By Graciela Vizcay Gomez
"The fact that we have 14 years of farm-level data from farmers across the US makes this study very special," Ciliberto said. "We have replicated the observations of the same farmers and we can see when they adopted the GM seeds and how that changed the use of chemicals."
Since 2008, genetically modified crops have accounted for more than 80 percent of the corn and soybean crops planted. American corn seeds are engineered with two genes: one kills insects that eat the seed, and one allows the seed to tolerate glyphosate, a herbicide commonly used in herbicides like Roundup. Soy is modified with only one glyphosate resistant gene.
Unsurprisingly, corn growers using the insect-resistant seeds used significantly less insecticides - about 11.2 percent less - than farmers who didn't use GM corn. Corn growers also used 1.3 percent less herbicide over a 13-year period.
In soybean crops, by contrast, they saw a significant increase in herbicide use, when they adopted genetically modified crops using 28 percent more herbicides than non-adopters.
Ciliberto attributes this increase to the proliferation of glyphosate-resistant weeds.
Commentators and other peer-reviewed studies have even claimed that the increase in pesticide use in GM crops has risen much higher, since the 1998-2011 data was reviewed in this new study from the University of Virginia. The period from 2011-2016 is when glyphosate resistant weeds have become a major economic problem for farmers in the United States based on their increased use and therefore money spent on cutting pesticides for their bottom line. .
"In the beginning, there was a reduction in herbicide use, but over time the use of chemicals increased because farmers had to add new chemicals because weeds develop resistance to glyphosate," Ciliberto said.
Corn growers, however, have not faced the same level of resistance, in part because they did not adopt genetically modified crops as quickly as their counterparts in the soybean industry. However, the study found evidence that both corn and soybean growers increased herbicide use in the last five years of the study, indicating that weed resistance is a growing problem for both groups.
From 2006 to 2011, the percentage of hectares sprayed with glyphosate only decreased from more than 70 percent to 41 percent for soybean farmers and from more than 40 percent to 19 percent for corn producers. The decline was due to farmers having to resort to combining glyphosate herbicides with other chemicals because glyphosate resistant weeds became more common.
"Evidence suggests that weeds are becoming more resistant and farmers are forced to use additional chemicals, and more of them," Ciliberto said.
The insects do not appear to have developed similar resistance, in part because federal regulations require farmers to have a "safe haven" in their fields that is free of genetically modified crops. Insects and worms in shelters have no need to develop resistance, and because they interact and reproduce with insects in other parts of the field, they help prevent the development of resistance genes.
Despite the decline in insecticide use, the continued growth in herbicide use poses a major environmental problem because large doses of the chemicals can damage biodiversity and increase water and air pollution.
Ciliberto and his colleagues measured the global environmental impact of changes in chemical use that have resulted from the adoption of genetically modified crops, using a measure called the environmental impact coefficient, or EIQ, to account for the impact of products. chemicals' on farm workers, consumers and the environment. Comparing adopters to non-adopters, they found little change in the impact on farm workers and consumers. However, the adoption of genetically modified soy correlated with a huge negative impact on the environment as increased use of herbicides also increased the pollution of local ecosystems.
Further investigation of the Environmental Impact Coefficient (EIQ) finding is being requested by independent scientists in the US and Europe using the even more accurate Pesticide Hazard Tool (Prime).
Overall, Ciliberto said she was surprised by the degree to which herbicide use had increased and concerned about the possible environmental impact.
"I did not expect to see such a strong pattern," he concluded.
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