In the summer of 2009, glyphosate-resistant weeds were reported on as many as 14,262 sites on up to 5.4 million acres, and the most recent summary indicates 30,000 sites infested on up to 11.4 million acres, according to Mortensen. In a period of three years, the number of reported sites infested by glyphosate-resistant weeds has increased nine fold, while the maximum infested acreage increased nearly fivefold.

"There is reason to believe this trend will continue into the future," he said. "The cost of forestalling and controlling herbicide-resistant weeds is estimated to cost farmers almost $1 billion each year, at an additional cost of $10-20 per acre."

Mortensen expressed concern about herbicide- and germplasm-development companies responding to the glyphosate-resistance problem by developing a new generation of genetically engineered crops in which glyphosate-resistant cultivars are being engineered to have additional resistance traits introduced into the crop's genome.

"These additional gene inserts will confer resistance to other herbicide active ingredients, including 2,4-D and dicamba," he said. "For a variety of reasons, it is quite likely that such crops will be widely adopted. Disturbingly, that would result in a significant increase of older, higher use-rate herbicides in soybean and cotton production.

"If they are adopted in the way I expect they will be, herbicide use in soybean production would increase by an average of 70 percent in a relatively short time after the release of these new genetically engineered, herbicide-resistant cultivars."

Vapor drift of more toxic herbicides has been implicated in many incidents of crop injury and may have additional impacts on natural vegetation interspersed in agricultural landscapes, Mortensen told lawmakers. Scientists have documented that non-target terrestrial plant injury was 75 to 400 times higher for dicamba and 2,4-D, respectively, than for glyphosate.

Together the herbicide and seed-breeding industries are moving to address the problem of resistance with crops that have been engineered to be resistant to multiple herbicide active ingredients, according to Mortensen. If these new crop introductions occur as reported, we should expect to see herbicide use continue to increase and a significant proportion of those added herbicides will be older, less environmentally benign compounds, he predicted.