Hitting Pigweed With Overlapping Residuals – Do’s And Don’ts

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Photo: ©2017 Debra L. Ferguson

In the 13 years since glyphosate-resistant Palmer amaranth was first discovered in Georgia’s Macon County, it has traveled far and wide – including an almost 1,200-mile trek to the north-central Kansas farm of Kent Stones.

Stones wasn’t terribly surprised when he spotted the aggressive weed growing in his field borders in 2015. Like many farmers north of the Mason-Dixon line he had followed the weed’s progress for several years, plus Palmer had been confirmed in 2014 within 50 miles of his farm.

He anticipated the weed’s arrival and had already modified his weed control program.

But he was alarmed at how quickly things got out of hand in his soybeans in 2016. “We were laying down successive layers of residual herbicide, one in early April and another in early June. But on about 75 percent of our crop, we went over three weeks without rain to incorporate that second layer. That allowed the Palmers to break through.”

Stones had to bring in chopping crews to take out the pest, and he spent about $100 an acre to clean out the toughest fields. Averaged over all the farm’s soybean acres, hand-chopping was around $26 per acre.

Don’t Be Afraid To Tweak Your Program

Stones, who farms corn, milo, soybeans and wheat, realized he needed to tweak his weed control program a bit more.

After those pigweed breakthroughs last year, he will move the second application up to May 20-25, or about 7 to 10 days earlier than in 2016.

“That earlier date range will give us a little more time to get rainfall incorporation before our first layer gets so weak that’s it’s no longer controlling the emerging Palmers,” he says.

While this change will reduce his window of control, Stones hopes his crop will reach canopy by the time the second layer of residual begins to weaken.

Stones will also try Xtend soybeans and LibertyLink soybeans to add some postemergence flexibility, although he is concerned about his lack of familiarity with the technologies’ varieties.

“We don’t know what their yield potential is on our farm,” he acknowledges. “We might get a field 100 percent clean of Palmer pigweed but run 10 to 12 bushels per acre less than our proven yield numbers, which would be more expensive than the weed problem, itself.”

Fortunately, 2016’s Palmer pigweed problems didn’t include any train wrecks, according to Stones.

“We have had almost zero yield loss to Palmer pigweed,” he says. “In fact, the last two years have been two of the highest-yielding farm averages ever for our soybeans. But we have spent more money. Our herbicide program has gone from $35 an acre four years ago to $55 to $75 per acre last year.”

Stones has learned a couple of valuable lessons since Palmer appeared on his farm. “Palmer more than 1.5 inches tall is probably uncontrollable with standard technologies,” he notes. “After they gain some size, the only thing that is going to take them out is a hoe.”

Don’t Skimp On Residual Rates…Ever

For Palmer amaranth control in soybeans, his consultant, Matt Hagny with Pinnacle Crop Technologies, recommends a weed control program with full rates of residual herbicide.

“Two weeks prior to planting, we apply the maximum three-fourths-pound rate of sulfentrazone,” Hagny specifies. “That’s our No. 1 weapon for residuals. Everything else is much less effective. Our Palmers are all triazine-resistant, so metribuzin is no help. We also apply Warrant preemergence for the second layer.”

For a third layer, Hagny recommends a 12-fluid-ounce rate of fomesafen or 24 ounces of Prefix, which is Dual and Reflex. That goes on as an early post at second trifoliate.

“We give it both barrels with residuals, then try to clean up escapes with Liberty or hand chopping,” Hagny continues.

“We sometimes chase some Palmer pigweed with PPO herbicides postemergence, but it’s very hit or miss,” Hagny says. “If they’re more than an 1.5 inches tall, the results are pretty flaky, although sometimes you can do a little better on some larger ones by mixing PPOs together – but it’s iffy.”

While a few of Hagny’s clients will plant Xtend soybeans, he generally discourages use of this technology.

“We rely on dicamba so much in the corn and sorghum part of our rotation that we can’t afford that much more selection pressure for dicamba-resistant Palmer pigweed by also using dicamba in our soybeans,” Hagny says.

Hagny says the timing of herbicide applications for his clients is firmly scheduled. “I’ve learned the hard way that no matter how carefully you scout Palmer pigweed in soybeans, you’re never going to find them all. My attitude is that we have to have pretty close to perfect control.”

Overlapping – It Works, In Theory

The idea of overlapping, or layering, residuals is simple enough, according to University of Georgia Extension weed scientist Stanley Culpepper. His approach:

  • Start clean, then apply your first residual.
  • Each residual thereafter should be applied before the previous residual begins to break down.
  • If everything goes according to plan, producers can prevent a lot of pigweeds from emerging.

Unfortunately, things don’t always go according to plan, Culpepper freely admits.

“The concept of overlapping residuals is a theory,” says Culpepper. “You cannot be completely successful with it. So you have to combine it with an overall sustainable program.”

Nearly 700 respondents to a recent survey of 1,000 Georgia producers conducted by Culpepper listed these six factors as important parts of a sustainable program:

  • Timely applications of herbicides.
  • Hand weeding.
  • Residual herbicides.
  • Extension information.
  • Start clean, stay clean.
  • Use different modes of action.

One example of what can go wrong with overlapping residuals in dryland fields is missing that rain needed to activate the residual herbicide. Culpepper’s best advice: choose your residual wisely.

“We use products that don’t break down very rapidly,” Culpepper explains. “For example, we apply a lot of Reflex in cotton preemergence. Data shows that if Reflex preemergence is activated 17 days after it’s applied, it still provides 90 to 95 percent of the Palmer amaranth control you would have had if it had been activated on the day it was applied.

“Yes, we have to figure out how to kill the escapes that have come up while the herbicide is not activated, but we have options, depending on cotton technology and application procedures – such as diuron, Liberty, Staple, dicamba, 2,4-D, as well as a few more products to help us.”

All components of a Palmer amaranth control program work in concert, Culpepper says.

Herbicide Resistance Info


“If we knock down 90 to 95 percent of the Palmer with overlapping residuals, the reduction in early-season weed competition along with better coverage and control with postemergence materials is most often achieved,” he notes. “Additionally, by using multiple types of herbicide chemistry in this program, we reduce selection pressure for the development of additional herbicide resistance.”

As a bonus, “there’s definitely less hand weeding toward the end of the season,” Culpepper says.

Evidence in Culpepper’s state shows that this program approach works. According to the Georgia survey, 96 percent of the responding producers believe they are doing a good job of managing Palmer amaranth, economics aside. That compares to 42 percent in 2013 and only 5 percent in 2010.