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9-Misc: Waterhemp in Missouri (USA) 'potentially resistant' to glyphosate

------------------------------- GENET-news -------------------------------

TITLE:  Waterhemp 'potentially resistant' to herbicide found in NW
        Missouri soybean fields
SOURCE: Agriculture Online, USA, by Duane Dailey (University of Missouri)
DATE:   23 Sep 2005

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Waterhemp 'potentially resistant' to herbicide found in NW Missouri
soybean fields

COLUMBIA, Mo. -- Common waterhemp with "potential resistance to
glyphosate herbicide" has been found in two soybean fields in northwest
Missouri by a weed scientist at the University of Missouri.

"Waterhemp grown from seed collected from suspect fields in 2004 shows
high tolerance to glyphosate in two greenhouse dose-response trials this
summer," said Kevin Bradley, MU Extension weed specialist. "The weeds
were found in fields planted to Roundup Ready soybeans continuously since
the new varieties were introduced in 1996.

"Common waterhemp is our No. 1 weed problem in corn and soybeans in most
of Missouri," Bradley said. "With the introduction of Roundup Ready
soybean varieties, glyphosate (the active ingredient in Roundup and
similar herbicides) became the No. 1 herbicide used in soybean fields."

Roundup Ready varieties are genetically modified to tolerate the
herbicide. Farmers spray glyphosate after the crop is growing, killing
the weeds, while the modified soybean plants are unharmed. The U.S.
Department of Agriculture reported that 89 percent of Missouri's 2005
soybean acreage was planted to these genetically modified soybean varieties.

Herbicide resistance develops in weeds when a plant in a weed population
has natural resistance to an herbicide, Bradley said. The plant is not
killed and produces seeds. Over time, with continued use of the same
herbicide, resistant plants dominate the population in a field as
herbicide-susceptible plants are killed.

Histories from the suspected fields supported the theory that the weeds
might be resistant, Bradley said. They have been in continuous soybeans,
and glyphosate has been the sole herbicide used since 1996.

"Problems developed over the last three years at one site and the grower
continued to use increasing rates of glyphosate," Bradley said.

To prevent development of herbicide resistance in weeds, extension
specialists first recommend alternating herbicides with different modes
of action. Second, Bradley said, a farmer can rotate growing corn and
soybeans. "Generally, when you rotate crops, you also rotate herbicide
modes of action."

MU specialists also encourage farmers to scout fields after spraying to
find any "escapes," and then eliminate these with mechanical or spot

Common waterhemp, also known as pigweed, produces thousands of tiny seeds
per plant. "Although waterhemp is not as competitive as some other
pigweeds, high numbers reduce yields in corn and soybean fields," Bradley

In MU greenhouse tests, some waterhemp continued to grow after being
sprayed with rates as high as 6 pounds of glyphosate acid per acre. "To
put this in perspective, the recommended rate for the size waterhemp that
we were spraying in these trials is 0.75 pounds of glyphosate acid per
acre," Bradley said.

By definition, an herbicide-resistant weed must have the inherited
ability to survive and reproduce following an application of an herbicide
that normally kills it.

While his greenhouse studies confirm survival following labeled
applications, Bradley is beginning inheritance studies to see if the
resistant trait is present in seeds collected from surviving plants. He
also will grow and treat plants under field conditions next season and
conduct surveys of the suspected fields.

In 2004, MU weed scientists confirmed a case of common ragweed, from
central Missouri, resistant to 10 times the normal rate of glyphosate.

If Bradley confirms inherited resistance in common waterhemp, that weed
will join ragweed, marestail and ryegrass as U.S. weeds found to have
developed glyphosate resistance.

Bradley has been called to fields where glyphosate resistance was
suspected. "Usually, we find some other cause for weed survival. Often
weeds came up after the last glyphosate application."

Glyphosate has no residual activity. It must be applied to growing weeds
to be effective.

Weeds grown in another suspected field in southwest Missouri did not show
glyphosate resistance in the greenhouse.

In his study, Bradley compared six biotypes of common waterhemp. The
control was seed from waterhemp in plots at the MU Bradford Research and
Extension Center near Columbia that have never been sprayed with glyphosate.

The three suspected biotypes were compared with two lines of waterhemp
that had shown "some tolerance" in tests several years ago. "All except
the two biotypes from northwest Missouri were killed with the recommended
rates of herbicide."

"This does not mean I recommend anyone moving away from Roundup Ready
soybeans altogether," Bradley said. "It is very effective and economical
for growers.

"However, the system must be used responsibly to sustain the technology."


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