Genetically engineered crops offer lower production costs, fewer pest problems, reduced pesticide use, and better yields, a new report finds. But their effectiveness will be dependent on improving weed-management practices.

“The Impact of Genetically Engineered Crops on Farm Sustainability in the United States” is the first comprehensive assessment of how GE crops affect all US farmers. The examination was funded by the National Research Council, which released the 253-page report today.

“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,” says David Ervin, professor of environmental management and economics from Portland State University in Oregon, who chaired the committee that wrote the report (news release). “However, these benefits are not universal for all farmers.”

Introduced in 1996, GE crops now make up more than 80% of soybeans, corn, and cotton grown in the US, occupying about half the nation’s cropland. These GE crops are designed to be resistant to the herbicide glyphosate, which has fewer adverse environmental effects than most other weedkillers. GE corn and cotton plants also are engineered to produce toxins from Bacillus thuringiensis (Bt), a bacterium that kills pests when it’s digested [corrected, 15/04].

Farmers who grow GE crops use fewer insecticides and herbicides that linger in soil and waterways, and they’re more likely to practice ‘conservation’ tillage, which improves soil quality and water filtration while reducing erosion. Improving water quality, according to the report, may represent the largest single environmental benefit of GE crops.

But weed problems will become more common as they evolve resistance to glyphosate or if weeds less susceptible to glyphosate become established in areas treated exclusively with that. Farmers may also return to toxic herbicides and tillage, which interrupts weed lifecycles but decreases soil quality and increases erosion.

“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 US agriculture and the environment now and in the future,” Ervin says. “Such gaps in our knowledge are preventing a full assessment of the environmental, economic, and other impacts of GE crops on farm sustainability.”

The report recommends more research and funding, both public and private, towards understanding weed-resistance problems and water-quality effects, assessing GE contributions to sustainable agriculture, and supporting ‘public goods’—such as plants that use less water and fewer fertilizers.