1.Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
2.Glyphosate-tolerant pigweed confirmed in West Tennessee
COMMENT
We previously reported how Roundup resistant palmer pigweed - considered one of the very toughest herbicide resistant weeds to deal with - had been found infesting 500 acres of Roundup Ready cotton in central Georgia, USA. Now it's been found in Tennessee.
Resistant weeds are raising farmers' costs, undermining no-till agriculture, which was coming in before Roundup Ready crops, and causing more chemicals to be poured on the land.
To get effective weed control farmers are already having to resort to poorly regarded chemicals like 2,4-D and getting out the plough. As a weed extension specialist notes below, "With glyphosate-resistant horseweed we've already seen a reduction in no-till acres."
He also notes that extra chemicals with Roundup Ready crops are now almost universal. An informal survey he did of retailers showed that in the last year, "they believe around 90 percent of our cotton had a pre-emerge (herbicide) put on. Primarily, the reason for that was control of glyphosate-resistant horseweed."
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1.Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
By - Staff Reports
Business Journal, September 24, 2005
http://bjournal.com/2005/content/article_views.php?ID=756&Author=56
JACKSON, TENN. - Researchers with the University of Tennessee Agricultural Experiment Station and UT Extension have confirmed that two populations of Palmer pigweed have survived properly applied applications of the herbicide glyphosate.
The weeds populations exist in West Tennessee in Lauderdale and Crockett counties. Larry Steckel, Ph.D. and UT Extension weed specialist, conducted the field trials. "We have been watching these fields since first receiving reports in 2004 of Palmer pigweed not killed by Roundup," Steckel reported. "Our results last year indicated a very small number of pigweed plants survived our applications, but this year Palmer pigweeds at both locations survived a full 22 ounces of Roundup WeatherMax." Steckel said plants at one location survived a 2X application rate (44 fluid ounces).
UT weed scientist Tom Mueller, a professor in the Department of Plant Sciences, coordinated greenhouse and laboratory studies of the tolerant populations. "In some ways the Palmer pigweed appears to be similar to glyphosate-tolerant horseweed/mare's tail," he said. "All the treated Palmer pigweed plants look the same for two or three days after application. They all wilt and turn yellow."
Mueller said about four days after spraying the tolerant plants stop wilting and start new growth from lateral buds. "Our preliminary laboratory analysis indicates the mechanism of action, or how the plant tolerates the glyphosate, appears to be the same in the Palmer pigweed and in the glyphosate-tolerant horseweed," he said.
Their findings provide confirmation of an announcement earlier this year by University of Georgia scientists and Monsanto. Both UT scientists agree that this is an important development for farmers throughout the state and nation.
"Palmer pigweed that is not killed by glyphosate will cause major yield losses and harvest headaches for soybean, cotton and other row crop producers," Mueller said.
Although glyphosate-tolerant horseweed spread rapidly over large areas of the Mid-south, Mueller and Steckel hope the weight of the Palmer pigweed seeds will slow the spread of the new herbicide-resistant pest. "It is less likely to spread on equipment and on the wind," said Mueller.
Because of the weed's widespread resistance to glyphosate, horseweed has become a major production problem, but good control options are in place for all crops, especially on fields that can be tilled. "The proper application of herbicides such as Clarity, 2,4-D, Gramoxone Max or Ignite allows farmers to produce pretty clean crops," Steckel said.
"However, we expect resistant Palmer pigweed will pose more problems for producers than horseweed."
The University of Tennessee discovery reinforces the importance of managing weed resistance to herbicides. "It is essential to use more than one herbicidal mode of action on your fields," said Mueller.
More details on this finding and recommendations on how to deal with glyphosate-resistant Palmer pigweed are available at the Web site: http://UTcrops.com.
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2.Glyphosate-tolerant pigweed confirmed in West Tennessee
By David Bennett
Delta Farm Press, Sep 23, 2005
http://deltafarmpress.com/news/050923-tolerant-pigweed/
Glyphosate-tolerant Palmer pigweed has been found in west Tennessee's Lauderdale and Crockett counties. The announcement comes on the heels of a similar finding in Georgia pigweed earlier this summer.
"The fields were in continuous, Roundup Ready cotton for many years - at least from the late 1990s on," Larry Steckel, Tennessee Extension weed scientist, said Sept. 23. "Roundup was the primary weed control on them although there have been some post-directed chemistries on them as well."
Were rates and sprayings properly applied?
"To my knowledge, correct, full-label rates were used. I'm very familiar with the farmers involved. They're very good at growing crops and don't cut rates. I'm confident this wasn't human error.
"Nowadays, we're putting Roundup on everything. It's led to unprecedented selection pressure. We were bound to find genes that could handle the chemistry."
Called to the fields in 2004, Steckel said it was immediately evident something wasn't right. "The way it looked - live pigweeds side-by-side with dead pigweeds at the same height - raised a red flag with me. When I checked the fields, pigweed was all that wasn't being controlled. My first thought was, 'Well, this could be the real deal.'"
There were plenty of pigweed in both fields. However, that alone didn't cause Steckel much worry. "Western Tennessee is covered up with Palmer pigweed. It isn't uncommon to see fields with a bunch of it. I get called to a lot of fields on suspicious weeds. After investigating, most of the time the escapes are due to rain after application, surfactant issues or something else. But none of that applied here."
This past spring, Steckel and colleagues decided to put out a number of trials: two in the questionable fields and two placed randomly in the counties. Normally, Palmer pigweed less than 6 inches tall can be "smoked" with a half rate of glyphosate, said Steckel.
"So in these tests, we looked at a half-rate, a full rate, a double rate and a 4X rate. At the two random sites, we got complete control on everything with the low rates."
In the two suspect fields that wasn't the case. "At the half-rate of Roundup WeatherMax, control was around 50 percent. At the full rate (22 ounces), control was around 80 percent. At the 44-ounce rate, we still had some escapes. At the 4X rate (88 ounces), everything was killed."
Tom Mueller coordinated greenhouse and laboratory studies of the tolerant pigweed populations. "In some ways the Palmer pigweed appears to be similar to glyphosate-tolerant horseweed (marestail)," said Mueller in a press release. "All the treated Palmer pigweed plants look the same for two or three days after application; they all wilt and turn yellow. However, at about four days after spraying, the tolerant plants stop wilting and start new growth from lateral buds. Our preliminary laboratory analysis indicates the mechanism of action, or how the plant tolerates the glyphosate, appears to be the same in the Palmer pigweed and in the glyphosate-tolerant horseweed."
In light of the test results, what are Steckel's recommendations?
"First, producers need to get more chemistry in the tank, more modes of action. And that's already been happening.
"I just did an informal survey of some retailers and, in the last year, they believe around 90 percent of our cotton had a pre-emerge (herbicide) put on. Primarily, the reason for that was control of glyphosate-resistant horseweed."
"Dual over-the-top of cotton postemergence will be a terrific tool. We'll be preaching that."
"Most importantly, Roundup rates shouldn't be cut. Producers must use the full rate and get good coverage."
Could the finding impact no-till acres?
"With glyphosate-resistant horseweed we've already seen a reduction in no-till acres. However, as successful as we've been with using pre-emerge herbicides, I think we'll see no-till acres rebound - especially when you consider the cost of diesel. Even with this new threat, I see that happening."
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