Herbicide-resistant weeds: current challenges, new tools

The ongoing evolution of herbicide-resistant weeds is a source of worry in Illinois, said University of Illinois professor of molecular weed science Patrick Tranel. He and research assistant Nick Hausman will be making a presentation on this topic during the 56th annual Agronomy Day at the U of I on August 16.

Recent examples in Illinois include biotypes of waterhemp (Amaranthus tuberculatus), Palmer amaranth (Amaranthus palmeri), and horseweed (also known as marestail or Conyza canadensis) resistant to glyphosate; a waterhemp biotype resistant to HPPD inhibitors; and waterhemp populations/biotypes that display multiple resistance to herbicides spanning several site-of-action groups. Survey data suggest that the majority of waterhemp populations now exhibit multiple-herbicide resistance.

When glyphosate-resistant crops were introduced, their initial success caused many weed-management practitioners to stop worrying about herbicide-resistant weeds. Now the increasing occurrence of glyphosate resistance has caused those concerns to be revived and research to find new weed-management tools to be revitalized.

"Within the next few years, we anticipate that new herbicide-resistant crops will be available," said Tranel. "These likely will include crops with genetically engineered resistance to 2,4-D, dicamba, or HPPD-inhibiting herbicides."

These crops will be stacked with other forms of resistance, such as resistance to glyphosate and/or glufosinate.

"Dow AgroSciences anticipates introducing its Enlist Weed Control System in corn in 2013 with soybean to follow later," Tranel said. "The Enlist system includes metabolic resistance to 2,4-D that will be stacked with glyphosate resistance. Coupled to the Enlist system is a new formulation of 2,4-D."

Monsanto is also developing crops with resistance to synthetic auxin herbicides stacked with glyphosate resistance, but their crops will be resistant to dicamba rather than 2,4-D. They have recently announced that they are on track for a 2014 launch of dicamba-resistant soybean.

Both Syngenta and Bayer are evaluating crops resistant to HPPD inhibitors. Soybean is the most important for the Midwest, but it is not expected to be available for at least two years.

Although these new crops will increase herbicide options for a given crop, the options will not include novel site-of-action chemistries -- they will use old chemistry, possibly with new formulations/variations.

"Most important, weed biotypes already exist that are resistant to these herbicides," cautioned Tranel. "Thus, it would be naive to expect any of these new weed-control tools to solve all of our current weed-resistance problems."

Another new weed-management tool recently announced by Monsanto is BioDirect Technology, which takes a biologically rather than chemically based approach. It is still in the very early stages of development, but it will be interesting to follow because it may eventually yield a novel approach to weed control.

Regardless of how novel the technology is, it will not be immune to resistance evolution.

"One of the things we learned from the Roundup Ready era is how to overuse something that seemingly is almost too good to be true," Tranel said. "If and when we begin adopting new weed-control options, we must not forget this lesson. Any weed-control option must be used wisely and judiciously, and as just one component of an integrated weed management strategy, if its effectiveness is to be preserved."