Since genetically engineered crops were introduced in 1996, at least nine species of weeds in the United States have evolved resistance to glyphosate, a main component in Roundup and other commercial weed killers, according to the report.
The weeds have become resistant to glyphosate largely because of repeated exposure, the assessment found.
"Genetically engineered crops could lose their effectiveness unless farmers also use other proven weed and insect management practices," advises the committee that wrote the report, formally called the Committee on the Impact of Biotechnology on Farm-Level Economics and Sustainability.
Pesticide sprayer spreads glyphosate on a field of soybeans. (Photo courtesy Monsanto)
Genetically engineered crops now constitute more than 80 percent of soybeans, corn, and cotton grown in the United States. In addition to glyphosate resistance these crops are engineered to produce Bacillus thuringiensis (Bt), a bacterium that kills susceptible insect pests.
"Many American farmers are enjoying higher profits due to the widespread use of certain genetically engineered crops and are reducing environmental impacts on and off the farm," said David Ervin, professor of environmental management and economics, Portland State University, Portland, Oregon, and chair of the committee.
"However, these benefits are not universal for all farmers," he said.
Improvements in water quality could prove to be the largest single benefit of genetically engineered crops, the report finds.
Insecticide use has declined since genetically engineered crops were introduced, and farmers who grow genetically engineered crops use fewer insecticides and herbicides that linger in soil and waterways, the report notes.
In addition, farmers who grow GE herbicide-resistant crops till less often to control weeds and are more likely to practice conservation tillage, which improves soil quality and water filtration and reduces erosion.
But no infrastructure exists to track and analyze the effects that genetically engineered crops may have on water quality.
The U.S. Geological Survey, along with other federal and state environmental agencies, should be provided with financial resources to document effects of GE crops on U.S. watersheds, the committee recommends.
The report notes that although two types of insects have developed resistance to Bt, there have been few economic or agronomic consequences from resistance.
"Practices to prevent insects from developing resistance should continue, such as an EPA-mandated strategy that requires farmers to plant a certain amount of conventional plants alongside Bt plants in "refuge" areas," the committee recommends.
Committee Chairman David Ervin (Photo courtesy National Research Council)
"And as more GE traits are developed and incorporated into a larger variety of crops, it's increasingly essential that we gain a better understanding of how genetic engineering technology will affect U.S. agriculture and the environment now and in the future," Ervin said.
"Such gaps in our knowledge are preventing a full assessment of the environmental, economic, and other impacts of GE crops on farm sustainability," he said.
Still, the committee was confident in its recommendation that farmers who grow GE herbicide-resistant crops "should not rely exclusively on glyphosate and need to incorporate a range of weed management practices, including using other herbicide mixes."
The higher costs associated with genetically engineered seeds are not always offset financially by lower production costs or higher yields, the report notes. For example, farmers in areas with fewer weed and pest problems may not have as much improvement in terms of reducing crop losses as farmers with more weeds and pests.
Even so, the report cites studies showing that farmers value the greater flexibility in pesticide spraying that genetically engineered crops provide and the increased safety for workers from less exposure to harmful pesticides.
Farmers have not been negatively affected by the proprietary terms involved in patent-protected genetically engineered seeds, the report says.
But some farmers have expressed concern that consolidation of the U.S. seed market will make it harder to buy conventional seeds or those that have only specific genetically engineered traits.
The committee recommends that research institutions receive government support to develop genetically engineered traits that could deliver valuable public benefits but provide little market incentive for the private sector to develop. These might be plants that decrease the likelihood of off-farm water pollution or plants that are resilient to changing climate conditions.
The committee recommends that intellectual property that has been patented in developing major crops should be made available for these purposes.
The study was funded by the National Research Council, an independent, nonprofit institution that provides science, technology, and health policy advice under an 1863 congressional charter.
Committee members, who serve pro bono as volunteers, are chosen by the National Academies for each study based on their expertise and experience and must satisfy the Academies' conflict-of-interest standards. The resulting consensus reports undergo external peer review before completion.
Click here to access the full report, "Impact of Genetically Engineered Crops on Farm Sustainability in the United States."
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