Corn is a staple food for a third of the human population and one of the most commonly grown grains in the world. It's also prominent in many Americans' diets — but not as a nutritious staple. In the US food supply, corn is used in livestock feed or corn derivatives appear as sweeteners or other additives with little nutritional value.

The first genetically modified corn was released in 1996. Since then, hundreds more have been developed. One biotech industry website names 307 distinct varieties of GMO corn, though the total number is likely much higher. In 2024, GMO corn made up an estimated 94% of the corn planted in the US, occupying more than 86 million acres. In fact, GMO varieties are so ubiquitous in the US that corn is considered a high-risk crop under the Non-GMO Project Standard.

Learn more about Understanding Risk Status

The bottom line is that corn is one of the most commonly used ingredients and one of the most impactful GMO crops. Let's dive into the different types of GMO corn — and where they might show up at the grocery store.

Which corn varieties are GMOs?

Corn is part of a family of cereal grain domesticated in Mexico close to 9,000 years ago. Indigenous Taino people called it mahiz, meaning "source of life" in the local dialect, from which we get the modern term, "maize."

Corn varieties can be grouped by the structure of the kernel's interior, which impacts how corn can be used. Most of the corn that is grown and consumed in North America is one of these three types: field corn (aka dent corn), sweet corn or flint corn.

Field corn

This is the most commonly grown kind of corn, making up about 99% of the corn grown in the United States. It is also the type of corn most likely to be genetically modified. 

Most field corn doesn't actually feed people. Instead, it is used in livestock feed or converted to ethanol for cars. The 10% of field corn that does make it into the human food supply is processed into corn derivatives such as starch or syrup, or harder-to-spot ingredients such as citric acid, cellulose, maltodextrin, flavorings and some vitamins.

Field corn is also called "dent corn" because as the corn dries, the soft starch inside the kernel shrinks, giving the kernels a dented appearance. In Italy, dent corn is called "dente di cavallo" or "horse's tooth" because the kernel resembles (you guessed it) a horse's teeth. 

Sweet corn

Sweet corn is what you buy in the grocery store on the cob, canned and frozen in bags — it's what most people think of when they picture corn. Sweet corn emerged from a natural mutation that prevents the sugars from turning into starch. It is harvested early when the kernels still contain a lot of moisture and sugar.

In 2011, Monsanto (now owned by Bayer) introduced the first GMO sweet corn, engineered to resist weedkillers and produce its own insecticide. Sweet corn makes up comparatively little of total corn production, growing on less than half a million acres of US farmland. GMO varieties comprise an estimated 10-25%. However, GMO sweet corn has been spotted recently in some grocery stores in the US and Canada, where it may be identified as "bioengineered" under the USDA's GMO labeling law.

What Is Bioengineered Food?

Flint corn

Flint corn has a hard outer layer that protects the soft endosperm, making the kernels "hard as flint," hence the name. Flint corn makes up very little of all corn grown in the US, and is unlikely to be genetically modified. It has a high nutritional value and, when dried, is used for corn meal, masa, polenta and grits.

Popcorn is a type of flint corn. However, popcorn is not considered a high-risk crop under the Non-GMO Project Standard for two reasons: 

1. There is no GMO popcorn commercially available at this time. 
2. Popcorn has a natural immunity from GMO contamination. 

For more on popcorn, read Will Biotech 'POP' Organic Corn's Best Defense Against GMO Pollen?

Additionally, some seed-saving resources list more corn varieties, such as flour corn, pod corn, heirloom corn, decorative corn, and more. Most of these are niche crops that make up very little (<1% combined) of corn acreage.

GMO corn traits: What are Bt and HT crops?

The most common traits engineered into genetically modified corn are herbicide tolerance and insect resistance. An early promise of GMOs claimed that growing them would reduce the application of chemical pesticides. However, that has been disproven, as we'll explore below.

Herbicide-tolerant (HT) corn is immune to weedkillers such as glyphosate, the active ingredient in Roundup. Farmers who grow HT corn can spray Roundup directly on their crops without harming the corn. In their first 20 years on the market, HT crops led to a 15-fold increase in herbicide use and contributed to the rise of herbicide-resistant weeds. 

