Herbicide resistance reaching critical mass

By Nat Williams

Weeds have been victorious in so many battles against herbicides, it may be time to say they’re winning the war.

Waterhemp and Palmer amaranth lead the way, with both exhibiting resistance to numerous modes of action. But it gets worse. Scientists at the University of Illinois（“U of I ”） have discovered strains of waterhemp that have developed resistance to chemicals not even applied to crops.

Weeds including waterhemp infiltrate a soybean field in southern Illinois. Scientists at the University of Illinois have discovered waterhemp plants that exhibit defenses against a chemical formulation never used, an alarming sign that herbicide resistance is becoming more prevalent.    Photo by Nat Williams

Resistance usually comes as a defensive response to application of specific herbicides. Agronomists have long warned that reliance on one mode of action over a long period is unwise because some weeds will eventually become resistant to the effects of the herbicide.

But Dean Reichers and other scientists at the U of I have discovered a much more troubling phenomenon. They examined how multiple-resistance waterhemp found in McLean County detoxifies the chemical designed to kill it, such as that in the products Callisto and Impact. They introduced a formulation including HPPD inhibitors that is not used on corn, and the weed species even exhibited defenses against it.

“We knew this chemical kills corn, so obviously they can’t be used in a cropping situation,” Reichers said. “We thought there is no way waterhemp can detoxify this, but it did. The question was, how is it doing this? This strategy is new and novel.”

According to the International Herbicide-Resistant Weed Database, weeds have evolved resistance to 21 of the 31 known herbicide sites of action and to 164 different herbicides. Herbicide-resistant weeds have been reported in 96 crops in 71 countries.

While the U of I research is alarming, Bryan Young of Purdue University is not surprised that herbicides are increasingly becoming ineffective as a weed-control measure. It’s a progression he and other weed scientists have observed for decades.

“When you use a herbicide, the most common way it doesn’t injure the crop is that crop has enzyme systems that might play a role in detoxifying herbicide,” Young said. “That same enzyme system may be found in the weed species. They just don’t have the activity of that enzyme to degrade it to the non-toxic form, so it ends up dying.

“It’s just a matter of time before that enzyme system gets enhanced so you have more of it because it allows for survival. Those that have a little bit faster metabolism of that herbicide are the ones that survive — enzyme systems that not only metabolize, say, 2, 4-D but also a bleaching agent like Callisto or ALS-inhibiting herbicides like Pursuit, Raptor and Accent. That might also be important for metabolizing herbicides that haven’t commercialized yet.”

Young doesn’t believe the development of genetically modified cash crops in systems such as Roundup Ready or Liberty Link is in any way responsible for the increase in herbicide-resistance weeds.

“It would have happened anyhow and probably a little bit faster,” he said. “Glyphosate worked pretty well for 10 to 15 years before you saw major problems with resistance. If you had to rely on older chemistry for 10 to 15 years without an opportunity to spray glyphosate, we’d be in a lot worse position today.”

Still, the U of I discovery does not bode well for the future efficacy of herbicides in areas where resistant weeds such as waterhemp and Palmer amaranth have thrived. Waterhemp has exhibited resistance to a total of seven modes of action, and Palmer amaranth is even more accomplished.

“We’ve been taught some pretty hard lessons that we can’t just rely on chemicals, because we’re running out of herbicides,” Reichers said. “That includes soil- applied and post-emergence formulations. Now it even includes chemicals never sprayed in a field.”

Fortunately, there are other options that show increasing effectiveness, including mechanical control. Weed-seed killers such as the Harrington Seed Destructor have been promising in eliminating weed seeds gathered and crushed during harvest.

Cultivation is another method that could be successful in some operations. And the use of cover crops is often a good means to combat troublesome weeds.