Gear Up For Palmer Amaranth

This is a fight you can’t afford to lose

Palmer amaranth is a weed straight out of a horror movie. Driven by a deep taproot backed by a fibrous root network, nearly impervious to summer heat, it can grow an inch or more per day. As it matures, its seedheads can tower eight feet off the ground, each plant waving hundreds of thousands of tiny seeds around and threatening to overwhelm the next year’s crop. University studies have documented yield losses to Palmer amaranth as high as 91% in corn and 78% in soybeans.

Jason Norsworthy, a weed science professor at the University of Arkansas, points out that a 16-acre field in his state that had three or four escapes of glyohosate-resistant Palmer amaranth—also called Palmer pigweed—in 2009 had nearly total crop failure in 2010 and 2011. In fact, he says, in 2011, “we killed more pigweed with the tire tracks on the sprayer than with Roundup.”

Infestations of Palmer amaranth have pushed weed control costs in some Southern fields as high as $200 per acre, and driven some farmers out of their fields completely. In short, even a small problem can quickly become an economic disaster.

Genetic Diversity

Palmer amaranth’s broad genetic diversity has allowed much of its population to develop herbicide resistance. Various populations of the weed have developed resistance to ALS inhibitors, microtubule inhibitors (such as trifluralin), triazines, glyphosate and HPPD inhibitors. In fact, many Palmer amaranth populations may be resistant to up to three herbicide site of action (SOA) groups at one time. Those resistance traits can spread like wildfire through a population as windblown pollen carries the dominant genes far and wide, notes Ford Baldwin, founder of Practical Weed Consultants in Austin, Ark., and former University of Arkansas Extension weed scientist.

The result is that bad news spreads fast, and multiple resistance can build up quickly. A 2010 survey in North Carolina, for instance, found that 98% of the state’s Palmer amaranth is resistant to glyphosate and 85% is resistant to ALS inhibitors.

Midwest Toehold

But Palmer amaranth is no longer just a Southern problem. Populations of the weed have become firmly established in Iowa, Illinois, Indiana, and Ohio. In 2010, Palmer amaranth resistant to multiple SOAs appeared in pockets in Michigan, according to Christy Sprague, Michigan State University Extension weed scientist, probably brought in with livestock feed. The weed has even gotten a toehold in northwestern Minnesota. Traveling in feed, on equipment or carried by floodwaters, Palmer amaranth’s tiny seeds represent a huge problem.

“If you’ve got a pigweed here or there in one year, the next year you’ll have a patch of an acre or so,” warns Baldwin. “Then you’ll have strips of it, and the following year you won’t put a combine in the field.”

Baldwin traveled to a Respect the Rotation field day in southern Illinois to warn Northern growers about the dangers of Palmer amaranth. He stood in a field where the weed was already running rampant, and predicted that the problem would soon be familiar to growers throughout the Corn Belt. By then, he said, glyphosate would be useful only as a grass herbicide.

Overwhelming Numbers

Bryan Young, professor of weed science at Southern Illinois University, pointed out that Palmer amaranth populations at the field day’s infested site were more than 10 plants per square foot. That’s nearly half a million Palmer amaranth plants per acre, which makes it easy to see how even effective herbicides can be overwhelmed. “With Palmer pigweed,” notes Baldwin, “95% weed control is crop failure.”

Huge populations and broad gene pools explain why resistant weeds—whether they’re Palmer amaranth or other pervasive species like marestail or waterhemp—are already looming in the field, even if they’re still under the radar.

“The resistant biotypes are in the field the very first time you apply the herbicide, so they’re already there,” cautions Jeff Stachler, Extension weed scientist at North Dakota State University and the University of Minnesota. “They’re just at ultra-low frequencies to the point that you can’t tell that they’re there—but they are there.” Depending on a herbicide’s site of action, resistance can develop after three to 20 applications, he warns.

Comprehensive Attack

Managing a voracious, fast-growing, genetically diverse, herbicide tolerant weed species takes a multi-pronged approach, starting from the very beginning of the season. That’s especially true for Palmer amaranth, which is nearly impossible to control with herbicides after it’s about three inches tall.

“We need to start cleaner than we have been in the past, either in corn or soybean production,” says Mike Weber, technical service manager for Crop Science.

