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Giant ragweed soon could cast a giant shadow on the world's most popular
herbicide, January 2007.

Researchers at Ohio State and Purdue universities have confirmed
glyphosate-resistant giant ragweed populations in Indiana and Ohio.
Glyphosate is the active ingredient in herbicides such as Roundup and
Touchdown, which are used for burndown weed control in no-till cropping
systems and postemergence in Roundup Ready soybeans and corn.

The weed species is the seventh in the United States to show resistance to
glyphosate.

"We've identified one giant ragweed population in Indiana and a few in Ohio
that are showing resistance to glyphosate," said Bill Johnson, Purdue
Extension weed scientist. "The population in Indiana is located in Noble
County, which is northwest of Fort Wayne. The field in which it was located
had been in soybeans six out of the last seven years, and the producer
relied solely on glyphosate for giant ragweed control."

The three Ohio fields with glyphosate-resistant giant ragweed are in central
and southwest counties.

Johnson and Mark Loux, Ohio State University Extension weed scientist, urge
farmers to alter their weed control strategies in 2007 to slow the
development of glyphosate-resistant weed populations. They recommend
starting with a weed-free cropfield at planting and using a program of
pre-emergence herbicides, followed by a series of timely postemergence
herbicide treatments.

Giant ragweed is the most competitive broadleaf weed in Indiana soybean
production, Johnson said. The weed can grow as tall as 15 feet, if left
undisturbed. Populations of three to four giant ragweed plants per square
yard can reduce crop yields by as much as 70 percent, he said.

Farmers annually plant millions of acres in crops genetically modified to
withstand glyphosate applications. While giant ragweed can complicate corn
production, it is a bigger problem in soybeans because there are few
alternative herbicides that provide effective control.

"The reason this is a problem in soybeans is because we have only four
effective postemergence herbicides for giant ragweed," Johnson said. "Those
are glyphosate, Flexstar, Cobra and FirstRate. If the giant ragweed
population is resistant to ALS inhibitors, we are left with only glyphosate,
Flexstar or Cobra. If the populations are resistant to glyphosate and
FirstRate, then we're left with either Flexstar or Cobra as a
post-treatment."

Like glyphosate, aceto-lactase synthase (ALS) inhibitors kill weeds by
preventing them from producing essential amino acids necessary for growth.
FirstRate is an ALS inhibitor. Flexstar and Cobra are postemergence contact
herbicides that attack a plant's cell walls.

Johnson and Loux have monitored suspected glyphosate-resistant giant ragweed
since 2004, when farmers in Indiana and Ohio reported weed populations that
were responding poorly to glyphosate applications. In some cases, producers
were treating their fields with the herbicide three or four times the same
year or when giant ragweed populations had reached 15-25 inches tall.

"Our on-farm field research in 2006 demonstrated that resistant populations
were not adequately controlled by glyphosate-based programs that have been
effective in other populations," Loux said.

Johnson and Loux expect glyphosate resistance to show up in more giant
ragweed, although it might not spread as easily as it has in marestail,
another problem weed.

"The wind can blow marestail seeds longer distances than giant ragweed,"
Johnson said. "Giant ragweed seeds are large and heavy, so we don't think
seed movement is going to be a huge issue. It is unknown whether the
resistance trait might be able to spread in giant ragweed pollen."

Producers have a big role to play in managing weeds to avoid glyphosate
resistance, Johnson said. They should start before planting their 2007 crop,
he said.

"If growers have fields with a history of poor control of giant ragweed with
glyphosate, they need to change their management tactics," Johnson said.
"One big key is to start out with a clean field, with tillage or an
effective burndown, which includes 2,4-D. Other keys to control include
using a residual herbicide, and targeting the first in-crop postemergence
treatment when the giant ragweed is between six inches and 12 inches tall.

"For the first postemergence treatment on 6- to 12-inch-tall giant ragweed,
they also should use the maximum labeled rate of 1.5 pounds of acid
equivalent per acre of glyphosate, or substitute tank mix FirstRate,
Flexstar or Cobra for glyphosate in that first treatment."

If plants survive the initial postemergence treatment, a second
postemergence treatment should be made three to four weeks after the first
treatment, before the weeds start to poke through the top of the soybean
canopy, Johnson said.

Additional recommendations can be found in "Management of Giant Ragweed in
Roundup Ready Soybean Fields with a History of Poor Control," by Johnson,
Loux, Purdue weed scientist Glenn Nice and OSU weed scientist Jeff Stachler.
The article can be downloaded at [agcrops.osu.edu].

[www.seedquest.com]

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