Over-reliance on glyphosate-type herbicides for weed control on U.S. farms has created a dramatic increase in the number of genetically-resistant weeds, according to a team of agricultural researchers, who say the solution lies in an integrated weed management program.

"I'm deeply concerned when I see figures that herbicide use could double in the next decade," said David Mortensen, professor of weed ecology at Penn State.

Since the mid-1990s, agricultural seed companies developed and marketed seeds that were genetically modified to resist herbicides such as Roundup -- glyphosate -- as a more flexible way to manage weeds, Mortensen said. About 95 percent of the current soybean crop is modified by inserting herbicide-resistant genes into the plants.

"We do understand why farmers would use the glyphosate and glyphosate-resistant crop package," Mortensen said. "It is simple and relatively cheap, but we have to think about the long-term consequences."

The researchers said that increased use of herbicides is leading to more species of weeds that also are resistant to the chemicals.

They report their findings in the current issue of BioScience, noting that 21 different weed species have evolved resistance to several glyphosate herbicides, 75 percent of which have been documented since 2005, despite company-sponsored research that the resistance would not occur.

"Several species have developed amazing biochemical ways to resist the effects of the herbicide," said J. Franklin Egan, doctoral student in ecology, Penn State. "If weed problems are addressed just with herbicides, evolution will win."

One way the weeds develop resistance is to make an enzyme that is insensitive to the herbicide, but still maintains cellular function, Egan said. Weeds have also developed ways for the plant to move the herbicide away from targeted enzymes.

"For instance, glyphosate-resistant strains of Conyza canadensis -- horseweed -- sequester glyphosate in leaf tissues that are exposed to an herbicide spray so that the glyphosate can be slowly translocated throughout the plant at nontoxic concentrations," Egan said. "To the horseweed, this controlled translocation process means the difference between taking many shots of whiskey on an empty stomach versus sipping wine with a meal."

In response to the increasing number of weeds resistant to current applications, companies are developing new generations of seeds genetically modified to resist multiple herbicides. This continual insertion of more genes into crops is not a sustainable solution to herbicide resistance, according to the researchers. They add that companies are creating a genetic modification treadmill similar to the pesticide treadmill experienced in the mid-20th century, when companies produced increasingly more toxic substances to manage pests resistant to pesticides.