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Resistant weeds continue devastating march through Mid-South farmlandResistant weeds continue devastating march through Mid-South farmland

Herbicide-resistant weeds continue to bedevil Mid-South.Liberty after Liberty a spraying recipe best avoided.New research shows importance of time of day when spraying.

David Bennett

January 27, 2014

8 Min Read
<p>A TENNESSEE COTTON field, pictured in early July 2011, is overgrown with resistant pigweeds. Stuck with a bad situation, the producer tilled the field up and replanted.</p>

The continuing spread, consequences and best ways to manage herbicide-resistant weeds were again key talking points of speakers at the annual Tri-State Soybean Forum. And hand-in-hand with such weeds is the need to protect herbicides that still work.

The point was driven home during the presentation of Larry Steckel, University of Tennessee weed specialist.

“We did a survey in Tennessee last year and 60 percent of our Liberty Link soybeans got nothing but Liberty on them,” during the Jan. 3 meeting in Dumas, Ark. University of Arkansas weed scientist Jason Norsworthy “ran a similar survey here in Arkansas and found similar results. Folks, we’re going to run Liberty into the ground if that’s the case. We’ve got to use other modes of action if we’re going to protect it and keep it around for any length of time.

“We’ve got to control (resistant pigweeds) early and up-front and overlay residuals as best we can.”

Steckel showed the audience a photo of a cotton field overgrown with pigweeds. “I’m sure a lot of you are very familiar with this type of situation. This is one of the many fields I was called to back in 2011. That was our high-tide for Palmer pigweed – the worst year, by far.

“I was called to this field on June 21, 2011. The farmer was about 24 hours too late spraying his Flexstar. If he’d been 24 hours earlier, he’d probably been pretty good.”

That night, about 8:00, Steckel scrolled through his phone log and found some 80 missed calls. “Almost all of them were about fields that looked like this. Most of (the callers) were retailers and county Extension agents talking about multiple problem fields. That was the first year that it dawned on me just how serious a problem this was.”

There’s no good answer in such situations, said Steckel. The farmer elected to till the field up and replant.

A photo taken on August 1 showed a cleaner field. “He used a residual the second time. The pigweed you see aren’t new germinators. Those re-rooted. You must be very aggressive with tillage on big pigweeds to affect them.

“Of course, another option is to try and keep beating at the weeds with herbicides. But that’s throwing good money after bad. So, you’ve got to take care of the problem up front the best you can with pre-emergence herbicides and overlaying residuals.”

Tennessee producers have handled pigweeds much better since 2011. “In 2012 and 2013 I’ve been pretty proud of where we are compared to where we were. It isn’t coming cheap, though, and it’s not coming easy.

“Typically, we go out with Roundup, dicamba and some kind of residual. Right behind the planter we spray gramoxone because you don’t dare leave a pigweed up and you want to take out any strangler marestail that got past the burndown.

“Then, we use a residual. I don’t have a big pick of the litter to use, although I like a herbicide with at least two effective sites of action for Palmer pigweed in the mix. We have the Valor-based products, Dual-based products, pyroxasulfone-based products, and even the Authority-based products. All would be good foundations, maybe get some metrabuzin in the mix.

“Then, try to overlay another residual over the top of the soybeans -- a Dual, a Prefix. One I like best post-emergence is Prefix.”


Steckel provided several herbicide program evaluations.

In the first scenario the producer has planted Roundup Ready 2 soybeans and has ALS-, DNA- and glyphosate-resistant Palmer pigweeds. What program to use?

  • Valor XLT.

“It has Valor and Classic in it. The group herbicide sites of action are 2 and 14. However, only one of those is effective on Palmer pigweed.”

  • Glyphosate post tank-mixed with Prefix.

“That gives us two effective sites of action: 14 and 15.

“So, for that group, we’ve got four sites of action but only three are effective for Palmer pigweed. And if the pigweed is already up the Dual isn’t going to help you.”

What’s a more robust plan?

  • Authority MTZ.

“That has metrabuzin and Authority in it, both with effective sites of action: 5 and 14. That’s for early pre.”

  • Prefix and glyphosate.

“Come back with prefix and glyphosate and that will give us six effective sites of action.”

Steckel acknowledged the financial hit producers can take with the herbicides. “I know I’m not the one writing the checks for these plans. I used to farm with my wife, Sandy, and remember writing checks for herbicides back in the early 1990s. It’s easier said than done. But in order to sustainably manage these weeds, particularly pigweed, we must put the investment in these residual herbicides.”

