is part of the Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Serving: Central

Glyphosate-tolerant pigweed identified in Arkansas

Judging by appearance, the trays of new-growth Palmer amaranth on Bob Scott’s greenhouse table don’t seem like they could be a formidable threat to Arkansas row crops. But early tests show these pigweeds are glyphosate-tolerant. In a farming environment largely made up of Roundup Ready crops that is big news.

Treated with the equivalent of 22 ounces per acre of Roundup WeatherMax, the pigweeds turned yellow and were stunted. Twenty-one days after treatment some had died while others were exhibiting re-growth. Using the same rate, known susceptible plants were dead in a week.

“It’s important to note that these pigweeds are still being killed with glyphosate,” says Scott, Arkansas Extension weed specialist. “It’s fair to say they aren’t resistant. But they have a high enough level of tolerance that we’re missing them in the field with normal rates of glyphosate.”

If those misses are allowed to mature and crossbreed, the concern becomes that more frequent and increasingly tolerant, perhaps resistant, populations will develop.

“We’re the third state (to discover) tolerant or resistant pigweeds. Last year, Georgia and Tennessee announced they’d found some. This is one bronze medal we could’ve done without.”

The Tennessee and Georgia populations have proven tolerant of 4X rates of glyphosate. The tolerance level for the Arkansas pigweeds is yet to be nailed down.

Mississippi County

Last fall, a producer found the tolerant pigweeds while harvesting his Mississippi County soybean field. The field had been in continuous Roundup Ready soybeans for five years and had been heavily dosed with glyphosate.

While unsure if the tolerant biotype developed in situ or floated in on floodwaters, the field was “a perfect candidate for tolerance buildup,” says Smith.

In addition to the tolerant pigweed, the grower’s farm also has giant ragweed (a weed difficult to control with glyphosate) and glyphosate-resistant horseweed.

“That’s the state’s glyphosate-trouble trifecta,” says Scott.

The grower first began noticing pigweed escapes in the fall of 2004. By last harvest season, the problem was worse and he called Mississippi County Extension agent Susan Matthews.

“Susan has had a lot of experience with glyphosate-resistant horseweed. When she went to the field and looked, the evidence was strong the plants had been up when glyphosate was sprayed.”

Trained eyes learn to look for skips, misses and other things that point away from tolerance or resistance. These pigweeds, randomly placed throughout 200 or 300 acres of soybeans, didn’t fit a theory of machine or applicator error.

Matthews striped several suspect pigweeds of seed and gathered soil from beneath the plants. Scott recently screened the samples.

“From our initial tests, we’re pretty sure this pigweed’s tolerance is heritable and is passed on to its offspring. The next test we need to do is a ‘lethal dose’ LD50 study — where the lowest dose needed to kill 50 percent of the population is determined.”

Plants are currently being transplanted for the lethality test. It will be summer before results are released.

Meanwhile, researchers are also set to conduct similar rate studies in natural populations since greenhouse plants are often more susceptible than those in the field.

“We need to get into the field for that test as well as to identify program approaches for management of this biotype in both cotton and soybeans,” says Scott. “From where I stand, it really doesn’t matter whether this pigweed crosses the line from tolerant to resistant. What really matters is it’s not being controlled with normal rates and is tolerant enough to cause major management problems. Right now, it’s very important to get onto the farms and find out how best to deal with it.”

The threat

The Arkansas Herbicide Resistance Weed Committee is closely involved in the approach to, and announcement of, the pigweed’s discovery. The nine-member committee — consisting of Extension personnel, independent consultants and industry representatives — has already released several brochures on resistance threats.

At least one board member, crop consultant Chuck Farr, believes he’s encountered tolerant pigweeds before.

The greenhouse-raised pigweeds appear “similar to others he’s seen throughout Mississippi and Crittenden counties,” says Scott. “He thinks this is a problem in more fields than has been recognized. Ken Smith (Arkansas Extension weed scientist and resistance committee head) and I agree with him. That’s one reason we decided to go public with this now.”

The committee wants producers to have time to digest prevention, maintenance and management recommendations before the season begins in earnest.

“This is time sensitive. I’m pretty sure there are other fields where pigweed escapes have occurred following glyphosate applications and farmers have been forced to re-treat with a tank-mix of other herbicides. We just haven’t heard about them yet. This should help shake out some of those locations.”