After a clean start, there’s no time to rest on your laurels—fighting weeds like Palmer amaranth requires a full-court press, all season long. Aaron Hager, University of Illinois Extension weed specialist, outlines a comprehensive plan of attack:

  • Plant into a clean seedbed.
  • Use full rates of a preemergence residual herbicide that is effective on Palmer amaranth.
  • Apply timely postemergence herbicide before Palmer amaranth seedlings are three inches tall.
  • Tank mix another residual herbicide with the postemergence application to extend control.
  • Come back with a second postemergence application if needed.
  • Remove surviving plants by hand or mechanically.

Of course, it’s vital to follow all the rules of good herbicide management, including employing at least two to three herbicide sites of action (SOA) and rotating herbicides to avoid using the same SOA twice in a row.

In an extensive bulletin on Palmer amaranth control in Indiana, Purdue University weed scientists Travis Legleiter and Bill Johnson also suggest harvesting heavily infested fields last, to avoid introducing the tiny Palmer amaranth seeds to new fields on equipment, and considering a cereal rye cover crop and deep tillage to aid in suppressing the weed.

Start Early

In Tennessee, where glyphosate-resistant Palmer amaranth and marestail are common statewide, University of Tennessee Extension weed specialist Larry Steckel says growers typically start their weed control program in soybeans 30 to 40 days before planting with a combination of glyphosate, dicamba and Valor. Follow the planter with Gramaxone and some type or pre-emerge herbicide for pigweed/amaranth, he adds, and come back with glyphosate or Liberty—depending on the variety of GM soybeans you’ve planted—tank mixed with a residual herbicide. That’s a costly program, he admits, and weeds still get through, requiring many growers to hire chopping crews.

“The take-home point is that on average, Tennessee growers’ herbicide bills have gone up 250% over the last six years, and we do not get the control you all probably enjoy now. So if you all aren’t to the stage we are yet with resistant weeds, anything you can do to keep then from getting established, you need to do,” Steckel says.

Recognize Resistance

The extraordinary genetic diversity of Palmer amaranth makes it prone to developing resistance to herbicides, including glyphosate. Some biotypes of Palmer amaranth contain as many as 100 copies of the EPSPS synthase gene in its cells, which overwhelms glyphosate by presenting too many sites for the herbicide to attack. Scarier still, a research team from Colorado State University reported recently in Weed Science that Palmer amaranth plants with such high levels of the EPSPS gene were just as robust as plants with normal levels. That means there’s no fitness cost, or disadvantage, to the plant from resistance. In turn, that means plants with the resistance gene are likely to thrive, and pass the trait on to future generations.

In order to properly manage herbicides and avoid applying a product to a population that’s resistant to it—the herbicide equivalent of spraying gasoline onto a fire—it’s important to recognize the signs of resistance.

Weeds that have developed resistance to ALS inhibitors, triazines or ACCas inhibitors typically show little or no damage from the herbicide, notes Jeff Stachler, Extension weed scientist at North Dakota State University and the University of Minnesota. But resistance to glyphosate can span a spectrum from narrowly surviving an application to showing no symptoms at all. That means that a resistant population following a glyphosate pass can easily be mistaken for the results of a poor application—a mistake that could haunt you for years.

Stachler points to some important questions that can help sort out application failures from resistance:

  • Was there rain, dust or an incorrect rate at application?
  • Do you see streaks that could indicate an application problem?
  • Did just one species survive? (That’s a big clue that resistance may be the issue.)
  • Is the species prone to resistance?

If you think the weeds may be resistant, send seed samples to your state university and control the outbreak—even if it means going in with hoes.

Free Online Resistance Course

Stay up-to-date on herbicide resistance management and earn continuing education credits for CCA status and many state licensing requirements through a free online course on herbicide resistance.

The program, hosted by Corn and Soybean Digest publisher Penton Communications, is accredited by the Weed Science Society of America. The course consists of five modules:

  • Current Status of Herbicide Resistance in Weeds
  • How Herbicides Work
  • What Is Herbicide Resistance?
  • Scouting After a Herbicide Application and Confirming Herbicide Resistance.
  • Principles of Managing Herbicide Resistance.

Visit to take the course online.


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