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ALS-resistant horseweed suspected in Arkansas plots

The problem of herbicide-resistant/tolerant weeds continues to pick up speed in the Mid-South. Calls to weed specialists are increasing, hard-to-kill weeds are popping up all over and everyone wonders what the remedy will be.

Northeast Arkansas has been “glyphosate-resistant horseweed central” for a long time, says Bob Scott, Arkansas Extension weed specialist. “Horseweed continues to dominate and threaten our crops. Unfortunately, horseweed is key in most Arkansas row-crop management decisions now.

“Today, we drove from central Arkansas to the northeast and every field we drove by had horseweed. Everyone is now at the party.”

Those already planted are past the window that would allow an “extremely effective” treatment. “All we can do now is burndown to try and allow the soybeans to get ahead and take over. If you've got horseweed in your crop, I suggest 0.33 ounce of FirstRate. That's the best thing we've looked at.”

Scott's recommendation “isn't the best program to control horseweed. But, postemergence, it's about all that's available. You can go up to 0.6 ounce (of FirstRate) total in-season. And if the horseweed is bad, you'll probably need 0.33 ounce twice.”

It would have been much better to have started a month or two ago with 8 ounces of Clarity in the burndown. That approach allows plenty of time on plant-back. It's also good to add in Synchrony or Valor as a residual to ensure a clean field at planting.

Last fall, Scott and colleagues planted research plots near Carlisle, Ark., — some 30 yards from I-40 — to check a variety of fall treatments for horseweed. They were able to keep the plots weed-free all winter. But between March and planting, as the herbicides began to break down, problems surfaced.

“In many fields where we used a residual and it had been clean all winter, we ended up having to burndown again prior to planting. What was accomplished? The clean fields do look better. But putting a pencil to it, if you've got to burndown again in the spring, why worry if they're clean in the winter?”

The study had about 20 different herbicides applied pre in the fall. Scott had been hearing that Envoke was a good residual on horseweed. And, in all other testing locations it did look “fantastic.”

But when Scott got the ratings back from the Carlisle plots, they showed zeros. “That alarmed me, and I thought, ‘That's not good. That can't be right.’ Envoke, Scepter Python, Synchrony: all zeros. You seeing a pattern forming? All those products are ALS inhibitors.”

ALS-resistant horseweed is not uncommon in other states. And now, it appears, Arkansas has the same.

“We took this horseweed into the greenhouse and sprayed FirstRate post. That was my first concern because FirstRate is our only option in soybeans.” The treatment didn't even turn the horseweed yellow.

“The funny thing is, you can spray glyphosate on this population of horseweeds and they'll die. But that's a small comfort. You can imagine our concern because all that's needed is an outcrossing between this ALS-resistant and a glyphosate-resistant horseweed. When that happens, we'll be in seriously bad shape.”

Dose response curves and other work needed to definitively claim resistance are yet to be completed. “But we've done enough testing to know with good certainty this population is resistant to ALS herbicides. Such horseweed has already been found in Ohio and other states. But it's particularly concerning to us because this weed is such a problem in Arkansas.”

If horseweed becomes resistant to both glyphosate and ALS herbicides, “we've got serious trouble. History has shown that from the first time a weed scientist finds such a resistant weed until it becomes a problem is a lot shorter than most think. We need to test to see how resistant, or tolerant, it is. Will it outcross with glyphosate-resistant horseweed? We assume it will.”

Scott says any outcrossed plants produced in the greenhouse will be destroyed. “We're being very cautious. There are no glyphosate-resistant pigweed locations being created right now in any ag areas. We are growing some for seed in labs and greenhouses — but never close to agricultural areas.”

Last year, Arkansas Extension confirmed glyphosate-resistant Palmer amaranth in Mississippi County. One year later, “every week, calls come in about fields where glyphosate isn't taking out the pigweed. It seems to be showing up in every county. I'm afraid we're standing on the edge of a major horseweed-level problem with resistant Palmer amaranth. Glyphosate tolerance seems to be going up overall. And individual populations are showing out at levels of treatment at 1.5X to 2X rates.

“Anyone at all familiar with pigweed knows how devastating and competitive it can be. Thousands of plants per square foot can result when they first come up. And we're not even to soybean-spraying time yet. I suspect we'll get a bunch more calls on this soon.”

Producers should be using some sort of resistance management program. One of the best things to do is begin with a pre. Syngenta's Prefix — a pint of Dual and a pint of Reflex — “is a product we tried on a farmer's field in Mississippi County. It did a great job and continues to look good in our plots. It has two modes of action that both work on pigweeds.

“I'm also getting a lot of questions on a Valor/FirstRate combination called Gangster. Early post-emerge, you can use things like Storm or Blazer. Later in the season, we'd probably recommend a full rate of FlexStar tank-mixed with glyphosate.

How does the spread of resistant/tolerant weeds in Arkansas compare to hard-hit areas of the Southeast? “We tend to believe these populations of resistance are developing (in situ). As these plants mate and outcross, the progeny are more resistant. It's just a steady buildup.

“Glyphosate resistance is very complicated with all the systems and pathways. You may be able to spray pigweeds on your farm with a quart of Roundup and it doesn't die. Then, you spray two quarts and it does die.

“But that doesn't mean you need to spray 2 quarts across all fields. That just continues the pattern of resistance until no rate works.”

The answer to resistance is different modes of action and production practices. “Producers must put different pressures on the pigweed. Even if you think you can increase rates, that's a very short-term solution.”

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