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, corn is used in livestock feed or corn derivatives appear as sweeteners or additives with little nutritional value.

The first genetically modified corn was released in 1996. Since then, another 290 distinct varieties of GMO corn have been created. In 2024, GMO corn made up an estimated 94% of the corn planted in the US, occupying more than 86 million acres. In fact, genetically modified corn is so ubiquitous in the US that it is considered a high-risk crop under the Non-GMO Project Standard.

Let's take a closer look at this high-risk crop. What are the most common types of corn? Which ones are most likely to be GMOs, and where might they show up at the grocery store?

Types of corn

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."

Today, many 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 falls into these three types:

Field corn is the most commonly grown kind of corn, making up about 99% of the corn grown in the United States, and the most commonly genetically modified corn. 

Most field corn doesn't feed people. Instead, it ends up in livestock feed or converted to ethanol for cars. The small amount of field corn that ends up in food for humans is processed into recognizable ingredients such as corn starch or corn syrup, or hard-to-spot derivatives 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 is what most people think of when they picture corn. Sweet corn is what you buy in the grocery store on the cob, canned and frozen in bags. 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 and GMO varieties comprise an estimated 10-25%.

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. 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. No GMO popcorn is available on the market at this time, and 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 types, 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 traits

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. However, 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.

GMO contamination

GMO seed poses serious threats to native varieties of corn grown in Mexico, where the crop originated. Contamination can have several 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.

Unfortunately, Mexico's attempts to prevent contamination by prohibiting commercial GMO corn cultivation have failed. Contamination by modified and patented DNA is well documented.

Mexico's right to choose

Corn is one of the most important crops in the world, and Mexico is its center of origin. Corn is central to cultural, agronomic and gastronomic life in Mexico, and its production is critical to food security and political stability. Because of the unique genetic inheritance within native maize, GMO contamination is a severe threat.

With so much at stake, Mexico has taken bold action to protect native corn varieties from contamination and to keep GMOs out of the human food supply. In 2020, then-president Lopez Obrador announced his intention to phase out GMO corn imports and glyphosate, the weedkiller most commonly used with GMO crops. The decree caused a major dispute with the United States, a significant producer of GMO corn and one of Mexico's largest trading partners. 

Facing significant pushback from the US, the Mexican government has since softened its position, prohibiting GMO corn from products intended for human consumption but allowing its continued import for livestock feed. Mexico's recently elected president, Claudia Sheinbaum, has pledged to uphold the restrictions, and the dispute is currently working its way through mediation. You can read more about this historic dispute in our article, Mexico Moves to Protect Its Cultural Heritage from GMO Corn.

Looking ahead

The story of GMO corn illustrates how the environment, economy and culture intersect in the food system. Mexico's ongoing 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 a macro version 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.