GENET archive


2-Plants: U.S. weed scientists on glyphosate-resistant pigweed

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

TITLE:  Mid-South weed scientists on resistant-pigweed
SOURCE: Delta Farm Press, USA, by David Bennett
        files attached: glyphosate.gif
        copied from
DATE:   17 Aug 2005

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following web page can be used to search for herbicide-resistant weeds by
country, active ingredient etc., it gives plenty of information about the

International Survey of Herbicide Resistant Weeds

Funded and Supported by the
- Herbicide Resistance Action Committee (HRAC), the
- North American Herbicide Resistance Action Committee (NAHRAC),and the
- Weed Science Society of America (WSSA).

The purpose of this survey is to monitor the evolution of herbicide-
resistant weeds and assess their impact throughout the world. Global
collaboration between weed scientists make the  survey and this web
site possible.

302 Resistant Biotypes, 181 Species (108 dicots and 73 monocots) and
over 270,000 fields

Mid-South weed scientists on resistant-pigweed

A probable case of glyphosate-resistant Palmer amaranth (pigweed) has
been found in Georgia. The discovery is no surprise: inexpensive
glyphosate, an increasing reliance on Roundup Ready crops and the weed's
diverse genetics led many weed scientists to predict such resistance long ago.

"Many of us said with the selection pressure we were putting on pigweed,
this would happen," said Bob Hayes, University of Tennessee weed
scientist. "For the amount of pressure, this announcement was right on
schedule. I hope this opens a bunch of eyes and shows how important a
diversified weed control program can be.

"When this pigweed gets (to the Mid-South), it could set us back. But
this is a big enough deal that many smart minds will go to work to find a
solution. I think it could dramatically change the cost of production.
But I'm not saying the sky's falling yet."

Hopes and fears

While no one knows exactly what glyphosate-resistant pigweed would mean
to the Mid-South, "it's fair to say it won't be good," said Ken Smith,
Arkansas Extension weed specialist. "Pigweed has a history of resistance.
It's a great candidate for it because it has a lot of genetic diversity,
it produces a lot of seed, it grows rapidly, it competes well and it
germinates over a long period of time. All of those factors mean we're
facing trouble."

Unlike glyphosate-resistant horseweed (marestail), Palmer amaranth seed
isn't windborne. For that reason, weed specialists don't anticipate a
rapid movement of the pigweed biotype found in Georgia.

"I believe we'll develop resistance in a local population," said Andy
Kendig, Missouri Extension weed control specialist. "That will probably
happen quicker than the Georgia weed will spread to this region."

The Southeast finding "definitely" changes the playing field, said
Arkansas-based Ford Baldwin, a weed scientist and Delta Farm Press
contributor. "Anyone who says he'll eradicate it is kidding himself. It's
good they're trying but weed resistance doesn't typically start in a
single field and then spread across the country.

"The same mechanism that caused resistance in Georgia is in place across
the country in umpteen acres. We're in a mono-glyphosate culture and this
was bound to happen. And it's bound to keep happening."

Years ago, Baldwin predicted Palmer amaranth would be the first weed to
become glyphosate-resistant. He was wrong -- marestail (horseweed) and
common ragweed beat pigweed for the dubious honor.

"This is the first serious (glyphosate-resistant weed), though," said
Baldwin. "Common ragweed is easy to control. Marestail isn't as easy to
control, but it can be done. Palmer amaranth, on the other hand, was hard
to control even before Roundup Ready came along. What is our alternative
if Roundup doesn't work anymore? That changes the game considerably.

"Resistant pigweed will impact agriculture in Arkansas in many more ways
than marestail has. Compared to resistant pigweeds, marestail is a minor
inconvenience. Resistant pigweed could be catastrophic."

Glyphosate dependent

Eschewing glyphosate, the most-used herbicide in the world, won't work,
said Kendig. "Just use less glyphosate isn't a workable solution for
growers. A large part of our research program is investigating
alternative chemistries. We've found if we delete glyphosate from our
weed-control programs, new weed problems pop up very quickly. We really
need and like the broad spectrum and strong control glyphosate normally

"Our example for the upper Mid-South is horseweed. Over the last few
months, we've used 2,4-D treatments, Clarity treatments, and Gramoxone
treatments on test plots with varying success. Some of the treatments
were inadequate as far as overall burndown. We still need glyphosate in
the mix.

