"What's scary is exactly a year after discovery it was already widespread in Delaware. The same pattern was seen in Tennessee. The first time I started seeing it while driving around Missouri was in 2003. The last couple of years, phone calls to me on this weed have been heavy."
---
from nlpwessex:
Three items:
1. No quick cures for glyphosate-resistant weeds
2. Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
3. Glyphosate-tolerant pigweed confirmed in West Tennessee
"The [glyphosate] resistant type [of horseweed] was first discovered in Delaware in 2000. In 2001, it was found in western Tennessee. In 2002, it was found in Missouri and Arkansas. What's scary is exactly a year after discovery it was already widespread in Delaware. The same pattern was seen in Tennessee. The first time I started seeing it while driving around Missouri was in 2003. The last couple of years, phone calls to me on this weed have been heavy."
Andy Kendig, Missouri Extension weed specialist
***
1. No quick cures for glyphosate-resistant weeds
Delta Farm Press, 27 September 2005
http://deltafarmpress.com/news/050927-glyphosate-resistant/
"We have been watching these fields since first receiving reports in 2004 of Palmer pigweed not killed by Roundup. 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.....we expect resistant Palmer pigweed will pose more problems for producers than horseweed."
Larry Steckel, University of Tennessee Extension weed specialist
http://bjournal.com/2005/content/article_views.php?ID=756&Author=56
2. Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
Business Journal, 24 September 2005
"Palmer pigweed that is not killed by glyphosate will cause major yield losses and harvest headaches for soybean, cotton and other row crop producers... It is essential to use more than one herbicidal mode of action on your fields."
Professor Tom Mueller, University of Tennessee weed scientist
http://bjournal.com/2005/content/article_views.php?ID=756&Author=56
3. Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
Business Journal, 24 September 2005
"The fields were in continuous, Roundup Ready cotton for many years - at least from the late 1990s on. 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.... 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.... 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...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.... First, producers need to get more chemistry in the tank, more modes of action. And that's been 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."
Larry Steckel, Tennessee Extension weed scientist
Glyphosate-tolerant pigweed confirmed in West Tennessee
Delta Farm Press, 23 September 2005
http://deltafarmpress.com/news/050923-tolerant-pigweed/
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No quick cures for glyphosate-resistant weeds
Sep 27, 2005 8:52 AM
By David Bennett
http://deltafarmpress.com/news/050927-glyphosate-resistant/
Delta Farm Press
For five years, glyphosate-resistant horseweed has moved swiftly across farmers’ fields. Despite the work of many great researchers there remain no quick remedies to the troublesome weed.
Mostly a no-till problem, the history of resistant horseweed (or marestail) is interesting. "The resistant type was first discovered in Delaware in 2000," said Andy Kendig, Missouri Extension weed specialist at the annual Delta Center field day outside Portageville, Mo., on Aug. 31. "In 2001, it was found in western Tennessee. In 2002, it was found in Missouri and Arkansas.
"What's scary is exactly a year after discovery it was already widespread in Delaware. The same pattern was seen in Tennessee. The first time I started seeing it while driving around Missouri was in 2003. The last couple of years, phone calls to me on this weed have been heavy."
There are good treatments available to deal with resistant horseweed but none are standalone cures. Through research findings on the weed, Kendig has come to several conclusions. Among them:
Fall herbicides aren't recommended
In a field with horseweed infestation ranging from 75 to nearly 150 plants per square yard - "a bad infestation" - none of Kendig's treatments did much good.
In January, after November treatments, "we found less horseweed but control was still a long way from adequate. Our best treatment was 25 plants per square yard. That's not good enough for a grower. Some of our March and April applications did a bit better job but with them, the presence of a residual herbicide didn't help."
Basically, the fall herbicides aren’t lasting long enough to do much good. In January, the November treatments may look clean. "But in March, the weeds cut loose. By planting time, we had a serious horseweed problem again."
A later date for burndowns?
Kendig sprayed 2,4-D and Clarity on a twice-a-month schedule from Feb. 15 through May.
"Basically, after a March 15 burndown with either 2,4-D or Clarity, horseweed germination was greatly reduced but still wasn't zero."
