A new study says that the use of herbicides in the production of three genetically modified herbicide-tolerant crops, cotton, soybeans and corn, has gone up rather than down. The counterintuitive estimate is based on an analysis of publicly available data from the U.S. Department of Agriculture's National Agriculture Statistics Service. 

Washington State University professor Charles Benbrook says this estimate is the first peer-reviewed estimate of the impacts of genetically engineered herbicide-resistant crops on pesticide use. Herbicide-resistant and insect-resistant crops have been terrific commercial successes and Benbrook looked at herbicide-resistant corn, soybeans, and cotton; Bacillus thuringiensis (Bt) corn targeting the European corn borer; Bt corn for corn rootworms; and Bt cotton for Lepidopteron insects.

In the paper, Benbrook writes that the emergence and spread of glyphosate-resistant weeds is strongly correlated with the upward trajectory in herbicide use. Marketed as Roundup and other trade names, glyphosate is a broad-spectrum systemic herbicide used to kill weeds. Approximately 95 percent of soybean and cotton acres, and over 85 percent of corn, are planted to varieties genetically modified to be herbicide resistant.

His model determined that herbicide-resistant crop technology has led to a 527 million pound increase in herbicide use in the United States between 1996 and 2011, while Bt crops have reduced insecticide applications by 123 million pounds. Overall, pesticide use increased by an estimated 404 million pounds, about 7%.

"Resistant weeds have become a major problem for many farmers reliant on GE crops, and are now driving up the volume of herbicide needed each year by about 25 percent," Benbrook said in a statement. The annual increase in the herbicides required to deal with tougher-to-control weeds on cropland planted to GE cultivars has grown from 1.5 million pounds in 1999 to about 90 million pounds in 2011. 

Herbicide-tolerant crops worked well in the first few years of use, Benbrook's estimate shows, but over-reliance may have led to shifts in weed communities and the spread of resistant weeds that force farmers to increase herbicide application rates (especially glyphosate), spray more often, and add new herbicides that work through an alternate mode of action into their spray programs.

He writes that if trends from the early days of GM products continue and genetically engineered forms of corn and soybeans tolerant of 2,4-D are approved, the volume of 2,4-D sprayed could drive herbicide usage upward by as much as 50%. The magnitude of increases in herbicide use on herbicide-resistant hectares has dwarfed the reduction in insecticide use on Bt crops, he says.

Citation: Charles M Benbrook, 'Impacts of genetically engineered crops on pesticide use in the U.S. -- the first sixteen years', Environmental Sciences Europe 2012, 24:24 28 September 28, 2012  doi:10.1186/2190-4715-24-24 (free to read)