What about Liberty?

“We use a fair amount of Liberty in Tennessee. It’s primarily what we use in cotton and we have about 15 percent of our acres in soybeans.

“A lot of folks want to compare Liberty to glyphosate. The comparison I use is Liberty to the PPO herbicides. Liberty is non-selective, primarily on broadleaves. Liberty won’t give any residual. Some will – like Reflex, Flexstar.

“They work a bit different in the plant but they’re very similar in that they’re contact herbicides. So, you need good coverage. That’s critical.”

And there’s a big difference, said Steckel. “If you spray pigweed that’s too big with Liberty and burn it you can often come back 7 to 10 days later with another shot and control it. That isn’t the case with (the PPOs).

“I hate even mentioning that because from a resistance management standpoint spraying Liberty after Liberty is awful. But it is a fact and in some of these train-wreck fields that’s where we are.”

In a system, Liberty is very robust where it is utilized with a few other herbicide modes of action. “One of my favorites is Prefix pre followed by Liberty. Put a bit of Dual with the Liberty and you’ve got a really good resistance management program.”

  • Authority MTZ pre followed by Liberty tank-mixed with Prefix.

“I really like Liberty tank-mixed with Prefix -- three different modes of action. It’s a bit harder on the soybeans. It’ll burn them but it’s very effective in my research.”

When to spray?

If Liberty had come out and we’d never seen Roundup, all would think it was an excellent herbicide. But compared to what Roundup used to do, “Liberty is pretty finicky,” said Steckel. “Relative humidity makes a big difference, how cold it is makes a big difference on how well it works.”

Recently completed research also shows that the time of day or night producers choose to spray makes a big difference on how consistently Liberty will control pigweeds. Besides Tennessee, the study was done in Georgia by Stanley Culpepper, in North Carolina by Alan York, in Mississippi by Jason Bond and in Louisiana by Daniel Stephenson.

“Our graduate students were sent out in the wee hours of the morning to spray Liberty. We took a quart of Liberty and sprayed pigweed that was border-line too big -- five to eight inches tall. We sprayed an hour before sunrise, a half-hour before, a half-hour after, an hour after, then two, four and six hours after sunrise. We did the same thing prior to sunset: six hours before sunset, four hours, two hours, one hour, a half-hour as it set, a half-hour after and an hour after.”

Fifteen days later, the students returned and sprayed at the same time. “So, these were sequential applications spaced by 14 days.”

The results proved dramatic on the level and consistency of control. “All of the locations together showed an hour before sunrise there wasn’t 40 percent control. At sunrise, we didn’t get 50 percent control. It was two hours after sunrise before we got good control. That happened at every single location -- an oddity in research.

“So what’s happening? I don’t know if anyone has gone out with a frog-gigging helmet in the dark and checked pigweed, but it’s ‘asleep.’ I never noticed that but the leaves roll up. I think that might be part of the issue -- we aren’t able to get the coverage we would otherwise.”

But that isn’t the whole story. “In fact, research done in Georgia shows that Liberty is a light-activated herbicide. Mechanically, how it works in the chloroplast of the cell is where the rub is, apparently.”

What about spraying around sunset?

“We got a bit more inconsistent results around sunset. In North Carolina and Georgia, as the sun went down so did the level of control. It wasn’t as dramatic as in the morning but it was there.

“In Tennessee and Mississippi, we got 99 percent control even after the sun went down. But after the sun goes down, it’s probably a good time to quit spraying. Of course, when you’re trying to avoid drift, when do you want to spray?”

Steckel says the findings have helped him in field diagnoses. “I’ve been called to fields where farmers have sprayed Liberty on two- or three-inch Palmer pigweed and it’s growing back. First thing I ask: ‘what time of day did you spray?’”

In closing, Steckel offered these tips for using Liberty.

  • Spray smaller.

“Of course, that’s easier said than done with something that grows as quick as pigweed. When it gets around eight inches tall, you can figure on a sequential application seven to 10 days apart.”

  • Fifteen gallons water minimum.

“With Liberty, Flexstar, Cobra, gramoxone, 15 gallons is clearly better than 10. In my work, I haven’t seen 20 gallons make a big difference over 15.” 

  • Spray at least two hours after sunrise.

  • Use nozzles 350 microns, or less, in size.

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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