Pigweed And PPO Resistance – Bad Situations Get Worse

Managing Palmer pigweed will become increasingly complicated now that PPO resistance has been confirmed in at least 4 states – Kentucky, Tennessee, Arkansas and Mississippi. That puts it squarely in the Delta and also on the far southern fringe of the Corn Belt.

Where PPO resistance abides, those plants quite likely carry some level of resistance to glyphosate and/or ALS-inhibitor-type herbicides, as well.

“It’s unlikely that the two (glyphosate and ALS resistance) are linked biologically. But nearly every Palmer sample we’ve ever sprayed in greenhouse tests ended up being glyphosate resistant and also resistant to ALS herbicides,” points out Jason Bond, Mississippi Extension Weed Scientist. “All this gets back to the volume of treatments and selection pressure.” 

Palmer amaranth was resistant to ALS first and then with enough glyphosate applications the weed gained resistance to glyphosate, as well, Bond says. “Now we’ve sprayed them with the PPO-type herbicides and have added another layer of resistance.”

In Tennessee where PPO-resistant Palmer amaranth has been identified in fields, about 25% of the plants at a couple of locations carried the resistance, according to Larry Steckel, Tennessee Extension Weed Specialist. 

PPO Resistance: “More The Rule” Going Forward

“Genetically speaking, that probably means the resistance in those fields was established two years ago,” Steckel says. “It’s safe to say that PPO resistance will be more widespread this spring and in a higher concentration in fields where it was confirmed in 2015.

“In 2016, PPO resistance will be more the rule than the exception across much of the Midsouth.”

Based on the most recent counts, PPO resistance has been confirmed in 20-plus counties in the 4 states. The problem gained attention – and wide confirmation – in 2015, although instances of PPO resistance were likely present but went undocumented. Poor weather conditions in recent years probably masked some resistant populations. Where pigweed grew through PPO materials, it might have been written off to timing or weather conditions that compromised herbicide treatments.

“We’ve been relying very heavily on this class of chemistry for pigweed control for several years now,” Bob Scott, Arkansas Extension Weed Specialist. “Unfortunately, as history has shown, when a class of chemistry works well — in this case, it seems to be a whole lot of Valor herbicide applied pre-emergence, followed by Flexstar applied post-emergence – we have a tendency to over-rely on it, to use it over and over again.”

Scott said the first samples of PPO-resistant pigweed in Arkansas – and maybe anywhere in the U.S. – were collected from Lawrence County in 2011 by a University of Arkansas graduate student, although the samples weren’t tested until 2013.

“It kind of got lost in a PhD student’s work in Fayetteville,” Scott said. “It was an isolated incident back then. Now it seems like the stuff is popping up everywhere.”

Bond, Steckel, Scott and their colleagues in other states are still trying to sort through their options. Mississippi has just rewritten its soybean weed control guide, for example, to reflect the potential for PPO-resistant pigweed.

Dealing With Resistance In Soybeans, Cotton

Liberty (and compatible glufosinate products) is the last herbicide that will control Palmer amaranth post-emergence in soybeans where PPO resistance is a factor, Steckel says. “Growers and consultants have been rushing to acquire LibertyLink soybeans to combat this new problem, based on conversations and comments at winter meetings,” he adds.

His recommendations for dealing with PPO resistance in LibertyLink soybeans:

  • Coverage with Liberty or equivalent glufosinate products is critical. Apply at least 15 gallons per acre and use nozzles that give good coverage. “I prefer some of the dual fan nozzles like the Wilger Y or the Greenleaf TADF,” Steckel says. 

  • The time of day glufosinate is applied directly influences pigweed control. “For best results, apply Liberty in a window from two hours after sunrise to about an hour before sunset.” 

  • In soybeans, consider mixing a PPO herbicide with glufosinate. “This tankmix provides more consistent Palmer control than either chemistry applied alone. It makes a lot of sense from a resistance management standpoint because glufosinate can control the PPO-resistant Palmer and the PPO herbicide will control those rare individuals that may be Liberty-tolerant.”

  • Utilize cultural weed control practices. Cover crops “should help provide control of horseweed and good suppression of Palmer amaranth. “Tennessee research in 2015 examined the effect of cover crops on Palmer amaranth in soybeans and “we found Palmer in a non-cover treatment took 8 days to emerge and reach 4 inches tall, while in a cover crop of cereal rye and vetch blend it took 30 days to reach the same stage,” Steckel reports. That delay, he adds, can be all the difference in making timely post-emergence Palmer treatments. 

  • Rotation with corn or grain sorghum, likewise, is a way to work over PPO-resistant Palmer amaranth with other chemistry.

Based on Midwest research, PPO residual applications still exhibit some level of control of PPO-resistant waterhemp, which is a pigweed species closely related to Palmer amaranth. 

“However, the level of control and length of residual is lower on resistant compared with susceptible populations,” Bond adds. “Preliminary data from the University of Arkansas shows a similar response in PPO-resistant Palmer amaranth.”

Given those findings, herbicide recommendations for Palmer control in Mississippi cotton won’t change in 2016, says Bond, although several critical components of the program are being strongly emphasized going into 2016. Among the recommendations:

  • Include paraquat (herbicide Group 22) with any preplant/preemergence herbicide application. Paraquat remains effective for Palmer amaranth control before crop emergence. However, widespread use of paraquat has placed extreme selection pressure on this species. 

  • Scout carefully to assess the effectiveness of preplant residual herbicide applications. This is critical to ensure no Palmer amaranth is emerged at planting. 

  • Apply fluometuron (herbicide Group 7) as a preemergence treatment at planting. “Fluometuron represents one of the few remaining herbicides modes of action effective for Palmer amaranth control,” Bond says. 

  • Include a chloroacetamide herbicide (herbicide Group 15) with the first postemergence application. 

  • Apply a layby herbicide treatment containing diuron (herbicide Group 7).