"Pigweeds are scary. We struggle to control them with conventional
chemistry. ALS provided good control for a couple of years. But there's
plenty of ALS-resistant pigweed now, so that's another problem with this."

Smith agrees with Kendig. A few years ago, when producers began using
Staple on pigweeds in northeast Arkansas, it worked "wonderfully. It
killed pigweeds going and coming. Then, all of a sudden, it stopped
working as well. Now, we believe half the pigweeds in the state are
resistant to ALS herbicides. So when glyphosate resistance becomes a
problem here, there's a 50 percent chance it will already be resistant to
ALS herbicides."

Baldwin, who rarely minces words, said he expects Arkansas will harbor
the resistant weed soon. "This is coming, and it's going to be a massive
problem. It's going to be especially bad on the sandy soils in the
northeast. Before Roundup Ready, producers couldn't control Palmer
pigweed. In our research plots, we couldn't either. When this hits, watch

Looking for solutions

To a man, the specialists said no magic bullet exists to deal with
resistant pigweeds. They agree a partial solution will be residual herbicides.

"A preplant residual is probably a good idea but it won't fit the bill
entirely," said Smith. "A preplant residual will last three weeks at the
most. Pigweed germinates all the way through the cropping system. Truth
is, putting another herbicide out up front or chucking another herbicide
into the tank will help, but it won't keep us from developing resistance."

Smith expects more acres will soon be planted in Liberty Link cotton.
"That cotton makes use of a different herbicide with a different mode of
action. It will be a good resistance management tool. I think producers
will eventually rotate Roundup Ready cotton with Liberty Link cotton."

If resistant pigweed moves into Arkansas in a "big way," the injury and
yield reduction will be "more than Asian soybean rust or boll weevils
ever thought about causing," said Smith. "Unfortunately, if we get into a
resistant pigweed with cotton, we have no post-emergence chemistry to go
over the top. If pigweed has already germinated, we can't take it down.

"Pigweed produces 500,000 seed per plant. When you're facing those kinds
of numbers, you can lose a crop. Pigweed can be devastating."


Smith also predicts the arrival of resistant Palmer amaranth will lead to
a reduction in conservation tillage systems.

"We've been proud of the con-till systems, and we should be. But I don't
see how resistant pigweed won't set us back in conservation efforts."

This belief ties into the rapid increase in farm size since 2000. "That's
happened largely because we can farm more acres under Roundup Ready
technology using con-till. If resistant pigweed hits that technology,
producers working 6,000-plus acres with a handful of employees are going
to be facing some tough choices."

Hayes said resistant pigweed could cause a move away from conservation
tillage. "We've seen that happen some in Tennessee due to resistant
horseweed. Of course, the current cost of diesel fuel could keep the
producers in con-till.

"Not everyone has Palmer amaranth. In Tennessee, it's predominately on
the Delta (western) side. It's true that in that area, resistant pigweed
could have a huge impact. But in the rest of the state, it may have no
impact at all."

Roundup Ready crops have changed the United State's row-cropping system.

"Farming has changed incredibly because of the Roundup Ready technology,"
said Baldwin. "I'm not sure we're capable of going back and farming like
we have in the past. Everything is tied to Roundup Ready now. And that's
fine: Roundup Ready is the best thing that's happened to us. But this is
a huge issue."

It's more important then ever before for producers to report any escaped
pigweed. Because of weed escapes, Smith has seen more rope-wick
applicators used this year than in the previous 10. He assumes the
escapes aren't due to resistance.

"Earlier this year, we had poor application weather and that led to
inadequate coverage or larger weeds when they were sprayed. That creates
a problem for finding early incidences of resistant weeds. It's hard to
be vigilant when there are escapes everywhere. But please try. If you see
any escapes, let someone know."

Always lurking

"Last fall, the focus was on the threat of Asian soybean rust," said
Baldwin. "But this was always lurking. Let me tell you, when we start
getting glyphosate resistance to weeds like Palmer pigweed, it'll make
rust look like a pup. Now, this weed problem isn't going to blow in
overnight like rust. But once you get it, that weed will be in your field
every year. It's going nowhere."

