Fighting ‘super weeds’

WALL LAKE-What’s the most important way to make money in row-crop agriculture? Weed control, said Mike Owen, an Iowa State University Extension weed specialist, who is urging farmers to take a new approach to herbicide resistance.

“In my experience, weeds are the most consistent, widespread challenge in the pest complex,” Owen said, “more so than insects or disease pressure.

“Herbicides will continue to be part of the solution, but we’re going to have to use them in a different way to manage herbicide resistance.”

Resistance is widespread in Iowa and it’s spreading at an increasing rate, Owen said, who spoke at a recent Extension meeting in Wall Lake. Even more disconcerting is the issue of multiple resistance.

Ongoing studies that began in 2011 show that approximately 29 percent of the waterhemp populations that ISU researchers have studied across Iowa have demonstrated resistance to three types of herbicides – glyphosate, atrazine and ALS herbicides.

In addition, 4 percent of waterhemp populations in ISU’s studies have shown resistance to five out of five of the most important herbicide classes.

Fool’s paradise?

It’s important to note that herbicides do not cause weeds to evolve resistance,” Owens aid. “It’s how the herbicides are used that causes weeds to evolve resistance.

“Weeds will inevitably respond to selection pressure. If you push them one way, they’re going to push back.”

Resistance can take many forms, from metabolic resistance, in which weeds can break down many classes of herbicides quickly, to elevated enzyme resistance, in which weeds elevate the amount of the enzyme targeted by a specific herbicide, resulting in more enzyme than the herbicide can impact.

Resistance is nothing new, added Owen, who said that concerns about herbicide-resistant weeds predate glyphosate by more than 50 years.

What’s different this time is that there are no silver bullets on the horizon. When resistance to triazine herbicides, which include atrazine developed many years ago, the debut of group 2 herbicides like Pursuit and Classic provided an effective solution, said Owen. But now, more than 112 weeds have developed resistance to triazines.

It has been 25 years since crop protection companies have developed a new site of action to combat weeds, said Owen. The last breakthrough included the HPPD-inhibiting herbicides like Callisto and Laudis. “When you’ve got herbicides that work, you’ve got to protect this technology, because new solutions are few and far between.”

It was easy to overlook this fact after Roundup Ready technology was introduced in the mid-1990s.

“We were living in a fool’s paradise and pushed the system pretty hard,” Owen said, “but weeds will inevitably respond to selection pressure.” There are 150 million acres of Roundup Ready crops planted in the United States each year.

Among Iowa’s 24 million acres of row crops – 14 million acres of corn and 10 million acres of soybeans – there are still approximately 8 million acres that only receive glyphosate, Owen said, citing a recent survey.

“Weeds’ genetic flexibility means that weeds will get around a specific control method,” Owen said, “if that’s the only control method you use.”