Sep 17, 2014
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Time for new thinking as tough weeds challenge glyphosate revolution

Farmers, field agronomists and weed scientists across the United States increasingly are concluding that populations of important weeds have adapted to glyphosate herbicide to the point that its days as a standalone broad-spectrum weed killer are numbered.

As a result, controlling weeds in agricultural crops is becoming more complicated. But growing awareness of the challenge and adoption of more sophisticated control strategies are helping growers tame the new tougher-to-control weed variants.

"Glyphosate will continue to be an important component of weed control programs," says Bill Johnson, a weed scientist at Purdue University. "As far as being the sole herbicide applied on a field, those days are numbered."

The severity of the challenge and the weed species that are no longer adequately controlled by glyphosate vary by geography. But the result is the same.

Take Arkansas, for example. A massive — and largely uncontrolled — outbreak of Palmer pigweed in the state in 2010 has convinced Extension Weed Specialist Bob Scott that virtually all Palmer pigweed (also known as Palmer amaranth) now should be considered glyphosate-resistant.

"The days of using glyphosate alone are over in Arkansas," Scott says. "Glyphosate is still an effective herbicide on many weeds, but not Palmer pigweed. In the future, it will be part of a programmed approach, much like before the days of glyphosate-tolerant crops."

Southwestern Minnesota agronomist Dorian Gatchell has reached the same conclusion. As control of lambsquarters and other broadleaf weeds with glyphosate-only programs worsened in recent years, his customers gradually began including herbicides with alternative modes of action into weed management programs.

"We went from a one-pass program to a two-pass and then to a three pass at high rates," says Gatchell, agronomy manager for Equity Elevator and Trading Co., Wood Lake, Minn. "It still didn’t control the lambsquarters. That should tell you something. Glyphosate alone is gone."

Other weeds to watch in various geographies include giant ragweed and waterhemp, Johnson says. "Issues with giant ragweed and waterhemp are growing very rapidly," he adds.

From difficult to control to resistance

The evolution of weed populations from easy to control to harder — or impossible — to control isn’t a mystery. Weed scientists have documented the process with a range of herbicides. For some weeds, such as ragweed, the difficult-to-control or resistance process begins with a naturally occurring variant in a weed population that is resistant to the herbicide. This one-in-a-million variant (or a billion, or a trillion or less, depending on the weed species and herbicide) then pollinates — potentially passing herbicide resistance to susceptible types. Then it and its cross-fertilized neighbors produce seed. Assuming the same herbicide is applied repeatedly, the population of uncontrolled variants gradually builds until it makes up a large percentage of that weed species in a field.

"Gradually, the weeds get harder to control," Johnson says. "At first you might notice patches where you are not effectively controlling the weeds. And the patches grow. And then (through continued selection for more resistant types), they become resistant."

There’s no set timeline for this to occur. It depends on the herbicide, the weed species and how selection pressure is applied over time. But there is a rule of thumb for glyphosate, Johnson says.

"Once a field has had 30 continuous applications of glyphosate, you really have to be on the lookout for problems," he says. "Typically, the situation is not recognized in production settings until you are in the second or third year past it being a significant problem."

Weeds to watch scorecard

To date, 12 weed species have been formally identified as being resistant to glyphosate in the United States. That includes the latest addition, annual bluegrass, which was identified as resistant at a single site in Missouri, according to weedscience.org, a site that tracks resistant weeds.

Most glyphosate-resistant weeds in the United States are broadleaf weeds, plus a handful of grasses. The resistant weeds list includes Palmer amaranth, tall waterhemp, common ragweed, giant ragweed, hairy fleabane, marestail (horseweed) and kochia. Resistant grasses include goosegrass, Italian ryegrass, rigid ryegrass, annual bluegrass and Johnsongrass.

But Johnson says lambsquarters, morningglory and velvetleaf also could be considered to be resistant, although they don’t meet the strict scientific definition for resistance. These hard-to-control weeds are widespread in the U.S. corn, soybean and cotton markets.

"These weeds have always been difficult for glyphosate to control, but we haven’t been able to prove it in the greenhouse," he says. "With weeds resistant to other herbicides, you could pour a jug on them and they would survive. With glyphosate, you seem to get injury symptoms, but the weeds don’t necessarily die."

The hot spot

Tommy Young, who farms near Tuckerman, Ark., experienced the resistant Palmer pigweed outbreak firsthand in 2010. But, luckily, it only affected the corners of irrigated fields, since he rotates herbicide chemistries on crops grown under pivot.

"What really floored me is that I did my initial spraying when the beans were 6 inches tall. It was almost like I forgot to put the chemical in the sprayer," says Young, who chairs the Arkansas Corn and Grain Sorghum Board. He followed with another glyphosate application before switching to a herbicide with an alternative mode of action, which resulted in acceptable control.

It would be hard to construct a better field experiment comparing a sophisticated resistance management strategy with continuous glyphosate use than in Young’s irrigated fields.

While Young grows both glyphosate-tolerant corn and soybeans under pivot, the weed control program in his corn relies primarily on herbicides other than glyphosate. The double-cropped winter wheat/soybean half of the rotation receives a complement of wheat herbicides, plus multiple glyphosate applications on soybeans. Meanwhile, field corners are in a close rotation of glyphosate-tolerant soybeans and winter wheat.

Unlike the resistant pigweed-infested field corners, weed control in irrigated portions was normal, with no signs of resistant weeds. Young says his "frightening" experience with resistant weeds underscores the importance of his decision to use alternative chemistries in irrigated glyphosate-tolerant corn.

"Years ago, it was preached that resistance was coming, and I believed it," he says. "With our corn, which is our money crop, we could afford to spend $18 to $20 more per acre. We thought it was worth it."

While Young appears to have staved off glyphosate-resistant Palmer pigweed on most of his acres, Scott, the Arkansas Extension specialist, says that beginning in 2011, most farmers across the state should assume Palmer pigweed in their fields is resistant to glyphosate.

"Arkansas now has the dubious honor of having the most glyphosate resistant weeds per capita in the nation," Scott says. "We all knew that the system we were on was not sustainable. Five or six years ago, we had essentially eliminated Palmer pigweed as a problem weed. Now, if you have Palmer pigweed in a field, you have to assume it is resistant at some level. After a year like 2010, I expect it to be everywhere in 2011."

The seriousness of the situation may have been reflected in attendance at a November 2010 Extension-sponsored session on controlling resistant pigweed. It attracted 700 to 800 attendees, compared with about 50 in recent years.

To join the discussion and find out more about the current state of weed management, visit talkweeds.com.

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