Weed resistance without a magic bullet: Syngenta tackles glyphosate-resistant weeds

Chuck Foresman is worried. As a farmer he wants glyphosate and Roundup Ready crops to remain effective in perpetuity. But as the recently named Syngenta lead on weed resistance management, he knows that isn't possible.

Foresman, Syngenta's head of weed resistance strategies, is proud of his company's efforts to alert growers to the options available to “delay the onset of resistance. You notice I said ‘delay’ because we aren't going to prevent it…The days of glyphosate, glyphosate, glyphosate are over.”

Resistance versus tolerance

Weed resistance can develop in a genetically diverse weed population that selects fit genes that can reproduce in the face of repeated exposure to a herbicide. The herbicide isn't causing a mutation to occur — the mutation already exists in the plant population.

“All you've done is spray a herbicide often enough so the plants that survive are capable of enriching the next generation of genes that can resist the herbicide.

“Now, ‘herbicide tolerance’ is a situation where you never did a very good job of controlling a weed in the first place. That isn't resistance.”

For example, nutsedge is difficult to control with glyphosate.

“If we find a nutsedge plant in a field (after a glyphosate treatment) it doesn't surprise me,” said Foresman. “Glyphosate never has been very good on nutsedge. I work for Syngenta so I'll tell you what is good on nutsedge: Envoke. You can't use Envoke on soybeans and corn but it's a fantastic herbicide in cotton. Now, if Envoke began missing nutsedge it would point to resistance.”

The list

To date, commercial use of glyphosate on millions of acres of row-crops has resulted in five resistant weeds: horseweed, rigid and Italian ryegrass, common ragweed, and, most recently, Palmer pigweed.

“Palmer pigweed (resistance) is a wake-up call, folks. We'd better be paying attention. This is our opportunity to discuss the seriousness of this. If the pigweed family becomes resistant broadly across the Unites States, it will affect the way growers farm, the weed management and cultural systems.”

Horseweed is the granddaddy of resistant weeds, said Foresman. First discovered in 2000, there are now about 3.3 million resistant horseweed-infested acres in the United States.

Horseweed's ability to travel is a key reason for the resistance explosion. The seed has a plume and can be picked up by the wind and blown anywhere.

“If a hurricane comes through at the same time seed is being dispersed, it can travel (far and wide). It's interesting that horseweed was first found on the east coast (in Delaware) in a Roundup Ready soybean field. The field had been treated year after year with glyphosate, numerous applications.”

A scant year after the Delaware discovery, the resistant weed was found in Tennessee.

“So it went from the east coast to Tennessee (that quickly). Now, there are 13 states that have reported glyphosate-resistant horseweed.”

A measure of how important glyphosate is: the amount used around the world. In 2004, there was over $3 billion worth of glyphosate used globally.

“Combined, the next 10 most popular herbicides just about measure up to the amount of glyphosate used…There is a river of glyphosate being applied year after year. Annually, over 180 million acres are treated in the U.S. alone.”

The reason it's popular isn't hard to understand. Glyphosate controls 170 different weeds.

“(Syngenta) has just over 30 weeds on the Envoke label. Many herbicides control either broadleaves or grasses. Envoke controls a bit of both — but, again, only 30 weeds.”

For this year, a Syngenta database has identified the percent frequency of complaint for certain weed escapes. “In other words, the number of times certain weeds were identified as needing another treatment,” said Foresman. “Fall panicum (along with) foxtail, giant ragweed and velvetleaf were on the list. But lambsquarters, waterhemp and Palmer pigweed amounted to about half my complaints. I predict a weed like lambsquarters will…be the next officially identified as resistant.”

Ready adoption

At least in part, the adoption of Roundup Ready crops was propelled by the patent expiring on Roundup (glyphosate). The subsequent production of cheap generic glyphosate has made it a great value at the same time Roundup Ready crops have become an integral part of the modern U.S. farming system.

“Farmers are able to grow more acres of corn, cotton and soybeans because of glyphosate and the Roundup Ready system,” said Foresman. “It's a fantastic technology.”

Foresman is from an Illinois farm. “Over the years, growing cotton has always been difficult. It's not easy. You're taking a perennial crop and making it an annual, basically. It's not very competitive early and you have to be careful about the herbicides you use around it.

