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Hard-to-control weeds common

Recently named LSU AgCenter state weed specialist, Bill Williams has stepped into a “common theme” of harder-to-control weeds, “whether in corn, cotton, soybeans, whatever. Teaweed and morning-glory too often get away. And an increasing problem has been with Palmer amaranth and some of the pigweeds. There are certainly resistance issues we’re concerned about.”

While Louisiana’s first official glyphosate-resistant weed is yet to be named, ryegrass is close to having the dubious honor.

In dealing with suspected resistance, “the number one issue is application timing,” Williams said at the recent Northeast Research Center field day outside St. Joe, La. “When we head out to look at a Palmer amaranth or common waterhemp-affected fields, 90 percent of the time we bring the weeds back to the lab, spray them and they’ll die.

“Over the years, glyphosate has sort of spoiled everyone. It’s common for folks to wait until the last minute — let all the weeds emerge and then make an application, or two. That works well with a lot of weeds.”

Williams pointed to weeds in the field behind him. “These are too large. Once they get this big, we’ve missed the boat. Not only are they going to be harder to control but they’ll already have removed yield.”

Pigweeds need to be targeted at 3 to 4 inches tall. Too often, they’re much larger before a herbicide is applied.

“They’ll wilt down — particularly Palmer and common waterhemp — the terminal will blow out. Then, they’ll sucker out at the bottom and grow large and they’re almost impossible to kill.”

Williams said Louisiana growers usually do a “great job of tank-mixing, using things like metolachlor/Flexstar and things like that. The problem with metolachlor and Sequence is they’re strictly a soil-applied herbicide. If you spray on growing weeds, it won’t help you any. And you’ll have lost the benefit of using a residual.”

Weed researchers have done many demonstrations with both teaweed and pigweed, primarily in soybeans. “We’ve had a lot of luck — growers have been very happy — when using something like Valor or Goal in a burndown program. That provides a two- or three-week window when the cotton or beans are coming up. Growers are able to delay an application a bit longer and it keeps the bigger pigweeds in check — they don’t come up at the same time the cotton or soybeans are emerging.”


Burndown/preplant is “probably what northeast Louisiana is having the most trouble with. Henbit is hard to control.”

Williams and colleagues thought that was solved a couple of years ago with things like Valor and Goal. However, this year the henbit began growing again.

“We’re not exactly sure why that is, but we’re doing a lot of work on it. We thought maybe the fact it begins to develop downy mildew in February and was interfering with control. But it turned out, if the disease was present on the henbit it was easier to control. That blew the original theory.”

Williams tried adding extra surfactant. It wasn’t able to completely overcome the regrowth but did help.

“I think the biggest thing with henbit is the lack of activity with Roundup/2,4-D. As long as both herbicides are kept at about 1 pound active ingredient, it works pretty well. Where there’s trouble is when 2,4-D rates are dropped to a quarter-pound or half-pound — where we’re just trying to pick up the primrose. That’s worked well in the past.”


Ryegrass populations “have exploded” in the state. “It’s a huge problem in the northwest part of the state and is moving (to the northeast). We’ve looked at a number of sites in Franklin Parish and others. It’s taking 10 to 12 pounds of glyphosate to kill the ryegrass.”

State weed scientists have not gone through the studies required to scientifically confirm glyphosate resistance. But Williams pointed out such resistance has been documented in neighboring states, in California and worldwide.

“I hesitate to go out and screen all these populations just to confirm resistance. It’s already obvious we can’t control it anymore — call it what you want, we can’t kill it.”

And there are not a lot of good programs to turn to. Resistance has already been confirmed with a lot of the ALS herbicides and some of the graminicides like Hoelon and Select.

What will control the ryegrass? “Really, we just need to try some things and see if it works. If you don’t have ALS-resistant ryegrass in corn, it can be taken out with Accent and Steadfast. Beacon works a bit.”

The biggest limitation is many growers have gone back to the Counter programs. “If you’re using Counter, then Accent, Steadfast, and Resolve are all out and can’t be used post-emergence in corn. If you have a ryegrass problem and are planning on using Accent, make sure you have an appropriate insecticide program.”

As for cotton and soybeans, “Select still works pretty well on most fields. There are some isolated fields showing resistance to those particular compounds.”

There will a label this year for fall-applied applications of Dual. The product “does a pretty good job. But I don’t really like that approach. For one thing, it keeps your beds clean all winter and they’ll erode. Second, you’ll spend $10 to $15 per acre to spray the whole field — and the whole field may not need it.”

Williams’ preferred approach: try to spot-spray areas where glyphosate failed in the normal burndown program.

“We’ve had very good luck with Gramoxone as long as the ryegrass is headed out. By using something like Gramoxone/diuron, it can be picked up. But it must be headed out. If it’s in the boot stage, or prior to it, it’ll just regrow. It has to be put out before flowering, so there’s about a two-week window in which to make the application.”

It’s all about timing, whether dealing with burndown or early-season weed control. “An Extension associate did a survey early in the spring in northeast Louisiana cotton and soybeans. Roughly, he found 70 percent of the soybean and cotton fields were unidentifiable. Those fields were cleaned up but, at some point, that will lead to resistance issues. It’s the same thing as cutting the glyphosate rate by half. Don’t let weeds get up to 10 to 12 inches tall.

“If you use something like Sequence to help with resistance, get the benefit of the residual. Make the application early when weeds are small. That’s why Dual is in there.”

Williams has checked several experimental compounds for weed control in corn. “Of course, we want to avoid resistance issues, so there’s a need to get away from only glyphosate-based programs.

Halex GT, a glyphosate-based program, has been “a very good product. Steadfast from DuPont does a good job. Laudis has done pretty well for us although it doesn’t have quite as much residual as Halex with Dual.

“We also looked at Corvis and Caprino. I’m not sure if they’ll be available next year. But they were very good standalone programs.”

So much corn, cotton and soybeans coming off before mid-September means “there’s a lot of time for weeds to grow. If you’re renting land, it’s difficult to go clean up fields. But your own land is worth cleaning the fields up in the fall. That not only will help with the next year’s weed control but it’ll help with insect populations by removing overwintering sites.

“A simple layby diuron/Linuron combination (half pound of diuron/half pound of Linuron) has been our best, overall. The problem with Envoke, Staple and products like that is when you’re dealing with (larger) teaweeds, they won’t do the job. Roundup won’t kill them either.”

Volunteers and grasses

Next to resistance issues and ryegrass, the next most important thing Williams has faced this year is volunteer crops. In mid-June, “we’re still getting calls on controlling volunteer corn, particularly in cotton. I believe Staple or Envoke is the way to deal with that. Select does a good job but if you’re dealing with other weeds, you might as well get more out of the application.”

In a layby situation, what is available that’ll hold grass for longer other than Dual or Sequence? “We’ve spent a lot of time looking at that. Dual will provide three or four weeks of control. In irrigated crops, it won’t last much longer. In dryland, it’ll last a couple of weeks if there’s a rain.”

Researchers have also checked diuron, excessive rates of atrazine and Dual. But there’s nothing available “that will hold from now until mid-August. Once the corn starts drying down or the cotton is defoliated, the first rain will push weeds out.

“In some ways, I kind of hope there won’t be a herbicide like that. If one comes out, you’ll be tied to a specific crop for the next year. That strong a residual will limit crop rotations, tie your hands.

“The closest thing we’re looking at that could help is some GAT corn and GAT beans. That may help out because there are some herbicides with very long residual control.

“The best weed control you can get right now is a crop canopy.”


TAGS: Management
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