Scientists Divided on Mexico’s Genetically Modified Corn Ban

More than just a staple crop, corn is at the heart of Mexico’s culinary richness and gastronomic tradition. Here, corn is present in every meal, from chilaquiles at breakfast to tacos at night, resulting in a per capita annual consumption of nearly 200 kilograms.¹ While white corn makes up most of what’s eaten in Mexico today, many small-scale farmers across the country grow a stunning array of native landraces in hues such as deep purple, bright orange, and fiery red for their own consumption, helping preserve the country’s vast maize diversity.

Concerns that transgenic corn—harboring genes inserted from unrelated species—could affect this rich diversity, along with social, ethical, and debated public health considerations, led to a moratorium on its commercial cultivation more than two decades ago.² This year, however, Mexico elevated the ban to the constitutional level, broadening it to include not only transgenic corn but also varieties genetically engineered through other methods, such as CRISPR, which edits genes within the plant’s own genome. The move has drawn mixed reactions from scientists.

“Usually, I consider a ban a very radical decision for governance of [genetically modified organisms], but in the context of maize in Mexico, this might be the best way to go forward in practical terms [to] guarantee that the biodiversity of maize will not be impacted,” said Sarah Agapito-Tenfen, a plant geneticist at the European Network of Scientists for Social and Environmental Responsibility (ENSSER).

But Luis Herrera-Estrella, a plant biotechnologist and genomicist at the Center for Research and Advanced Studies of the National Polytechnic Institute (Cinvestav) and Texas Tech University, disagreed. “[The reform] is closing the door to technology for one of the country’s most important crops, at a time when we’re not developing alternative technologies to replace the potential benefits of transgenics,” he said, referring to uses such as protecting crops from insect pests and herbicides and improving yield.

In Mexico, the debate over genetically modified (GM) corn is long-standing, starting with the 1998 moratorium.^2,3 Although the ban was lifted in the 2000s—allowing limited cultivation, mostly in northern states—legal challenges reestablished restrictions in 2013. More recently, a 2023 presidential decree went further, banning GM corn for human consumption and effectively restricting its import for that purpose. As the world’s top importer of US corn, Mexico faced a formal trade challenge from the United States, which it ultimately lost in late 2024.

In the aftermath, President Claudia Sheinbaum enacted a constitutional reform in March 2025, banning GM corn cultivation and requiring evaluation for all other uses. The reform also commits the government to promoting rural and cultural development to ensure the well-being of farming communities. At its core is a bold constitutional mandate: to safeguard Mexico’s unparalleled corn biodiversity. Whether restricting agricultural biotechnology will achieve that goal, however, remains hotly debated.

Transgene Presence in Native Corn No Longer in Question

Since the early days of the moratorium, a key concern was that transgenes might spread to local landraces. The 2001 report of transgenic DNA in native corn in the southern Mexican state of Oaxaca, therefore, set off alarm bells.⁴ At the time, some researchers challenged the strength of the evidence, fueling a heated debate.^5-8 However, in recent years, mounting evidence from various studies reporting transgene presence in local corn across the country has led to a growing scientific consensus.^9-12 Last year, the Mexican government reported that they detected transgenes in 25 percent of samples collected from 856 corn grain, seed, and flour storage facilities across the country.

June Simpson, a plant molecular geneticist at Cinvestav, led one of the most extensive analyses on transgenes in local landraces.¹¹ The project originally aimed to study the natural genetic diversity across landraces in Mexico. “There are different phenotypes, which look very alike, but when you look at them at the genomic level, they can be very diverse,” she explained. To better understand this diversity and support conservation efforts, her team collected more than 3,000 corn samples from 15 states. Then, they decided to screen for transgenes as well. “We realized, if we’re going to have all these samples and we’re going to process them, it would be very simple just to include the markers for the transgenes within those samples,” Simpson said.

The scientists detected transgenes in corn from all the states they sampled, with frequencies ranging from four to 52.9 percent. They did not find a direct correlation between the presence of transgenes and the number of permissions granted for transgenic corn cultivation in specific states during the lifted ban in the late 2000s and early 2010s, making it unlikely that this period of authorized cultivation accounts for the widespread presence of transgenes across the country.

