David Bennett, Associate Editor

July 30, 2008

8 Min Read

For years, weed scientists in the South have warned that using a sole herbicide in Roundup Ready cropping systems would inevitably lead to a resistant weed boom. It was preordained by nature, they insisted. And, as more weeds were selected out for resistance, the scientists were proven correct.

Having developed an abundance of resistant pigweed and resulting problems, Georgia was the resistance trip-wire. But it turns out Georgia doesn’t have that much of a head start on the Mid-South. The increasingly common sight of chopping crews hoeing their way across Mid-South fields is testament to that.

Most years, Larry Steckel gets three to five calls on glyphosate failures. Earlier this summer, the veteran University of Tennessee row crop weed specialist was getting five per day.

“Glyphosate-resistant Palmer amaranth is blowing up, mainly in cotton and soybeans. Resistance was confirmed in three counties last year. We’re up to at least 10 counties now — all on the west side of the state.”

Tennessee’s Mississippi River counties were hit first. Now, the problem is moving inland with Crockett, Gibson and Carroll counties home to the resistant weeds.

Much of the resistance is being seen in scattered plants or, more typically, areas the size of a couple of pickup trucks. Pigweeds in such areas have been sprayed two or three times with Roundup and “aren’t even bothered.”

Does Steckel have a handle on the tolerance/resistance levels?

“That’s a huge concern. In the past, when you applied 22 ounces of Roundup WeatherMax to a resistant pigweed, it’d at least cause symptoms. Now, in some cases, we can spray 152 ounces and not see any symptoms.”

The rapid spread of the resistance has “absolutely shocked” Steckel. “It’s hard to believe how quickly and strong the resistance has become and spread.”

Having been an Arkansas Extension weed specialist for years, Ken Smith thought he’d “quit being surprised at what weeds are capable of. But, let me tell you, these resistant pigweeds are so much worse than we thought they’d be.”

(Editor’s note: for more on Smith’s resistance studies, see http://deltafarmpress.com/mag/farming_high_incidence_arkansas/index.html)

And it isn’t as if growers aren’t doing anything to slow down resistance. Some producers “have done everything right and still aren’t able to manage these pigweeds,” says Smith.

“At this point, I suspect we won’t be able to harvest some fields. Other fields have pigweed-infested areas that will be left alone during harvest. Some cotton fields are severely infested. Combines will run through pigweeds better than a picker. Pigweeds and cotton pickers don’t mix well.”

Recently, Smith was in a Marianna, Ark., soybean field. “The grower had used only Roundup and probably got 30 to 40 percent control. Now, about 60 percent of the pigweeds in that field just kind of giggle at glyphosate.”

With resistance issues on the front burner, in mid-July growers and consultants traveled to central Arkansas’ Newport Research Station for a plot tour.

“The main reason I’m (conducting plot work here) is because this place is a pigweed-infested hell,” said Bob Scott, Arkansas Extension weed specialist, to the crowd. “It’s a carpet of Palmer amaranth where we work plots.”

The plots contain very few Touchdown or Roundup treatments.

“If we have a theme it’s ‘pigweed control without glyphosate.’ The reason for that is I’m up to about 12 or 14 fields that I’ve personally been called on where one or two applications of Roundup PowerMax or Touchdown have failed to kill pigweeds.”

Such incidents are well past the point of novelty, said Scott.

“Glyphosate-resistant pigweed is here in the state in a big way. (A chemical company representative) told me he knows of 40 to 50 fields that have been retreated with full rates of Flexstar following the failure of Roundup to kill pigweeds. Those fields are all over the state, not in one area.”

In the plots, viewed in a rain, Scott showed where a shot of Flexstar was applied to 8-inch pigweed.

“It was about 60 (percent control) and now is a zero — they just re-grew. It burned the tops out of the pigweeds and messed them up pretty well. They just turned around and came back.”

Flexstar is an “excellent pigweed material” if the plants are only several inches tall. “All these pigweeds we’re missing with two shots of Roundup PowerMax are being followed with Flexstar, largely because there’s no other choice. There’s nothing else to recommend other than a hoe.”

Also on the tour: several plots highlighting LibertyLink and Ignite.

“The main thing to check in the plots is that Ignite isn’t glyphosate. It is a replacement technology but it can’t be used the same way Roundup is used. You can’t wait to spray Ignite on larger pigweeds. They must be sprayed when they’re small. You can see that in the plots.

“That can be a challenge for growers — especially if you walk a lot of acres. There isn’t as much of a grace period. We’re heading back to 1996 when weeds had to be sprayed” when very small.