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

TITLE:  Missouri's pigweed problem increases
SOURCE: Delta Farm Press, by Andy Kendig
DATE:   19 Dec 2003

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Missouri's pigweed problem increases

Since he farms on weekends, Jake Fisher, superintendent of the Delta
Research Station in Portageville, Mo., has good insight into issues
growers face. He recently asked what we were doing about pigweed. Of
course, he knew the answer (we're working on it), and we knew what he
meant (keep working on it!).

We continue work on Palmer amaranth control in corn, rice and soybeans.
But I especially want to mention work in cotton targeting pigweed
problems. These pigweed studies are being funded by Cotton Incorporated's
state support program.

Ten years ago, pigweed was considered easy to control. A good
recommendation was "spray something." Today, we consider the weed
difficult, and the most intensive treatments seem to fail.

It's also strange to consider what's happened since Roundup Ready has
become popular. Ten years ago, morningglory was our biggest problem. Then
we switched to glyphosate, which is considered to be weaker on
morningglory and stronger on pigweed. But it's morningglory concerns that
have faded while pigweed is capturing headlines.

Here's one more angle on pigweed: in northern Missouri, growers complain
of the waterhemp species of pigweed while we complain of the Palmer
amaranth species. These two pigweeds have some differences, but the big
similarity is they are both causing headaches. Waterhemp seems to be
troublesome north of US Highway 60 and Palmer dominates south of the highway.

Reid Smeda (a weed scientist in Columbia, Mo.) planted waterhemp at
Portageville, and it went away after three years.

In soybeans, Roundup Ready has been the answer. If you have pigweed
problems, Roundup Ready is probably the way to go. Just keep your eyes
open -- some of the waterhemp pigweed has been acting like it's
glyphosate-resistant. The chemists can't agree on what's actually
happening regarding the resistance claims. But whatever you call it, the
weed sometimes survives treatment.

Many say that Palmer amaranth is a good candidate to develop glyphosate
resistance, and it is. But while we've all been watching pigweed, mare's
tail turned out to be the dark-horse candidate that won the resistance race.

Roundup Ready cotton has also been a God-send for pigweed control.
However, I'm not so sure that it isn't causing some of the pigweed
problems. Roundup Ready cotton fields often have good pigweed control,
with an occasional big escape every 200 feet or so. These escapes die if
you spray glyphosate on them, so they aren't resistant. What we are
probably seeing is that plants right in the drill are physically missed
by the post-directed spray.

Also, the use of residual herbicides has declined greatly, allowing
Palmer amaranth to continue to germinate. Palmer will grow up to a foot
per week, and glyphosate has no residual activity, so an occasional plant
does seem to be slipping by.

In cotton, we've had some good luck with early postemergence Dual/
glyphosate mixes. With post-directed treatments, it's more important when
you spray than what you spray. Residual herbicides should be used in
post-directed, hooded and layby treatments. But if you spray on time,
most will work.

We might want to consider making more applications of residual herbicides
and fewer directed applications of straight glyphosate. Also, we still
see benefits from pre-emergence herbicides even though they aren't popular.

Palmer amaranth is also a problem in corn. In most cases, the cause is
that most corn herbicides are designed for cooler Iowa weather. Our
warmer, wetter, longer summers wear out the best herbicides. This gives
rise to all kinds of late-season weeds that germinate when corn dries out
and ceases to shade the soil.

Also, a lot of our pigweed have ALS resistance, and ALS chemistry is
popular in corn.

Recently, Ford Baldwin mentioned Palmer as being troublesome in rice. The
big cause there is that Command happens to be weak on pigweeds, and
Command has displaced propanil (a good pigweed herbicide). For the
record, Facet, Ricestar, Regiment, Londax, Permit and Clincher also do
not control pigweed.

We've tended to ignore broadleaf weeds until flood-up time, but maybe we
need to be jumping on them sooner in a Command program. The flood will
control pigweed, but if they are thick, they will severely inhibit rice
growth before flooding.

We will continue to work on pigweed control programs and share our
results. However, for the foreseeable future, and regardless of the crop,
we should recognize the following basic truths:

- Many herbicides provide excellent short term pigweed control.
- Success requires a well-planned program using more than one herbicide.
- In many cases, residual herbicides are key.
- Good weather and good luck helps.
- Ninety-nine percent control of a million weeds leaves a lot to be desired.

Andy Kendig is Extension Weed Specialist at the Missouri University Delta
Center in Portageville, Mo.


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