Columbia, Missouri
July 11, 2006
Herbicide tests in a western
Missouri soybean field have confirmed that tall waterhemp is the
sixth glyphosate-resistant weed in the U.S. and the ninth such
weed in the world.
"Our field trials follow full greenhouse tests in which this
tall waterhemp survived higher-than-recommended rates of
glyphosate and produced seed that grew into plants that also
were resistant," said Kevin Bradley,
University of Missouri
weed scientist.
Bradley and MU graduate student Travis Legleiter are testing
several soybean herbicides on the resistant weeds found in a
field near the Missouri River in Platte County.
"In the field we used up to eight times the labeled rate,"
Legleiter said. Notepad in hand, he kneels to count weeds in one
high-rate section of the field plot.
"You can see the waterhemp was injured initially but recovered,"
he said. Plants are light green to yellow in the center, a
symptom of glyphosate treatment. But the bulk of each plant's
leaves are green and healthy.
"They are regrowing and will likely survive to produce resistant
seed," Bradley said.
Their confirmation this week placed tall waterhemp on the
international herbicide-resistant weeds Web site,
www.weedscience.org. The site, dedicated to information on weed
resistance, lists 183 weed species that have been proven to be
resistant to one herbicide or another.
Glyphosate is the active ingredient in Roundup and other
herbicides. Bradley and other scientists have been concerned
about weeds developing resistance to the herbicide due to its
popularity when used with Roundup Ready soybean, corn and cotton
seed.
Bradley said the field history reads like a recipe for
developing resistant weeds: back-to-back seasons of one crop -
soybeans - and continuous use of the same herbicide - glyphosate
- since 1996.
MU weed scientist Reid Smeda has been working on another field,
in central Missouri, containing glyphosate-resistant common
ragweed for several seasons. That field has been in a
soybean-soybean-wheat rotation with almost sole use of
glyphosate herbicide for weed control.
While all resistant weeds are worrisome, Bradley said resistant
tall waterhemp is especially troubling.
"Waterhemp is one of Missouri's toughest weed problems. It has
developed resistance to a number of other soybean herbicides."
That resistance has been known to spread quickly. Waterhemp
plants are either male or female, which means females rely on
pollen shed from surrounding male plants.
"If the resistant trait is carried in the pollen, which we are
fairly confident it is, then you have pollen traveling to fields
all around the resistant plants." Each female waterhemp plant
produces hundreds of thousands of seeds, ensuring a ready supply
of plants for the following season.
Bradley and Legleiter have found good news in their field plots.
The glyphosate-resistant waterhemp is killed by a number of
popular pre-emergence soybean and corn herbicides. The pair plan
at least two seasons of examining whether the resistant plants
can be brought under control economically in continuous soybeans
- using pre-emergence and post-emergence herbicides - or whether
it is better for farmers facing resistant weeds to alternate
plantings of corn and soybeans. The rotation opens up a wider
array of herbicides labeled for use in corn.
The eight other confirmed glyphosate-resistant weeds throughout
the world include buckhorn plantain, common ragweed, goosegrass,
hairy fleabane, horseweed or marestail, Italian ryegrass, palmer
amaranth and rigid ryegrass.
RELATED RELEASE:
Monsanto recommends using a soil residual
herbicide in Roundup Ready soybeans to control
glyphosate-resistant waterhemp |