GM: weed and insect problems loom

Herbicide and pesticide resistance are leading to an increased use of these chemicals.

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US farmers planting genetically modified herbicide-tolerant (HT) cotton, soya bean and maize crops have increased the use of herbicides on their farms. This is according to Prof Charles Benbrook of Washington State University, who has analysed statistics provided by the US Department of Agriculture.

About 95% of the soya bean and cotton crop and more than 85% of maize in the US include varieties genetically modified to be herbicide resistant. Prof Benbrook reports in Environmental Sciences Europe that increased herbicide use is related to the spread of glyphosate-resistant weeds. Marketed as Roundup and other trade names, glyphosate is a broad-spectrum herbicide.

“Resistant weeds have become a problem for many farmers reliant on GM crops, and are now driving up the volume of herbicide needed each year by about 25%,” says Prof Benbrook. He adds that the annual increase in the herbicides required to deal with tougher-to-control weeds on crop lands planted to GM cultivars has grown from 682 000kg in 1999 to about 41 million kilograms in 2011. There are now two dozen weeds resistant to glyphosate.

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Many of these are spreading rapidly, and millions of hectares are infested with more than one glyphosate-resistant weed.
Resistant weeds drive up herbicide use by 25% to 50%, and increases farmers’ weed control costs by at least as much.He says the industry’s primary response to the spread of glyphosate-resistant weeds has been to develop new HT varieties resistant to multiple herbicides, including 2,4-D and dicamba. These older phenoxy herbicides pose greater human health and environmental risks than glyphosate, he cautions.

Pesticide use has also increased. During the first six years of commercial use, from 1996 to 2001, HT and Bt (genetically engineered to produce the natural insecticide Bacillus thuringiensis) crops saw a 14 million kilogram, or 2%, reduction in pesticides. However, the spread of insect populations resistant to the Bt toxins in GM maize and cotton has led to an increase in insecticide use. Overall, pesticide use in 2011 was about 20% higher on each hectare planted with a GM crop, compared with pesticide use on those not planted to GM crops.