“But I'll tell you what: I could raise cotton — or at least take care of the weed control aspect because of the glyphosate program. Roundup Ready has made the crop less complicated and growers love it.”

But are they loving it into an early grave?

“I want to point out how popular Roundup Ready crops have become. In 2004, 90 percent of soybean acres, 22 percent of corn and 80 percent of cotton was (Roundup Ready). There is huge acceptance and (Roundup Ready) corn acreage is still growing.”

At the same time, the overall effectiveness of glyphosate seems to be slipping. Since the mid-1990s, “labeled rates for glyphosate have climbed. From the Roundup label, rates on foxtail have gone up 3X, on barnyardgrass the increase is 2.75X, ryegrass 2X, waterhemp 1.5X, and horseweed 1.5X. Over the years, we keep raising the rate. The effectiveness today isn't as good as it once was.”

Another Foresman chart concerns modes of action available to deal with weeds in 1996 versus 2003. In 1996, there were seven modes of action broadly used in cotton. In 2003 — “and I can project that onto today” — glyphosate use is dominant.

“As we go forward, I believe we'll see more of the residual herbicides used. Growers are astute enough to get some of them back into their programs. And that's important because using them will help us sustain glyphosate longer.

“Often I'm asked, ‘Why is Syngenta so interested in the glyphosate resistance?’ The reason is glyphosate helps us keep Syngenta products available in the marketplace. Glyphosate controls nearly all the weeds our herbicides control. Glyphosate is a mode of action that's effective on the same weeds we're trying to control. And it isn't just Syngenta products — it's the whole industry. Being able to keep glyphosate out there and viable helps sustain all the other modes of action.”

Like growers, EPA also likes glyphosate.

“It's green. EPA has been very easy to work with in respect to the addition of new uses and labels. This product is used from burndown to end-crop to harvest aide. Both agriculture and society at large benefit from the use of glyphosate and glyphosate-tolerant crops.”

Multiple resistances

For the first case of any suspected resistant weed species, Foresman believes it's important to do good heritability work. Once it's proven that resistance is present, “I don't know if you need to do heritability work for every other population found around the United States or world. What's important is to demonstrate the mechanism works to pass on (resistant) genes to the next population. That's what's important.”

The typical grower doesn't recognize resistant populations unless it's about 30 percent of his weeds.

“You'll say, ‘Well, there's a couple of weeds here but the sprayer must have missed them.’ Or, ‘Those weeds were just too big when first sprayed.’

“Well, lo and behold, those escapes go ahead and produce pollen and seed. The next year, resistant plant numbers (rise) and continue to build. Once that 30 percent threshold is met, that's when most will recognize, ‘Hey, I may have a problem.’”

As far as resistance to multiple herbicides, an Ohio population has confirmed resistance not only to glyphosate but also ALS.

“They'd been using First Rate to knock down this population. If we end up with weed populations resistant to multiple modes of action, (there is additional trouble).”

For example, consider narrow-row soybeans with a population of waterhemp or pigweed uncontrollable with glyphosate, a triazine or ALS. How will a producer take out the weeds?

“I don't know how he'd do it. You can't cultivate.

“I bring this up because there are already multiple modes of action resistance in some of these weeds. And to tack on glyphosate makes the weed problems even more complex.”

Foresman said weeds that will become resistant to a herbicide are likely to have already demonstrated such ability.

“That's because the genetic variability of these weeds is very broad. So broad, in fact, that a mutation already exists in the population. You're spraying millions of acres with a herbicide and that one mutation is surviving. That's how it happens.”

With both male and female plants, pigweeds are obligate out-crossers and aren't choosy about a mate. Research has shown pigweeds can pass on resistance genes between species — for example, from a Palmer pigweed to a regular pigweed.

Foresman is concerned about resistance passing from Palmer pigweed to waterhemp. “Can that happen? Nobody knows. But it's happened (between other pigweed species).”

One thing is for sure: There is no magic bullet.

“If we abuse this technology, we lose it,” said Foresman. “And shame on us if that happens.”

(For more information about Syngenta's resistance work visit www.resistancefighter.com)

e-mail: [email protected]

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