With the widespread adoption of genetically engineered crops — accounting for 94 percent of all soybeans and 93 percent of all corn in the U.S. in 2015 — the use of herbicides rose, although insecticide use in GE corn declined, according to a new study published in the journal Science.
The study, led by economist Edward Perry of Kansas State University, relied on a large, farm data set that spans 1998-2011, tracking an average of 5,424 farmers of corn and 5,029 of soybeans. “Unlike previous studies, this new analysis relies on directly observed herbicide usage rates, as well as a statistical model that controls for unobserved factors,” Science said.
The study found that adopters of glyphosate-tolerant soybeans used 28-percent more herbicides than non-adopters. In contrast, farmers who planted glyphosate-tolerant corn reduced their herbicide by 1.8 percent relative to non-adopters. When the usage data was weighted by its environmental impact (or Environmental Impact Quotient), herbicide use in soybeans was flat compared to non-adopters while it declined 9.8 percent with GE corn, “reflecting the relatively lower EIQ values for glyphosate.”
But over time, the study found, farmers who adopted glyphosate-tolerarant crops “used increasingly more herbicides relative to non adopters.”
The study also found an 11.2 percent drop in insecticide use among adopters of insect-resistant Bt corn, relative to growers who did not plant the genetically modified crop. This figure was not affected when the results were weighed by environmental impact, or EIQ. By 2011, 50 percent of those insecticides consisted of seed treatments with neonicotinoids, the study said.
The study also discovered a sharp increase in the use of herbicides other than glyphosate after 2007. “We find clear evidence of increasing herbicide use by glyphosate-tolerant variety adopters over time for both soybeans and maize, a finding that we attribute in part to the emergence of glyphosate weed resistance. No such pattern appears for maize insecticide use over time, consistent with the evidence that non–Bt maize refugia have been broadly effective as a means to prevent the onset of pest resistance,” the authors write.