As the Mid-South wheat crop turns golden and heads toward harvest, growers should be on the lookout for another problem weed.
Last year, through a screening program in force for years, University of Arkansas researchers discovered about a dozen samples of ryegrass collected in the state were both ALS-resistant and Accase-resistant. Those are the two main modes of action — or groups of chemistry — used to control ryegrass in wheat.
“That means this ryegrass is resistant to products like Osprey, Finesse, and PowerFlex as well as Hoelon,” says Bob Scott, Arkansas Extension weed specialist and Delta Farm Press contributor.
“Over the years, we’ve focused on Hoelon-resistant ryegrass. At the same time, growers have gotten away from Hoelon and begun using other products in wheat.”
Scott and colleagues are very concerned about the ryegrass finds and have set up a graduate student-run program trying to find out how widespread the new resistance is.
The student has “just begun a sampling process in the state. We’d like to do a comprehensive survey of ryegrass populations to see where we’re at. It’s fair to say all these incidents of new ryegrass resistance in such a short time span make us nervous.”
Researchers would like to get 150 to 200 samples of ryegrass across the state. Then, they’ll do a screening program checking on susceptibility/resistance to three families of chemistry:
• Hoelon and Axial XL (Accase inhibitors)
“That’s because we have a lot of failures in burndown. Mississippi researchers have already confirmed glyphosate-resistance in their ryegrass.”
• ALS products (Osprey, PowerFlex, Finesse and Finesse G&B).
Scott encourages readers to do a search of ryegrass on the Web. “It won’t take long to find that, worldwide, ryegrass is one of the most widely resistant weeds there is. Australia has ryegrass that’s essentially resistant to every herbicide known to man.”
(For more on Australian ryegrass, see: Will U.S. become resistance leader?.)
Based on what is found in the screening program, “we may try to come up with programs to curtail some of this resistance. We may need to get serious about implementing ryegrass resistance programs in the state.”
Where were the dozen Arkansas ryegrass locations?
“They were randomly scattered across the Delta — from Monroe County all the way north to Lawrence County.
Did the farmers with this ryegrass say they’d seen it for the first time last year? “Yes, and that’s what is even more frightening about this. We haven’t been using ALS chemistry for ryegrass control for very long and this happened.”
It took many years for Hoelon-resistant ryegrass to develop. Apparently, this ALS-resistant ryegrass was already in the field — “perhaps already in the natural population. It could be from highway right-of-way applications of that family of chemistry. Who knows? But it seemed to pop up overnight.”
Used twice a growing season for many years, Hoelon has been over-relied on for ryegrass control. As a result, it’s “easier” to understand how Hoelon resistance developed.
However, Osprey and PowerFlex haven’t been available for very long. To have failures with them so quickly “indicates the resistance genes were already out there. That’s one of the things the survey being conducted — and some of the subsequent work (University of Arkansas associate professor) Nilda Burgos will do from a genetic standpoint — is set to tackle.”
The survey will include not only ryegrass in production fields, but ryegrass in highway right-of-ways, pastures, some commercial seed sources and other places the resistance traits may be coming from.
Once again, glyphosate-resistant horseweed is “rearing its ugly head,” says Scott. The rains, cool temperatures and wet soils have resulted in a struggle to get burndown applications out and crops in. Horseweed has taken advantage.
“It’s being reinforced that if we miss the dicamba/Roundup burndown timing, it creates a mess. Farmers are left trying to burn down the horseweed with Ignite, Gramoxone and other products. Too often, they end up fighting horseweed all year long.”
For burndown applications of Ignite and Gramoxone, Scott recommends “switching to flat fan nozzles and using as much water as possible” for best results.
One thing that will be interesting to follow this growing season: fields that have been switched from Roundup Ready beans to LibertyLink. In those, farmers “will be able to use a couple of applications in-season of Ignite. This should do a slightly better job than two applications of glyphosate plus FirstRate on glyphosate-resistant horseweed.”
In addition, Dow has already announced “a few of their products could be in short supply this year, including FirstRate. You can make two applications of FirstRate at a third of an ounce in-season. That’s about the only thing we have to go postemerge in soybeans for horseweed.
“I’m hearing from a bunch of farmers about failures. I recommend they get ready to come back with a lot of FirstRate. But if that’s short, options are few.”
Resistance management specialists
Increasingly, weed scientists are becoming resistance management specialists without the title, says Scott. He doesn’t see that changing anytime soon.
“The foreseeable future looks like a string of resistance issues. For the rest of my career, I don’t see folks like me developing a lot of new chemistries — although we’ll do that if possible. It seems more and more my role is developing methods using existing technology and chemistries to deal with resistance.
“In the past, weed science was more ‘What’s new? What’s coming along? How can we best use it?’”
However, because of the lull in research — “there hasn’t been a new mode of action announced in a while — we’ll have to deal with these (burgeoning) resistance problems with the tools we already have.”
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