New GMO Controversy: Are the Herbicides Dangerous?
Corn sold at grocery stories may be genetically modified.
Credit: Corn picture via Shutterstock

Although genetically modified organisms (GMOs) don't appear by themselves to have ill effects on human health, the herbicides used on these crops could be an overlooked health threat, some researchers say in a controversial new opinion piece.

People have been manipulating genes in plants for centuries, but arguing that this means GMOs are safe "misses the point that GM crops are now the agricultural products most heavily treated with herbicides, and that two of these herbicides may pose risks of cancer," Dr. Philip Landrigan, a professor of preventive medicine at Mount Sinai School of Medicine in New York, and Charles Benbrook, a crop and soil scientist at Washington State University, wrote in an opinion article published in the Aug. 20 issue of the New England Journal of Medicine (NEJM).

Most of the corn and soybeans grown in the United States are genetically engineered. This means that products made from these crops are also GM foods, everything from soda to tofu.

Many of today's GM crops have been engineered to be resistant to weed killers, and this has led to an overreliance on these chemicals, Landriganand Benbrooksaid. The authors argued that because some studies have linked cancer risk to the herbicides used on GM crops — in particular, a widely used herbicide called glyphosate (sold under the brand name Roundup) — the United States should reconsider creating labeling requirements for GM foods.

Labeling is essential for "assessing effects of chemical herbicides applied to GM crops," the two researchers wrote. 

However, other experts disagreed with the opinion and said that the majority of studies have failed to find a link between glyphosate and cancer, and that this herbicide is much safer than chemicals used on crops in the past. "The whole [NEJM] article is dubious," said Kent Bradford, a professor of plant science at the University of California, Davis. Bradford noted that one of the authors of the new article (Benbrook) has received funding from and served on the board of the Organic Center, a nonprofit organization that says it brings together "evidence-based science on the health and environmental benefits of organic food and farming."

Landrigan and Benbrook wrote that the emergence of weeds that are resistant to herbicides led farmers to increase their use of these chemicals on crops. And in 2014, the Environmental Protection Agency (EPA) approved the use of a new weed killer called Enlist Duo. This chemical contains glyphosate as well as an herbicide called 2,4-Dichlorophenoxyacetic acid (2,4-D), a component of Agent Orange used in the Vietnam War. (A different component of Agent Orange, called dioxin, has already been linked to cancer.) [7 Medical Myths Even Doctors Believe]

The EPA estimates that the approval of Enlist Duo will result in a 3- to 7-fold increase in the use of 2,4-D in the United States, according to the NEJM article.

Landrigan and Benbrook argue that this approval was based on flawed studies, which were commissioned by the manufacturer and did not examine whether the substance could have effects on people's hormones, or the regulation of human genes.

In addition, earlier this year, the International Agency for Research on Cancer classified glyphosate as a "probable human carcinogen" and 2,4-D as a "possible human carcinogen," the NEJM article says.

This decision on glyphosate was based mostly on research done in animals, with studies finding links between glyphosate and tumors in rodents. Some studies have also suggested that people who work with glyphosate may be at higher risk for non-Hodgkin lymphoma. The decision on 2,4-D was based mostly on studies done in lab dishes and in animals; this research found that 2,4-D could cause oxidative stress, which is thought to increase cancer risk by damaging DNA.

"These developments suggest that GM foods and the herbicides applied to them may pose hazards to human health that were not examined in previous assessments," Landrigana nd Benbrook said.

However, Bradford said that most review studies, including studies from the European Union and the U.S. EPA, have concluded that glyphosatedoes not pose a risk of cancer. "Glyphosate is well known to be a very nontoxic compound," Bradford said.

Bradford also noted that herbicides are used on all crops, not just those that are genetically modified. For example, 2,4-D is used on U.S. wheat crops, which are not genetically modified, he said.

What's more, studies from the U.S. Department of Agriculture have found that herbicides that were used in the past were up to 16 times more damaging to the environment than is glyphosate, Bradford said. "We should be happy that we were able to move to a much less environmentally impacting herbicide," he said.

Margaret Smith, a professor of plant breeding and genetics at Cornell University in New York state, said she agreed with the authors of the NEJM article that use of GM crops resistant to herbicides, and use of glyphosate, have gone up in recent years. "There's no question" that there's been an increase in these factors, Smith said.

But Smith also said that glyphosate has a relatively benign impact on the environment, and that its use replaced more environmentally hazardous products. In addition, the increased use of herbicides on GM crops has meant that farmers don't need to plow their fields as often as a means of killing weeds. This reduction in plowing has environmental benefits, because plowing increases soil erosion and runoff, Smith said.

However, the downside of herbicide use on GM crops is that when agriculturalists use the same pest management tactic for years, they start to see pests (such as weeds) that are resistant.

"As the use of Roundup has become broader and more common, we now see more evolution of weeds that are less susceptible to it," Smith said. "That's a very predictable thing."

The authors wrote that they believe that the EPA should delay the implementation of Enlist Duo until more independent studies on the herbicide are completed.

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