"Genetic engineering makes it possible to take a crop that was formerly susceptible to glyphosate and genetically transform it to be resistant to the plant-killing effects of the herbicide."

The adoption and widespread use of genetically engineered, herbicide-resistant crops has greatly changed how farmers manage weeds, enabling them to rely solely on a single-tactic approach to weed management (application of glyphosate). Unfortunately, Mortensen noted, this approach has resulted in an unintended, but not unexpected, problem — a dramatic rise in the number of weed species that are resistant to glyphosate and a resulting decline in the effectiveness of glyphosate as a weed-management tool.

"During the period since the introduction of glyphosate-resistant crops, the number of weedy plant species that have evolved resistance to glyphosate has increased dramatically, from zero in 1995 to 19 in June of 2010," Mortensen said.

This list includes many of the most problematic weed species, such as common ragweed, horseweed, johnsongrass and several of the most common pigweeds — many of which are geographically widespread.

"In practice, the problem of glyphosate resistance goes far beyond a species count," Mortensen said. "More important, perhaps, is the increase in acreage infested with glyphosate-resistant weeds. The reported extent of infestation in the United States has increased dramatically since just November of 2007, when glyphosate-resistant populations of eight weed species were reported on no more than 3,251 sites covering up to 2.4 million acres."