In writing an article about Palmer amaranth resistance to PPO inhibitor herbicides, it’s tempting to say – paraphrasing Mark Twain – the reports of their demise have been greatly exaggerated.
While any evidence of pigweed resistance in the postemergence PPO inhibitors – Blazer, Cobra, Flexstar, Prefix and Sharpen – is cause for concern, say weed scientists, few believe they are truly dead. PPO resistance has been confirmed in pigweed in Arkansas and Tennessee.
What happens after this depends on how farmers react to the developments and what they do to protect the PPO herbicides, which became the workhorses for cotton and soybeans after Palmer amaranth began developing resistance to glyphosate.
“Larry Steckel (University of Tennessee) and Bob Scott (University of Arkansas) have shown the resistance gene is there – it’s been confirmed in four counties,” says Frank Carey, field market development specialist for Valent USA Corp., which markets several PPO herbicides. “More samples are coming in and will be tested. I’m glad we’re catching it early.”
Both Steckel, Extension weed scientist at the West Tennessee Research and Education Center in Jackson, and Scott, Extension weed scientist for Arkansas, have suspected resistance was occurring in some locations in their states. But actual resistance wasn’t confirmed until testing was completed at the University of Illinois in July.
Ironically, the testing program in Illinois was developed to help document resistance in common waterhemp, a distant relative of Palmer amaranth, to the PPO inhibitor herbicides more than 10 years ago.
Two classes of chemistry left
Confirmation of PPO-inhibitor resistance in Palmer amaranth is troubling to weed scientists in the South because it leaves only one or two classes of chemistry – 1) mitotic inhibitors such as Dual, Warrant and Zidua and 2) Liberty – that are labeled for use on cotton and soybeans.
In the Midwest, farmers have continued to use the PPOs, primarily by spraying weeds as early as possible, using one and often two residual herbicides on every acre and improving the coverage of the contact-herbicide PPOs.
“This shows that we can manage through it,” says Carey. “But it will take a concerted effort by growers in a number of areas to preserve the effectiveness of these herbicides.”
Since the discovery of glyphosate resistance in pigweed in south Georgia in 2005, weed scientists have recommended adding PPO herbicides such as Reflex or Flexstar (fomesafen) to postemergence tank mixes to control pigweed escapes.
Concerned about the development of resistance to PPOs, the specialists recommended rotating the PPOs with other herbicides, and, one, Eric Prostko with the University of Georgia Extension Service, questioned whether growers should save the PPOs for their two most important economic crops – cotton and peanuts – and not use them in corn and soybeans.
Spraying weeds when they are small is something weed scientists have been preaching for years, but it will be critical for preserving the efficacy of the PPO inhibitors.
Too many survivors
“One of the problems with controlling Palmer amaranth after it gets 4 inches tall is that, by then, it has so many auxiliary buds, the herbicide won’t kill them all,” he says. “If you don’t kill the whole plant, it will keep growing from the surviving buds.
Carey says he remembers a colleague telling him early in his career with Valent that “air-conditioned tractor cabs would lead to problems with controlling weeds with herbicides. That’s because when it’s hot and dry enough to need air conditioning, it may be too hot and dry for herbicides to work effectively.”
While healthy, growing plants will help, good coverage is essential for the PPOs to be effective, and that means growers may need to increase the amount of water in their spray tanks.
“That was what was so good about Scepter and glyphosate,” says Carey. “If you got herbicide anywhere on the plant, you killed it. The PPOs are contact herbicides. That’s why you have to have coverage.”
Carey concedes it may be difficult for growers to go back to applying 15 to 20 gallons of water per acre with their herbicides. “But we’re having to go back to old technology with some of these herbicides, and that’s true of the recommendations for coverage.”
As part of their efforts to improve coverage, growers should also lower the spray boom and slow down their sprayers, making sure they’re applying the material directly and evenly to the target weed species.
Other steps that can help preserve the efficacy of PPOs:
- Use alternate herbicide modes of action. The University of Georgia’s Prostko says growers should try not to apply a PPO herbicide more than once per crop per year. Weed scientists recommend growers become familiar with the different groups of herbicide and not use a specific mode of action more than once in a year.
- Do not apply a PPO herbicide alone. “Use it in a mixture with other herbicides such as Fierce or Prefix, and overlap your residuals,” says Carey. “You can start with a solid pre-emerge herbicide like Fierce within two weeks of the planter. You can tank mix that with Gramoxone and get the small Palmers.”
- Spray weeds early – when they’re less than 2 inches tall.
- Apply pre-emergence herbicides. Weed specialists in the Midwest recommend putting residual herbicides on every soybean acre.
- Rotate crops and herbicides.
- Narrow the row spacings on crops such as soybeans so the plants achieve canopy sooner and reduce the need for multiple postemergence sprays.
- Clean equipment thoroughly as often as possible to keep Palmer amaranth seed from spreading.
“In the Midwest, with waterhemp that’s less than 2 inches tall, you can still kill it with Cobra or Reflex,” says Carey. “If it’s taller than 2 inches, Liberty on Liberty Link soybeans is what’s left. The only difference if you have PPO resistance is whether you use the Roundup Ready system or Liberty Link. Otherwise, it’s the same: Use multiple modes of action and rotate chemistries. Three is better than two, and two is better than one.”
To learn more about PPO inhibitor resistance, visit http://www.gaweed.com/slides/PAC-08/PAC-08.pdf.