American researchers have completed the longest study of popular agricultural genetically modified organisms (GMOs) to date. The 14-year study found that GMO crops increased the use of herbicides on American farms by 28 percent.

The study examined data from 5,000 soybean and 5,000 maize farmers in the United States. The GMO soybean and maize farms each comprised two GMO varieties – one pest-resistant variety, and one herbicide-tolerant variety.

The researchers found that while the use of insecticides decreased by about 11 percent on GMO farms due to the pest-resistant varieties of the crops themselves, use of herbicides, mainly glyphosate marketed by Monsanto as Roundup, increased by more than twice as much due to herbicide-resistant GMO seeds.

“In the beginning, there was a reduction in herbicide use,” said University of Virginia economist Federico Ciliberto, who led the research team, in a statement. “But over time the use of chemicals increased because farmers were having to add new chemicals as weeds developed a resistance to glyphosate.”

The groundbreaking study is particularly interesting given its long duration. The study examined data on GMO crops beginning in 1998 and spanning until 2011.

“The fact that we have 14 years of farm-level data from farmers all over the U.S. makes this study very special,” Ciliberto said. “We have repeated observations of the same farmers and can see when they adopted genetically modified seeds and how that changed their use of chemicals.”

While the decreased use of pesticides is an advantage of GMO crops, the overwhelming increase of glyphosate use with GMO crops is worrying. The World Health Organization declared in 2015 that glyphosate “probably carcinogenic to humans” after extensive research by the organization’s cancer arm.

“The widespread adoption of GMO corn and soybeans has led to an explosion in the use of glyphosate – a main ingredient in Monsanto’s Roundup and Dow’s Enlist Duo,” said Ken Cook, president and co-founder of Environmental Working Group, at the time of WHO's announcement. “Consumers have the right to know how their food is grown and whether their food dollars are driving up the use of a probable carcinogen.”

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Tractor spraying soybean farm image via Shutterstock