Genetically engineered crops have provided substantial" environmental and economic benefits to American farmers, but overuse of the technology is threatening to erode the gains, a national science advisory organization said ... in a report. The study was issued by the National Research Council, which is affiliated with the National Academy of Sciences. David E. Ervin, the chairman of the committee that wrote the report, ... warned that farmers were jeopardizing the benefits by planting too many so-called Roundup Ready crops. These crops are genetically engineered to be impervious to the herbicide Roundup, allowing farmers to spray the chemical to kill weeds while leaving the crops unscathed. Overuse of this seductively simple approach to weed control is starting to backfire. Use of Roundup, or its generic equivalent, glyphosate, has skyrocketed to the point that weeds are rapidly becoming resistant to the chemical. That is rendering the technology less useful, requiring farmers to start using additional herbicides, some of them more toxic than glyphosate. One critic, Charles Benbrook, said the conclusion that the crops help farmers might not be true in the future. That is because the report relies mostly on data from the first few years, before prices of the biotech seeds rose sharply and the glyphosate-resistant weeds proliferated.