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Fighting ‘super weeds’

Tips for managing herbicide resistance

January 6, 2013

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."

Article Photos

-Messenger photo by Darcy Dougherty Maulsby
Mike Owen, an ISU weed specialist, asks for a show of hands among producers who have fields with zero weed problems. He spoke Dec. 19 on managing against resistant weeds in Wall Lake.

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.

Fact Box

Rethinking weed control

It's vital to take a new approach to weed management, said Mike Owen, an Iowa State University Extension weed specialist, who urges growers to:

Use more diverse weed management solutions. Look for ways to rotate effective herbicides with various sites of action to "upset the ecological apple cart," he said. "The more diversity you can bring to your weed control program, the better off you'll be."

Target weed control solutions. Just as growers tailor specific hybrids and fertility programs to their acres, it pays to use field-by-field weed management strategies, since certain fields may require extra care.

Pay attention to detail. While effective rates and application timing are critical, spraying additives are also important in some cases, said Bob Hartzler, an ISU Extension weed specialist and professor of agronomy. When tank-mixing glyphosate with Cobra or Laudis, for example, crop oil concentrate is required for the mixture to be most effective.

Rethink tillage. Tillage has important implications for herbicide resistance, Owen said. As tillage systems have changed in the last 20 to 30 years to include more conservation and no-till systems, marestail and common waterhemp have become more challenging.

While tillage is not a good solution on highly erodible land, it may be useful in other settings, said Owen, who noted that some growers are using a rotary hoe designed for high-residue areas.

Realize that little things can make a big difference. One weed control strategy by itself may not provide 100 percent, 80 percent or even 50 percent weed control, but lots of "little hammers" used together can pay off in the long run, Owen said. Also, put more emphasis on return on investment rather than cost when evaluating a complete system of pre-emerge and post-emerge crop protection products, he added.

"In most cases," Owen said, "we're seeing a higher ROI with the more costly programs, which makes the higher investment worthwhile."

Herbicide resistance is one of the biggest issues facing farmers in 2013, Owen said, who encourages growers to consider all the weed control strategies available.

"The evolution of herbicide resistance is not a herbicide problem - it's a management issue.

"If you can learn to manage weeds in general more effectively, you'll also manage herbicide-resistant weeds better."

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."



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