal under the provisions of the USMCA trade agreement, which include provisions for national policies enacted in the public interest if they are carried out in a transparent way and using the least trade-distorting measures available. Mexico also asserts its right exceptional treatment under the USMCA for measures that fulfill its obligations to respect and uphold Indigenous rights and cultures.


Mexico has demonstrated with rigor and evidence that its GM corn restrictions are legal under USMCA, do not significantly impact U.S. corn exporters, and are fully justified by the hundreds of peer-reviewed studies that show evidence of risk to human health and the environment from high levels of consumption of minimally processed GM corn with glyphosate residues. The U.S. has yet to show that its GM corn is safe for Mexicans to eat. 

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