Corn that is modified for insect resistance is known as "Bt corn," after the toxin Bacillus thuringiensis. Bt corn produces insecticide in every cell in the corn plant, poisoning insect pests that eat the corn. Because the insecticide is inside the cells, it can’t be washed away. 

An estimated 83% of the corn grown in North America is genetically modified with both HT and Bt traits. 

Corn has also been engineered for other, less common traits, such as improved photosynthesis, drought resistance, or improved performance in livestock feed or ethanol.

We're currently monitoring the development of a new generation of genetically modified organisms created using emerging gene editing techniques such as CRISPR, TALEN, ZFN, ODM and RNAi. The Non-GMO Project contributed to a research paper on these "New GMOs," which found that two of the three new GMO crops currently in commercial production are corn varieties engineered for herbicide tolerance and pest resistance; GMO developers are also working on an additional ten new GMO corn varieties promising a range of traits. While new GMO crops are still GMOs according to international definitions, they don't face the same regulatory hurdles as traditional, transgenic GMOs, making them more likely to enter the supply chain unlabeled.  

The consequences of GMO contamination in corn

Corn is, to be blunt, a promiscuous crop. Each stalk generates up to a billion pollen grains, which can drift 1/2 mile away on a gentle breeze. Insects and human activity increase the pollination radius. In favorable conditions, corn varieties may cross-pollinate generously with other stalks in the region.

When pollination moves genetically modified DNA to a non-GMO corn plant, it results in a contamination event. Contamination can have serious negative impacts, including the following:

• Financial risk — A farmer whose organic or non-GMO crops are contaminated can lose market access, causing economic hardship
• Legal risk — Most GMO seed is patented, and the farmer whose crop was compromised is potentially liable for contamination events. 
• Loss of biodiversity — Contamination from GMO seeds or pollen can change the characteristics of a non-GMO crop, impacting a crop's suitability for food, industrial or breeding purposes.

The issue of GMO contamination is particularly important in Mexico, the birthplace of corn. Unique native corn varieties and their wild relatives can still be found in Mexico today. Cultivating and stewarding non-GMO corn provides plant breeders with the tools they need to develop climate-smart food crops to feed a growing population. Unfortunately, contamination of non-GMO corn by modified and patented DNA is already well documented. 

Protecting Mexican people and land from GMOs

Corn has undeniable global significance as a staple food crop. However, in Mexico, it is uniquely foundational to cultural, agronomic and gastronomic life. Corn was first discovered and domesticated in this region, and native varieties are still cultivated there today. Mexican people consume roughly ten times as much corn as Americans, making it essential to political stability and public health.

Recently, Mexico undertook bold actions to protect native, non-GMO corn varieties from contamination by GMOs and to safeguard citizens from the potential health impacts of GMO corn and related chemical herbicides.

In 2020, then-President López Obrador announced his intention to phase out GMO corn imports and glyphosate, the weedkiller most commonly used with GMO crops. After intense pushback from the US — a major GMO corn producer and one of Mexico's largest trading partners — the Mexican government ultimately softened its position to exclude GMO corn from products intended for human consumption. 

In December 2024, a mediation panel convened under the USMCA trade agreement ruled in favor of the US, effectively stating that the States can compel Mexico to accept GMO corn for tortillas and other human food. While deeply disappointed, the Mexican government agreed to comply with the ruling. 

However, Mexico's desire to protect its environment, culture and the wellbeing of its citizens continues — at least to the degree permitted by the trade panel. Earlier this year, the government amended its constitution to prohibit the cultivation of genetically modified corn seeds. 

What can the past tell us about the future?

The history of GMO corn illuminates the intersection of environmental, economic and cultural forces in the food system. Mexico's dispute with the United States over GMO corn imports is more than just a trade disagreement. It highlights the tension between corporate lobbyists and national autonomy, industrial agriculture and agroecology. It is an international embodiment of the struggle over your right to choose whether or not to consume GMOs. 

With 94% of US-grown corn coming from GMO seed, it might feel impossible to avoid corn derivatives – but that’s why the Non-GMO Project was created. Under the Non-GMO Project Standard, products that contain corn as a major or minor ingredient face special scrutiny, including testing, ingredient tracing or segregation. Together, we can help preserve the precious 6% of non-GMO corn acreage in the US, ensuring that future generations can access a more natural food supply.

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