The largest, most comprehensive analysis of genetically modified crops suggest the practice has led to a reduction in insecticide use but a spike in the employment of herbicides.

A team of researchers at the University of Virginia, led by economist Federico Ciliberto, compiled and analyzed the growing methods of 5,000 soybean and 5,000 maize farmers in the United States from 1998 to 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 in a news release. "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."

Researchers found that the adoption of genetically engineered crops among maize growers has slightly reduced the use of both herbicides and insecticides.

Farmers who grow GM soybeans, however, are now using 28 percent more herbicides than soybean growers who haven't adopted genetically modified crops.

"I did not expect to see such a strong pattern," Ciliberto said.

Over time, weeds on GM soybean farms have developed a resistance to herbicides, forcing farmers to use more and more herbicides to achieve the same weed-killing effect. The increasing use of herbicides is bad news for the environment, researchers say.

Intense herbicide spraying can leach poisonous chemicals into surrounding ecosystems, reducing biodiversity and contributing to air and water pollution.

Ciliberto and his colleagues calculated the environmental impact of increased herbicide use on farm workers, consumers and the environment. Though the adoption of GM crops has had little effect on workers and consumers, the environment is paying a price.

Soybean growers were much quicker to adopt GM seeds, which researchers believe explains why maize farmers haven't yet seen the same levels of herbicide resistance on their farms. In time, herbicide use on maize farms may look similar to use on soybean farms.

The new research was published this week in the journal Science Advances.