When tolerant pigweeds get into cotton fields — “and they will,” says Smith — there are no over-the-top herbicides to control them. “Not long ago, I posed this scenario to some great cotton consultants who walk fields daily: There’s Roundup Ready cotton sprayed with glyphosate. Everything dies except the pigweed. What are you going to do?

“I wish I could tell you someone had a good answer. None of us did.

“This isn’t the only tolerant population we’ll find. I hope I’m wrong, but I foresee multiple locations being found this year. I don’t think that’s going too far out on a limb, either.”

If cotton producers were to lose Roundup and had to go back to “the old way of farming,” says Bill Robertson, Arkansas Extension cotton specialist, “it would be incredibly tough. We’ve made a lot of changes since the Roundup Ready system came in. It’s impacted the way we farm, the way we spread equipment and employees around acreage, the timeliness of applications. The logistics would be completely screwed up if we had to go back to a more conventional weed control program.”

Another member of the resistance committee, Zach Shappley, calls the pigweed find “very serious.” As a Monsanto technology development representative, Shappley says his company wants its Roundup Ready technology to be “available as a benefit over the long haul. Being pro-active on this will maintain (its) longevity. We encourage growers to use (chemistries other than glyphosate), other tank-mixes and other modes of action in their Roundup Ready systems. They need to start that now and continue it into the future. Spraying glyphosate alone is the wrong approach.”

Distances in space

Whatever happens with the northeast Arkansas pigweed, spraying programs will continue to include glyphosate. That might surprise people outside agriculture.

“But the glyphosate molecule controls so many other weeds and is so cost-effective that producers can’t really get away from it,” says Smith. “Instead, they’ll have to manage around the pigweeds and do things to try and prevent resistance.”

If resistant/tolerant pigweeds get a foot or 2 feet tall in soybeans, “you can just about forget it,” says Scott. “At that point, we can burn them back and make them mad but not kill them.”

Why did the pigweeds show up in northeast Arkansas? Could there be a connection to the Tennessee population?

“There’s no telling,” says Scott. “(There’s a) tremendous amount of diversity in pigweed populations. It’s been said before: pigweeds have no morals. They’ll crossbreed — boom, boom, boom. That leads to hybridization. And when you’ve got tremendous diversity among three or four species that can hybridize, that leads to an astronomical amount of genes and traits.”

Scott believes the cause of tolerance was continuous use of the same herbicide on a broad genetic base. “We’ve been putting selection pressure on our crops. This is ‘Evolution 101’ — except with unnatural selection.”

Dick Oliver, a weed scientist with the University of Arkansas, has done pigweed trials and found it’s not unusual to have eight to 10 pigweeds emerging per square inch of soil. As the weeds compete, some die out while others survive. If an inch of soil has 10 pigweeds, “imagine how much selection pressure there has been on 3 million acres of Roundup Ready soybeans,” says Scott. “You’d probably need one of those incomprehensible numbers like they use to measure distances in space.”

Cure/prevention: a wash

Another problem the pigweed presents: the cure for resistance costs as much as prevention.

“So imagine you’re a farmer struggling to make it,” says Scott. “You’re spending everything you can already without adding a pre-emerge treatment. When anyone asks what the cure is, I say, ‘You need a pre-emerge out.’ The response is, ‘Well, why don’t I just wait to (treat) when I have the problem?’”

No herbicide program provides thorough control of all weeds.

“It’s a numbers game. When a resistant weed escapes initial treatment there are often no alternatives for cleanup. In today’s agriculture there’s no room for major losses on even a small percentage of acres.”

Long-term, one could argue the extra expense of prevention maintains land values and popular technology. Currently, though, farming is short-term enterprise.

“It’s a year-by-year thing for many producers. It’s hard to blame them for taking advantage of the technology. That’s where the economy and desperation have pushed us.”

The takeaway message, says Smith, is “keep fields as clean as possible.” It is extremely important that any escapes be identified and dealt with.

“The farmer who first discovered resistant horseweed on his place told me, ‘When Roundup Ready crops came into our program, I thought we’d received a gift. But it wasn’t a gift — it was just a loan. And now we’re going to pay it back.’”


Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.