Weed scientists suspected giant ragweed might have developed resistance to PPO herbicides as early as 2015. Resistance to glyphosate in giant ragweed was confirmed much earlier, as was resistance to ALS herbicides. Now, University of Wisconsin-Madison researchers Felipe Faleco and Rodrigo Werle, working with Pat Tranel at the University of Illinois, have confirmed that a population of giant ragweed in Wisconsin found in 2018 was resistant to PPO chemistry.
According to sources, it's the first-ever proven resistance of giant ragweed to PPO Group 14 herbicides in the world. And it's also the first time multiple herbicide resistance was documented in Wisconsin.
Tranel led the molecular work to identify the mechanism of resistance to the herbicides. The researchers concluded that a genetic mutation within the giant ragweed species likely conferred resistance within the plant to PPO herbicides.
Glyphosate resistance within giant ragweed has been documented within the U.S. and Canada for a long time. ALS herbicide resistance also has become widespread. These researchers began their search for PPO resistance after reports of weed control problems with giant ragweed in Wisconsin in 2018, which didn't fit known resistance patterns.
They collected samples in 2018 and worked with ALS and PPO postemergence chemicals in their studies.
"We stopped pursuing proving it when dicamba-tolerant and Enlist soybeans with tolerance to 2,4-D arrived because we knew they would control resistant giant ragweed anyway," explains Bill Johnson, Purdue weed control specialist. So Johnson is not surprised that other weed scientists now confirm resistance to PPO chemistry in giant ragweed.
Flexstar, a PPO herbicide, was used extensively for giant ragweed starting around 2005, after many giant ragweed populations displayed glyphosate resistance. After about five years of use for giant ragweed control, Flexstar gradually became less effective, Johnson says.
"We assume that PPO resistance in giant ragweed is likely widespread," he notes. "It is another thing to consider when developing weed control strategies for giant ragweed."
The giant ragweed challenge
Giant ragweed is a tough competitor everywhere, but especially in the eastern Corn Belt. "It has been perhaps the No. 1 weed farmers fought going back to the '60s and '70s," Johnson says. "It is a large-seeded broadleaf weed, and it germinates in multiple flushes throughout the season, especially if you keep getting rains.
"We have residual herbicides with activity, but no soybean residual herbicide that is super-effective against it. Plus, residual herbicides don't last forever. So, that forced us to concentrate on controlling it with post herbicides.
"We can do reasonably well postemergence in corn because we have dicamba herbicides. Dicamba is excellent against giant ragweed. This weed has always been tougher to control all season long in soybeans, because we didn't have dicamba.
"Now, it appears that we won't have that postemergence option even in Xtend and Xtendflex soybeans. We do expect to use dicamba as a burndown."
Controlling giant ragweed in soybeans
Here are Johnson's best step-by-step recommendations for battling giant ragweed in various soybean systems:
No-till, reduced tillage. To kill emerged giant ragweed plants, consider a burndown of dicamba or 2,4-D ester. "Both are very good on giant ragweed," Johnson says. "We only expect to be allowed to use the dicamba soybean products as burndowns in '25."
You may also want to consider adding Sharpen to kill weeds faster and pick up marestail, he notes.
Xtend and Xtendflex soybeans. If you select soybeans with these traits and apply 2,4-D burndown, wait at least seven days before planting, depending upon label instructions of the 2,4-D product applied, Johnson says. You can apply XtendiMax, Engenia and Tavium dicamba products ahead of soybeans with the dicamba-tolerant trait without a waiting interval. However, you cannot apply other dicamba products, such as Banvel, Clarity and DiFlexx, ahead of soybeans without waiting. Always read labels first!
Enlist 3 soybeans. There is no waiting period between spraying approved 2,4-D products and planting Enlist E3 soybeans. Do not apply dicamba ahead of Enlist soybean varieties because they don't have dicamba tolerance.
Conventional tillage. "If you work ground early and giant ragweeds regrow before planting, kill them," Johnson says. "Either spray them with a burndown or till them out, but don't ignore them and hope to control them later."
Timing postemergence sprays
"Giant ragweed gets a jump and often reaches the 4-inch height first," Johnson notes. "Time spraying around the first weeds to reach 4 inches. Too many people wait until later weeds reach 4 inches, and then giant ragweeds are taller and tougher to control."
Post, Enlist system. Be ready with 2,4-D and/or Liberty at full rates. "Then you might add glyphosate for grasses, and it may get some giant ragweeds - many carry low resistance to glyphosate," Johnson says. "Add a Group 15 herbicide for residual waterhemp control."
"Liberty works on giant ragweed, but not as well as 2,4-D on larger plants."
Post, Xtend system. Liberty is the main weapon if ragweeds are PPO resistant. "So, now you must get giant ragweeds while they're small and still on label," Johnson says.
