Ken Smith is unsure of the extent of glyphosate-resistant/tolerant Palmer amaranth in Arkansas. But the Extension weed specialist is positive farmers are finding more and more of the pest.
“After spraying, more farmers just aren’t getting a response on pigweeds. They’ll spray and most pigweeds die while others just get puny. But, most alarming, is when we spray and the pigweeds don’t even get sick.”
Smith was alerted to just such a field in Lincoln County in late May. After spraying and failing to control pigweeds, “an extremely astute” farmer applied a second dose of glyphosate. The pigweeds weren’t hurt that time either, so he called Smith.
Smith found a very clean 100-acre field with a single blemish: a spot of perhaps 1,500 square feet where Palmer amaranth had taken over. The pigweeds were solid with at least several hundred plants.
Since it’s restricted to that small of an area, Smith suspects the plants probably came from the seed of a single plant in 2006.
“The standard procedure is to spray them with our sprayer. We did that and the pigweeds still weren’t affected. So those weeds have been sprayed three times and they’re still green and growing.”
Were normal rates used? “Yes, normal rates, although we sprayed one strip at a double rate. Those pigweeds didn’t appear to fare any worse. We collected some of the pigweeds for the greenhouse, to study offspring. And when we left, we sprayed a gallon of Roundup WeatherMax.”
A week after spraying the WeatherMax, Smith reported, “I’m very sad to say the pigweeds weren’t bothered at all. That’s after three shots at regular rates and another gallon. My goodness, the implications are not good. We’ll suggest the grower disk the weeds up and put out a soil residual to try and keep anything from germinating.”
Are calls on suspect pigweeds more frequent this year? “Much more. Several days ago, I was in Lee County visiting a farmer. He’d already been over his cotton twice with glyphosate and had no luck with the pigweeds. Pigweeds were running over the field.
“When we visited him, he was on a third trip across the field — with Roundup and Staple, at that point. I checked the field and told him, ‘You’ll get a few more of these pigweeds with that mix. Unfortunately, you won’t kill them. They may become very sick, but they’ll put out adventitious buds down the stem and they’ll come back.
“The farmer lamented the fact that he’d done everything he knew to do without success. And this was a rather large area — the field is probably 200-plus acres.
“Such scenarios are much more common this year. I probably get a call on tolerance concerns every third day. In fact, we’re getting so many of those calls we’re backed up in visiting the locations. I’m going to as many as I can.”
Is resistant/tolerant Palmer amaranth spreading in situ or from seed or pollen movement? “I think we’re developing our own populations by reliance on glyphosate. We’ve selected pigweeds out for almost 10 years. We’ve just scooted tolerance levels up the scale.
“And once a few pigweeds get by, the horse is out of the barn and running wild. These pigweeds will produce massive amounts of seed, and they’re absolutely prolific.
“But I also think this is spreading with pollen. I think that avenue of spread is more dangerous than we originally thought.”
What about suspected ALS-resistant horseweed being found in Arkansas? “Unlike pigweeds, we’ve never considered horseweed to be very genetically diverse. With Palmer amaranth, you expect tolerance/resistance issues — not as much with horseweed. But there’s at least one Arkansas location with ALS-resistant horseweed.
But the ALS chemistry is very site-specific in how it works within a plant. “Again, when using it we’re selecting horseweeds that can handle the herbicide. FirstRate, which we’ve been using in soybeans successfully, will probably work for a few years before losing effectiveness.
“The thing is, this population of ALS-tolerant horseweed was found by accident. It’s probably a safe bet that there are other locations in the state that haven’t been found yet.”
How good is the possibility of the ALS-resistant horseweed also picking up resistance to glyphosate? “That’s very likely to happen, almost a given. I’d be very surprised if it doesn’t happen.
“The good part of this is most horseweed control is still built around the dicamba molecule. Farmers want to get into the field early with dicamba. And as long as dicamba has been in the market — since the 1970s — not many weeds have become resistant to it. So there is a rather successful tool still available on horseweed.”
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