GENET archive


9-Misc: Glyphosate-tolerant pigweed confirmed in West Tennessee (USA)

                                 PART I
------------------------------- GENET-news -------------------------------

TITLE:  Palmer Amaranth (Amaranthus palmeri) Showing Reduced Sensitivity
        to Glyphosate
SOURCE: The University of Tennessee, USA
DATE:   Sep 2005

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Palmer Amaranth (Amaranthus palmeri) Showing Reduced Sensitivity to Glyphosate

Researchers with the Tennessee Agricultural Experiment Station and UT
Extension have confirmed two separate populations of Palmer amaranth
(pigweed) that survived properly-applied applications of Roundup
herbicide. These two populations are located on cotton fields in Crockett
and Lauderdale Counties. This past summer we conducted numerous field and
greenhouse trials confirming the reduced sensitivity of these Palmer
pigweed populations to glyphosate. Last year a very small number of
pigweed plants survived a glyphosate application in these fields. This
year some Palmer pigweeds at both locations survived a full 22 oz/A of
Roundup WeatherMax, and at one location some plants survived a 2X (44
fluid ounces) rate.

County Extension Agents Richard Buntin of Crockett County and Jerry
Parker of Lauderdale County alerted us to these fields in 2004. This past
summer with their help we conducted field trials on Palmer pigweed in
four different fields in two counties. One field in each county was
chosen due to suspected glyphosate-resistance. The other field in the
county was chosen to help ascertain the level of glyphosate tolerance in
a random field. As it turned out the random chosen field in each county
contained a "normal" population of Palmer pigweed that was readily
controlled with glyphosate. The normal Palmer pigweed fields were
separated from the glyphosate-tolerant fields in Crockett County by 3
miles and in Lauderdale County by 15 miles. All the Palmer pigweed we
evaluated had been in fields that were continuous cotton. The Palmer
pigweed in these fields was about 3 to 5 inches tall at the time of
application. The "normal" Palmer pigweed populations showed 100% control
with all rates of Roundup WeatherMax applied including the low 1/2X (11
fluid ounces/A) rate. The two tolerant Palmer locations showed 10 and 23%
Palmer pigweed survival at the 1/2X rate, 2 and 5% survival at the 1X (22
oz/A) rate, 0.5% and 4% survival at the 2X (44 oz/A) rate and complete
control at the 4X (88 oz/A) rate (Figure 1.).

Though overall Palmer pigweed control was slightly better, the greenhouse
analysis reflected the results in the field. Palmer pigweed grown from
seed in these fields showed two-fold (GR 50 = 2X) more tolerance to
glyphosate than Palmer pigweed grown from seed collected from a known
susceptible Palmer population from Jackson. In some ways the Palmer
pigweed appears to be similar to glyphosate-resistant horseweed. 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 4 days
after spraying the tolerant plants stop wilting and start new growth from
lateral buds.

Dr. Tom Mueller, weed scientist at UT Knoxville, conducted the laboratory
analysis. The preliminary data indicates that the mechanism of action, or
how the plant tolerates the glyphosate, appears to be the same in the
Palmer pigweed as in the glyphosate-resistant horseweed. It appears to be

This level of tolerance to glyphosate of the Palmer pigweed in these
Tennessee fields is a good warning. As most are well aware we have few
good options for controlling Palmer pigweed in cotton and soybeans
postemergence. This 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. Please refer to the Weed Control Manual
for Tennessee Row Crops (PB1580) for management recommendations for
Palmer pigweed.

Contact Information:
Tom Mueller, 865-974-8805,
Larry Steckel, 731-425-4705,

                                  PART II
------------------------------- GENET-news -------------------------------

TITLE:  Glyphosate-tolerant pigweed confirmed in West Tennessee
SOURCE: Delta Farm Press, USA, by David Bennett
DATE:   23 Sep 2005

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Glyphosate-tolerant pigweed confirmed in West Tennessee

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

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."

                                 PART III
------------------------------- GENET-news -------------------------------

TITLE:  No quick cures for glyphosate-resistant weeds
SOURCE: Delta Farm Press, USA, by David Bennett
DATE:   27 Sep 2005

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No quick cures for glyphosate-resistant weeds

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

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

"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

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."


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