So, how did transgenes end up in local landraces? Simpson speculated that a major source is corn seeds imported from the US for industrial use, such as food processing. Farmers often do not distinguish between transgenic and nontransgenic corn, which can lead to unintended planting, she noted. Traditional farming practices may spread these seeds further. “In rural communities, they have a very strong tradition of exchanging seeds between the different communities locally,” said Simpson. If transgenic seeds enter the mix, they grow, cross-pollinate, and farmers continue sharing them. Other researchers have suggested that illegal markets of transgenic corn seeds also contributes to this dispersal.¹³

But, while the presence of transgenes in local landraces is no longer debated, their impact remains contentious.

Are Transgenes Endangering Local Landraces?

Although the specific genetic modifications in a GM organism are well characterized, several scientists argue that this does not guarantee a full understanding of their broader consequences—an important consideration in understanding the potential impact of transgene transfer to wild crops. For instance, researchers have documented unintended changes in genetically engineered crops.^14,15 Some evidence also suggests that certain modifications may impose a metabolic cost on the plant.¹⁶ Additionally, scientists have raised concerns about the potential transfer of insect-resistance genes to problematic weeds.¹⁷

Furthermore, it is unclear how one or more transgenes might affect other organisms with which the plant interacts. These genetic modifications are entirely new to the environment and could impact biodiversity, said Ana Wegier, a conservational geneticist at the National Autonomous University of Mexico. Beyond that, she added, many transgenes are designed for specific ecological functions—such as dealing with insects—so they may affect more than just their intended targets. Indeed, while GM crops may pose low lethal risks to most pollinators, scientists have documented indirect effects that warrant further investigation.^18-20

Carlos Blanco, an entomologist at the University of New Mexico, said that transgenes can affect the density of beneficial fauna. However, he noted that in the context of a pest outbreak, farmers would still have to use insecticides, which also harm nontarget organisms and, in some cases, contaminate the soil. Blanco pointed to GM cotton as an example. For decades, the crop was devastated by the tobacco budworm. Now, thanks to GM cotton cultivated in northern Mexico and the southern US, the pest is nearing extinction and the environmental impact of insecticide use has declined, he noted.²¹

Overall, the evidence of GM crops’ effect on biodiversity remains inconclusive.²² As for the specific impact of transgenes on Mexican corn diversity, there is no obvious evidence of significant loss. “At least, the results we have don’t suggest that the transgenes or the transgenic materials are running over the landraces [or] that they’re becoming the predominant genotypes,” said Simpson. And, according to Herrera-Estrella, this is unlikely to occur. He noted that improved hybrid corn varieties developed without transgenic technologies have been crossing with landraces for decades, yet these native varieties have not disappeared.

The real threat, Herrera-Estrella added, lies elsewhere: rural migration and the high costs and low yields associated with cultivating local landraces. Protecting these native crops, he said, requires social programs that directly address those challenges.

Editing the Future—or Choosing Caution?

Transgenic modifications are no longer the only form of genetic engineering. With the advent of new technologies like CRISPR, a wider range of genetic alterations is now possible. The new reform acknowledges it, stating that “[the cultivation of corn] in the national territory must be free of genetic modifications produced using techniques that surpass the natural barriers of reproduction or recombination.” Wegier, who participated as an academic specialist in negotiating the law’s amendment, said it is a positive step that the reform is not limited solely to transgenesis, but also includes all forms of genetic modification through agricultural biotechnology.

But, for Herrera-Estrella, the constitutional change is a drawback. It bans all use of gene editing, a technology that will likely bring a far greater revolution than transgenics, he said. “It shuts down all existing and future possibilities.”

Agapito-Tenfen, meanwhile, does suggest caution about CRISPR, noting that there are very few studies on the effects of its use to edit crops. “The majority of papers that try to navigate the impacts are performed by the people who are developing the technology—and I don’t mean just big biotech, it’s even research institutes—and these people are not trained to do risk assessment,” she said. For her, that is currently one of the limitations in the field: “There is no funding for risk research.”

Finally, Wegier concluded that GM corn is not needed in Mexico. “To me, it’s a false notion to even think we must turn to modern biotechnology as an alternative before exploring all the real options we have—ones that don’t carry the same risks,” she said. “Mexico grows corn from north to south, from the mountains to the coast, under all types of moisture conditions.” For her, the solution to current challenges lies in that rich diversity of native corn, supported by agroecological approaches and farmer-led innovations.

Simpson said that while she generally supports transgenic technology, Mexico’s unique status as the center of corn diversity calls for caution. Still, even if using transgenic corn is not essential, she added, “I don’t think we should completely close the door on it.”

NOTE: The interviews with Wegier, Herrera-Estrella, and Blanco were conducted in Spanish.