It’s more important than ever to mix modes of action and apply several at once
The pigweed monster is only growing stronger. Long-suspected PPO herbicide resistance in Palmer amaranth has arrived in Midsouth fields with grim implications for weed control and post options.
With PPO resistance confirmed in two Tennessee soybean fields in 2015, followed by confirmation in Arkansas, weed scientists are trying to gauge how big of a blow PPO resistance will be to agriculture. “Farmers are starting to look for an alternative from a post-emergence standpoint, but there’s nothing out there for soybeans, which are primarily Roundup Ready,” says Larry Steckel, University of Tennessee weed specialist.
Some farmers are looking to LibertyLink and hoping for the arrival of Enlist or Xtend technology.
In the mean time, farmers will have to manage Palmer with the herbicides already available, Steckel says. “Next year, they will have to plan to use even more pre-emergent herbicides. That will be the only ironclad way to control pigweeds,” he adds.
Thriving Palmer on properly managed soybean fields has heightened Steckel’s suspicions for several years. On their best day, PPOs are barely adequate, in most cases, from a post-emergence standpoint—controlling up to 3" Palmer with a full rate. Steckel recommends using Boundary (Dual Magnum and metribuzin) or Intimidator (Dual Magnum, metribuzin and Reflex) at pre-emergence, followed with Zidua or Dual Magnum at early post-emergence.
The echoes of glyphosate-resistant pigweed don’t bode well for the spread of PPO resistance. Glyphosate resistance emerged seemingly from nowhere in fields and exploded a few short years later. “PPO will probably spread this fall through machinery at harvest,” Steckel says. “It’s also possible the gene was spread through pollen from males to females, as we’ve seen in Palmer. Therefore, a combine isn’t even needed to spread PPO.”
In 2011, Bob Scott, University of Arkansas Extension weed specialist, suspected the presence of PPO resistance in Arkansas fields, but isolated suspicions have turned to multiple failures in 2015. Calls from farmers are pouring in at a fast rate to Scott’s office, but he’s extremely limited as to what to recommend. “Unless we get new technology labeled in time for 2016, if a guy comes to me with PPO resistance in his fields, I don’t see any other choice but to switch to LibertyLink beans.”
There are still plenty of fields in Arkansas without PPO resistance, where Valor and FlexStar will work, but how long will that last considering the pace of spread in 2015? “If everyone goes ahead and pretends like this problem isn’t happening and grows conventional and Roundup Ready beans next year, we could be headed for a real problem,” Scott warns.
What should a farmer do if he or she suspects PPO resistance? Scott doesn’t believe field testing is necessarily the answer. He advocates going ahead and treating a suspect field as positive for PPO resistance and gives farmers two options:
1. Plant LibertyLink soybeans and use a chemistry program that involves Dual and metribuzin, followed by an early timing of Liberty.
2. Rotate to corn, grain sorghum or rice, which Scott says is a better option. “Rotate away from beans and PPO chemistries. If you stay with the crop, grow a LibertyLink bean,” he adds.
The initial parallels between PPO resistance and glyphosate resistance are undeniable, Scott says. “We found our first Roundup-resistant pigweed in 2005, and a few years later, we had a tremendous problem. Are we on the same track with PPO? Here we are in 2015 staring the down the barrel as fields pop up everywhere,” he says.
When applications and good timing line up but Palmer amaranth doesn’t die, the surviving pigweed has all the hallmarks of PPO resistance. Jason Bond, Mississippi State University Extension weed scientist, is currently investigating multiple Palmer failures. He hopes some Midsouth fields deemed failures are actually the result of mistimed applications due to unusual weather patterns. “No matter how you look at it, PPO resistance will be a big deal in 2016 on Roundup Ready acreage, especially for a guy who commits to Roundup Ready thinking PPO will work, but it doesn’t. On soybeans, if it’s straight Roundup Ready and the PPO fails, there will be tremendous problems,” Bond says.
There’s no reason to believe PPO resistance won’t spread fast and wide, but Bond hopes to provide education to keep pace. “I don’t think PPO resistance brings the fundamental shift that glyphosate resistance caused, but if this blows up big—and it could—it will sideswipe people in 2016,” he says.
Glyphosate resistance touches all levels of an operation and causes changes in management. However, PPO resistance zeroes in on a primary mode of action used in defense. “It’s a big deal on mode of action, but not on the whole farm management side like glyphosate resistance,” Bond says.
The Midwest has dealt with PPO-resistant waterhemp for years and without question, multiple resistance issues has changed weed management, describes Kevin Bradley, University of Missouri Extension weed scientist. “I wouldn’t be surprised if pollen transfer can occur between waterhemp and Palmer amaranth. It’s already been proven in the lab and greenhouse that a small amount of transfer can take place. But when you’re dealing with millions of plants, a tiny amount of transfer can be enough.”
The fight against PPO-resistant waterhemp is getting worse as pollen transfers this resistance trait from one waterhemp plant to another. “It depends on what Midwest state you’re in, but waterhemp populations with PPO resistance in Illinois are greater than 50%. In Missouri, we did surveys that showed we were around 30% several years back, but we might be up to 50% now,” Bradley says.
PPO resistance in Palmer amaranth means more of what weed scientists have been advocating for years—the necessity to mix modes of action and apply several together.