I started reading the dog food bag the same week I noticed two nearly identical formulas side by side. Both bags said GMO-free on the front. One cost about twice as much per pound. I wanted to know what the extra money bought, and the label refused to tell me.

That question sent me back through the history of a phrase that began in the produce section and the cereal aisle before it ever reached the pet store. Genetically engineered crops arrived in American fields during the 1990s. Federal farm data show that by the early 2000s, more than 90 percent of U.S. corn and soybean acres were planted with genetically engineered seed. Herbicide-tolerant soybeans and insect-resistant corn became the base of countless processed foods, livestock feeds, and eventually pet food formulas.

Consumers who wanted to avoid those ingredients had one early, dependable route: the national organic seal, which took effect in 2002 and banned genetic engineering as part of its standards. Organic pet food came along more slowly because the grain and meat supply chains were separate from human food and costly to verify. A third-party non-GMO verification label launched for human food in 2007, and pet food followed once enough owners started asking the same question at the pet store they had been asking at the grocery store.

People brought the label to the pet aisle because the ingredients were already shared. The corn, soy, wheat, and pea protein in many pet foods came from the same commodity streams as human food. When those streams were largely genetically engineered, a non-GMO claim on a dog food bag offered a way to opt out.

What the label does not say

Most of the animal proteins in pet food are not genetically engineered animals. As of now, only one genetically engineered food animal has been approved in the United States: an Atlantic salmon cleared in 2015. It has not become a common pet food ingredient. The chicken, beef, turkey, and fish in most bowls are not themselves genetically modified.

The genetically engineered ingredients in pet food are almost always plants. Corn, soy, canola, and sugar beets are the most common genetically engineered crops in U.S. agriculture. When a pet food contains corn or soy, the GMO question is real for those ingredients. When a formula is built on chicken and rice with no corn, soy, or canola, the GMO-free label may be covering ground that was never genetically modified in the first place.

A bigger gap sits in animal feed. A chicken raised on genetically engineered corn and soy is not itself genetically engineered, but its feed was. A non-GMO label on a chicken-based pet food can mean two different things. It can mean the plant ingredients in the bag were verified non-GMO, or it can mean the company also traced the feed given to the animals that became the meat. Most non-GMO labels do not specify. The federal organic standard does, because organic livestock must eat organic feed.

Ask a company whether its non-GMO claim includes animal feed, and the answer reveals how deep the verification goes. Some companies clarify that their claim covers plant ingredients only. Others say they trace feed but cannot point to a third-party audit for that feed. The phrase GMO-free, without a standard behind it, lets a company define its own scope.

A rulebook with gaps

No federal rule defines the phrase GMO-free on pet food. The agency that oversees pet food safety and labeling has not issued a standard for non-GMO or GMO-free claims. State feed regulators follow model rules set by a national feed control association, and those model rules do not define the phrase either.

Human food has a disclosure standard that pet food does not. A federal rule requiring disclosure of certain bioengineered ingredients on human food labels took effect in January 2022. Pet food was not part of that rule. Once I learned that the standard covered the granola bars in my pantry but not the kibble in my dog's bowl, I stopped assuming the two aisles followed the same rulebook.

What fills the gap are private certifications. The federal organic seal and the most widely used third-party non-GMO verification program are the two I look for. The organic route is a federal standard that prohibits genetic engineering and requires organic feed for livestock. The third-party program uses supply chain audits and testing to verify ingredients, with action thresholds based on ingredient risk. A bag that says GMO-free without one of those seals may still be accurate, but you are taking the company's word for what the phrase means.

What I look for now

I stopped treating GMO-free as a nutrition claim. It is a supply chain claim. The food still needs to meet the nutritional profile for my dog's life stage, and the ingredient list still carries the real information. A grain-free, potato-heavy formula can be non-GMO and still not be the right fit for every dog.

When I read a bag, I ask four questions.

• Which plant ingredients are in the formula? If corn, soy, canola, or sugar beets appear, the GMO question has substance. If the formula is built on rice, coconut, pumpkin, and meat, the phrase may be doing less work.
• Does the label show a third-party seal? The federal organic seal and the third-party non-GMO verification label are standards with defined requirements. Unverified text on a bag is a marketing choice, not a certification.
• Does the claim address animal feed? If the food is meat-first and the answer covers only the plant ingredients, I know the label covers a narrow slice of the supply chain. That may be fine. I want to know.
• Would I choose this food if the phrase disappeared? A food should earn its place on nutrition, palatability, and how my dog responds to it. The GMO-free claim should sit further down the list.

Where the label might go next

The next decade will complicate this phrase. Gene-edited ingredients are already showing up in pet food research. Unlike older genetic modification, which moved genes between species, gene editing can make small changes within a plant or animal's own genome. Some companies are working on hypoallergenic proteins and cultivated meat for pet food. These products may not be classified as GMOs under current definitions in some countries, which means the label language will need to shift.

I also expect the feed question to become a bigger differentiator. As more shoppers realize that GMO-free does not automatically mean the meat came from animals fed non-GMO feed, some pet food makers will lean into that transparency. Others will stay quiet. Shoppers will keep pushing the label forward, just as we did when it traveled from our cereal boxes to our pet bowls.

For now, I read the back of the bag first, not just the front. I ask about feed. I look for a seal. And I remember that the label tells a story about supply chains, not a guarantee of a shinier coat or a settled stomach. The food itself tells me the rest.