This leads to a difficult question: does Kendig recommend producers wait until after March 15 to burndown?
"The limitation is the 2,4-D has a one-month preplant interval. If you get up to March 21, you’ll be forced to wait to April 21 to plant cotton. Depending on the rate used, Clarity has a two- or three-week preplant interval. This is definitely something for producers to think about, though."
Before making any decision, Kendig cautioned the data is from one year only.
"One thing that scares me is every year is different. A slightly different weather pattern could mean the 'magic date' will be April 1 instead of March 15."
FirstRate, Envoke, Ignite
Kendig is frequently asked how to deal with resistant horseweed in soybeans. "Fortunately, for soybeans, the herbicide FirstRate has been pretty good. There were some failures when the horseweed was large or drought-stressed. But FirstRate is the standard recommendation and mostly does a good job in soybeans."
In cotton, Kendig and colleagues have tested Envoke several times. "Once it didn’t work very well on knee-high horseweed. Last year, we tried Envoke again and it looked better. However, the second time around, we targeted horseweed that was only 8 inches tall. We need some further testing."
The Liberty Link system with Ignite has also garnered a lot of attention. Ignite as a burndown treatment has been somewhat inconsistent in Kendig’s research.
"But there’s no question when growing Liberty Link cotton, once temperatures warm up Ignite is an option for control of horseweed."
Gramoxone mixtures
Gramoxone alone releases horseweed. However, Gramoxone tank-mixtures with Caparol, Cotoran and Direx can provide upwards of 75 percent control. Other mixes have also shown promise.
"Something else we may also consider is a good disk and hipper. That may be the answer for some of our horseweed problem."
Tossing glyphosate not the answer
To prevent resistance from developing, Kendig has tried to remove glyphosate from burndown treatments. It hasn’t worked.
"Glyphosate controls too many weeds to throw it out entirely. For resistance prevention, think in terms of Band-Aid chemistry. There could be potential in beans and cotton to replace a glyphosate application with a pre-emerge treatment.
"But I caution folks against saying, ‘I’m taking glyphosate out of my burndown.’ If you do that, you’re giving up weed control."
Resistant Palmer amaranth
In late July, a "probable" case of glyphosate-resistant pigweed (Palmer amaranth) was confirmed in central Georgia. The weed is suspected to be confined to several locations. Across the nation, ears of weed scientists pricked up.
"On Palmer amaranth, the list of things you can add in Roundup Ready cotton is short. Prowl pre-emerge or incorporated Treflan/Prowl is an option, but incorporation isn’t very popular."
As a pre-emerge herbicide, Prowl has been "rather inconsistent. It depends on a good, activating rainfall. There’s no question in my mind that incorporating the yellow herbicide does a very good job. However, I also recognize that using it requires a lot of time and diesel fuel. Because of that hardly anyone is doing it now."
Now that resistant pigweed has likely been found, "my views have changed a bit. Is it time to put a half rate of Cotoran back in behind the press wheel?"
If the resistant pigweed spreads, Kendig said, lay-by chemistry will play a major role in control.
"(Roundup Ready) Flex cotton should be widely available in about five years. A big concern weed guys have is hoods and lay-by equipment will go to fence rows. I hate to hear that and feel it’s a bad situation.
"Our lay-by chemistry is key in managing palmer amaranth. We’ve done a lot of work with all the lay-by materials. Some are a little better than others. But the big thing with these materials is getting them sprayed on time. The Flex cotton, ideally, will allow us always to get an ideal height differential."
What about Zorial?
"It’s a good, soil-applied treatment, and we’ve tested it extensively. But it’s hard to find at agriculture chemical dealerships now that growers have essentially stopped using pre-emergence herbicides."
---
Tennessee Researchers Confirm Glyphosate-Resistant Pigweed
September 24, 2005
By - Staff Reports
http://bjournal.com/2005/content/article_views.php?ID=756&Author=56
Business Journal
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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Glyphosate-tolerant pigweed confirmed in West Tennessee
Sep 23, 2005 2:53 PM
By David Bennett
http://deltafarmpress.com/news/050923-tolerant-pigweed/
Delta Farm Press
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 been 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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