Observers also checked residual options in the plots — either conventional or LibertyLink.

“Valor and Prefix, two relatively new products, look very good on pigweeds. Using pre-emerge (products), there are some things we haven’t done and there are probably some younger farmers/consultants that may not remember having to use some of the residuals. If you aren’t familiar enough with them, get educated.”

Despite screening many samples, “we still haven’t confirmed any resistance to Ricestar or Clincher. Many things can make those chemicals not work properly — drought, weed size, rate.”

Scott and colleagues have also screened barnyardgrass for ALS resistance to Newpath. That’s a major concern, “but, so far, those have all proven negative. Currently, some barnyardgrass is being screened for glyphosate resistance. We’ve done the same before, but the grass has been susceptible. However, suspicious samples keep rolling in — we got some today.”

Steckel is currently looking at many tank-mix options with Roundup to control glyphosate-resistant pigweeds.

“In soybeans, Flexstar does a good job. If we catch them early enough — say 6 inches, or less — control will be close to 90 percent. Resource mixed with Roundup provides about 80 percent control. Blazer gives about 85 percent control.

“We also tried ALS inhibitors like Classic, Harmony and Pursuit. Those tend to provide a homerun or a strikeout. Some of these weeds are also ALS resistant, evidently.”

In Arkansas, glyphosate-resistant Palmer amaranth is now widespread. Smith suspects flooding is a major cause.

“We were looking at the map where most of the resistance has shown up. I’m not suggesting this problem is being carried around exclusively by water. But it sure appears that the worst resistance populations are following the rivers. It would be interesting to do some sampling to see how many seed are in the rivers.

“With all the flooding we had this year, though, I’ve been apprehensive about seed traveling. Well, I’m afraid this resistance problem is about to get much worse.”

Steckel agrees. “After walking so many of these problem fields, I’m fairly confident much of this problem was spread through flooding. That isn’t the case everywhere but it certainly is in the river counties where the problem is most widespread.

“You can kind of follow the drift line to the problem — where high water reached. And growers who have never had this problem, after the flooding, suddenly have resistant pigweeds.”

Cropping scouts have done “great work” keeping growers updated on resistance issues, says Steckel. “They’ve done yeoman’s work. Several of them have told me that of the fields they walk, 50 to 70 percent have pigweed escapes.”

This cropping year has been tough so far, says Smith. “It’s been almost impossible to get all the herbicide applications out on time. So, I’m not sure how aware growers are about this resistance weed explosion. By harvest everyone will, though. By then, the magnitude of this will hit home.”

Another sign the resistance problem is growing: Smith has been asked repeatedly to provide contact information for chopping crews. “Growers want every chopping crew they can hire. But finding them is tough. Honestly, though, some of these fields I’m seeing are so overwhelmed with pigweeds even a chopping crew wouldn’t be enough. After you’ve sprayed a field three times with Roundup and still have a pigweed every square foot, chopping crews aren’t the answer.”

Steckel says he’s only seen a couple of fields that are “total train wrecks. One in Shelby County looked very similar to the hard-hit fields in Georgia. That field was sprayed three times with a 22-ounce rate of WeatherMax and had little success. They’ll try and salvage that field but it’ll be very difficult. Next year, I’m afraid there will be more fields like that.”

Chopping crews are also out in Tennessee.

“And in some areas, cotton farmers have sacrificed some of their crops. I know of a farmer with a large resistant area — maybe 100 yards long and 30 yards wide — and he applied Gramoxone and Caparol and took it all out.

“One of the biggest concerns with this is, quite frankly, it could run us out of cotton. In soybeans, at least we have some options. In cotton those aren’t there — once pigweed is up, it’s safe.

“That’s my greatest fear: losing cotton. Between commodity prices, plant bug numbers appearing to pick up and this resistant-Palmer amaranth explosion, cotton is an increasingly tough sell.

Of course, I’ve been checking with a lot of my counterparts in Georgia and North Carolina about this. They’re putting a pre down and, where it’s activated with water, it provides pretty good success. But in Tennessee we don’t have that much irrigation. It’s scary to think the pre activation is dependent on a timely rain.”

Those still able to control pigweeds with Roundup need to be extremely careful about being clean at harvest, says Smith.

“They don’t need one pigweed go to seed. There’s enough resistant pollen floating around in the state now to provide resistance to (progeny).”

e-mail: [email protected]

About the Author(s)

David Bennett

Associate Editor, Delta Farm Press

David Bennett, associate editor for Delta Farm Press, is an Arkansan. He worked with a daily newspaper before joining Farm Press in 1994. Bennett writes about legislative and crop related issues in the Mid-South states.

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