Researchers plan for resistant weeds

There is no silver bullet. Finding a cure for glyphosate-resistant horseweed will be a long, hard slog through test plots and laboratories. And remember this, say weed scientists: at the same time horseweed is being studied, other weeds are being selected out for resistance.

“In Tennessee, once we got the (horseweed) problem and realized how serious it was, we developed a pro-active strategy,” says Bob Hayes, weed scientist and superintendent of the West Tennessee Experiment Station in Jackson. “We wanted to let people know what was going on, to educate everyone we could.”

However, too often people hear but don't pay much attention until the problem is in their own backyard.

“As long as it was a Tennessee problem, few were concerned,” says Hayes. “Then, all of sudden, horseweed (also known as marestail) became a problem in other states and began impacting growers everywhere. If there's a lesson to be learned it's this: just because you don't have a problem now, doesn't mean you won't get it and shouldn't be preparing for it well in advance.”

Weed workshop

To aid in such preparation, Hayes and over 50 other researchers and chemical industry representatives attended a workshop in Tunica, Miss.

“Everyone threw their work on the table, and we all realized that there's a tremendous amount left to do,” says Ken Smith, Arkansas Extension weed scientist. “The meeting helped identify deficiencies in knowledge as much as show us what's known.

“We do know this: instead of a $6 to $10 per acre burndown treatment in Arkansas, we soon could be at $20 to $23. Crops just got more expensive to grow.”

In Tennessee, Hayes has found treatment costs have increased, although not quite as high as numbers Smith cites. “We've been looking at adding Clarity to glyphosate as a burndown. That costs around $7 or $8 per acre. If you have to put in a pre-emergence herbicide (which wouldn't be a bad strategy to manage against other weed resistance) it'll cost another $8 to $10 per acre.”

Another topic broached at the meeting was a central clearing house for information on resistant weeds.

“I think that's something that will develop out of this,” says Smith. “It would be great if everyone could come to a central location and see what the data is saying and how it compares to findings in their areas. Even if it was just for Delta states, it would be very beneficial.”

Three points

There are three main things Smith took away from the meeting:

  • The rapid spread of horseweed. In 2000, resistant horseweed was found in Delaware. In 2001, it was in Tennessee. In 2002, it was in Kentucky, Ohio, New Jersey, Indiana and Maryland. In 2003, it was found in Arkansas and Mississippi. This year, North Carolina, Missouri and Alabama have confirmed the weed's presence. “In Arkansas alone, we've gone from three counties with the problem last year to 10 counties. It's spreading very quickly.”
  • The need for a residual herbicide. “We're not going to manage this weed effectively without a residual program in front of cotton planting.”
  • A warning. “This should be a wakeup call about how quickly and easily these weeds become resistant.”

With the use of so much glyphosate, not only has a resistant horseweed been selected but also one that germinates throughout the spring.

“Back in the good old days, we could spray horseweed in a burndown program and be done with it for the year,” says Smith. “It wouldn't show up again all year. In this latest shift, though, we've selected out a biotype that germinates later and later in the spring.”

When that happened, early applications of glyphosate over the top of cotton lost potency.

“We can spray early and be clean at planting time and then have a flush of horseweed. We then spray glyphosate on up to four-leaf cotton. But by the time the cotton is at the stage where we can post-direct under it with an effective product, the horseweed has already grown too tall. We can't cover and spray it properly.”

This has necessitated re-evaluating soil residual herbicides. Valor and Clarity have some soil residual but also carry 21- and 30-day preplant intervals.

“We don't know how far Valor, for example, will carry us,” says Smith. “There's a deficiency of knowledge that must be addressed. It'll be at least through next season before we have some answers.

“Most likely, we'll be using glyphosate for the grasses and other trash weeds — henbit, chickweed. Then, we'll put in another product like Clarity to take down the horseweed. On top of that, we'll have to put a residual in the tank to hold the crop past planting until post-direct. Once we get into post-direct programs, we can handle the weeds. But it's a long time from mid-March burndown until cotton is at six-leaf stage.”

The lurking fear, say both men, is what will happen if other resistant weeds develop.

“We can't say we have weed resistance in many other species,” says Hayes. “However, everything is leading us to the conclusion that we will get resistance in other weeds.”

Like pigweed?

“Growers need to know that if they keep using glyphosate as the only herbicide to control pigweed, they're setting themselves up for resistance… we're putting tremendous selection pressure on pigweed across enormous geography.

“And when we get resistant pigweed — unlike resistant horseweed — we don't have tools to manage it. That's especially true in cotton. Some herbicides have activity, but they won't be the answer like Clarity is for horseweed.”

Pigweed may not be the next resistant weed. It may be morning-glory. If it is, herbicides like Staple and Envoke will be available, says Hayes.

But pigweed remains the real weed demon. Pigweed is incredibly prolific and has wide genetic diversity that makes it easier to become resistant — particularly when so much pressure is placed on the weed with “one molecule,” says Smith. “If there are any resistance management techniques to employ, they need to be used in pigweed. If pigweed becomes resistant it will revolutionize farming — and not in a good way.

“If a farmer sprays pigweed twice and it doesn't die, he should call us immediately,” pleads Smith. “We need to stay on top of this weed.”

What could a scenario with both resistant horseweed and pigweed look like?

“You know, we once farmed without glyphosate,” says Smith. “But we can't go back: we don't have enough cultivators, enough tractor drivers. We don't have enough farmers either. A typical farmer back then was farming 1,000 acres. Today, that same farmer may be farming 5,000 or 10,000 acres. Because of the convenience glyphosate has provided, producers continue to take in more and more acres. We just can't go back.”

And with all the pigweed seed in the soil bank currently, “I don't know how we'd farm. Hopefully, someone will find an answer through research. As part of our horseweed trials next year, we'll be focusing on two things to find needed answers: resistance management and how to farm pigweed country without glyphosate. Finding answers will take time, though.”

Hayes says while none were announced at the meeting, there are likely new products in the chemical companies' pipelines to help with resistance. But best not hold your breath waiting for them, he says.

“Who knows if a product will be released in five years? 10 years? What will the cost be? If that product is the only kid on the block, they'll charge all the market can bear. How much can a cotton producer afford on top of his existing program? The chances are any new product will be an add-on to glyphosate programs anyway, not a total replacement.”

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