development and commercialization of genetic-engineering technology in crops. The discussion turns next to the future agenda of GE-crop applications, including general patterns of crop-trait development, implications for future weed-resistance management, and the potential role of GE crops for biofuels. The penultimate section highlights two subjects of research that the committee believes deserve more resources and effort: water quality and social impacts of GE crops. In closing, we discuss options for strengthening public and private research and development to exploit the potential of genetic-engineering technology to contribute more fully to environmental, economic, and social objectives.

KEY FINDINGS

The evidence shows that the planting of GE crops has largely resulted in less adverse or equivalent effects on the farm environment compared with the conventional non-GE systems that GE crops replaced. A key improvement has been the change to pesticide regimens that apply less pesticide or that use pesticides with lower toxicity to the environment but that have more consistent efficacy than conventional pesticide regimens used on non-GE versions of the crops. In the first phase of use, herbicide-resistant (HR) crops have been associated with an increased use of conservation tillage, in particular no-till methods, that can improve water quality and enhance some soil-quality characteristics. That farmers who practice conservation tillage are more likely to adopt GE crops suggests the two technologies are complementary.

At least one potential environmental risk associated with the first phase of GE crops has surfaced: Some adopters of GE crops rely heavily on a single pesticide to control targeted pests, and this leads to a buildup of pest resistance regardless of whether GE crops or non-GE crops are involved. The governmental regulation of GE Bt crops through refuge requirements seems to have proved effective in delaying buildup of insect resistance with two reported exceptions, which have not had major consequences in the United States. Grower decisions to use repeated applications of particular herbicides to some HR crops have led, in some documented cases, to evolved herbicide-resistance problems and shifts in the weed community. In contrast with Bt-crop refuge requirements, no public or private mechanisms for delaying weed resistance have been extensively implemented. If the herbicide-resistance problem is not addressed soon, farmers may increasingly return to herbicides that were used before the adoption of HR crops. Tillage could increase as a pest-management tactic as well. Such actions could limit some of the environmental and personal safety gains associated with the use of HR crops. The newest HR varieties likely will have tolerance to more than one herbicide, and



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