"Specifically, several companies are actively developing crops that can resist glyphosate, 2, 4-D and Dicamba herbicides," said Mortensen. "Such genetic manipulation makes it possible to use herbicides on these crops that previously would have killed or injured them. What is more troubling is that 2,4-D and Dicamba are older and less environmentally friendly."

Egan said there are several problems with the treadmill response. First, weeds will eventually evolve combined resistance to Dicamba, 2, 4-D and glyphosate herbicides. Globally, there are already many examples of weeds simultaneously resistant to two or more herbicides.

Increased use of 2, 4-D and Dicamba applied over the growing corn and soybean means much more of these herbicides will be applied at a time of year when many sensitive crops like tomato and grapes are most vulnerable to injury. Such injury results when these herbicides move from the targeted field during or following an application.

Overuse of chemical weed killers may increase chances that farmers will use the herbicide during inappropriate or non-recommended weather conditions, leading to herbicides drifting from the targeted area and killing or harming other plants and crops.

Egan also said that if farms become too reliant on herbicides, farmers will find it more difficult to use integrated weed management approaches.

Integrated weed management includes planting cover crops, rotating crops and using mechanical weed control methods. Farmers can use herbicides in this management approach, but must use them in a targeted, judicious fashion.

The researchers, who also worked with Bruce D. Maxwell, professor of land resources and environmental sciences at Montana State University; Matthew R. Ryan a post-doctoral student at Penn State; and Richard G. Smith, assistant professor of agroecology at the University of New Hampshire; said that in previous studies, integrated weed management had lowered herbicide use by as much as 94 percent while maintaining profit margins for the operations.

"Integrated weed management is really the path forward," said Egan. "We believe these methods can be implemented, and we already have a lot of show that they're effective and straight forward to incorporate."