206
-Foraging Around, Lancaster Farming, Saturday, April 21,2001
Herbicide-Resistant Weed Identified In First State
NEWARK, Del. At the annual
meeting of the Weed Science Society
of America early in February, Dr.
Mark VanGessel, cooperative exten
sion specialist for weeds and crops
at the University of Delaware (UD),
announced that a weed-resistant to
the herbicide glyphosate had been
identified in Delaware fields.
Marestail (also known as hor
seweed), an annual weed that grows
up to six-feet tall, is commonly
found in undisturbed no-till or
fallow fields, said VanGessel. He
also serves as an associate professor
for weed and crop management at
UD College of Agriculture and Nat
ural Resources.
“We found resistant marestail in
three fields in Delaware and suspect
it may be present in three or more
fields here, and possibly in Mary
land and New Jersey as well. This is
the first report of a broadleaf plant
resistant to glyphosate,” he said.
According to VanGessel, the
weed has appeared in fields that
have been planted exclusively in
herbicide-resistant soybeans. In
each of these cases, burndown and
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over-the-top treatments of glypho
sate were used for at least three
years, using no other herbicides in
the rotation.
“In one of these fields, I saw dead
marestail plants right next to mares
tail that looked like nothing had
happened to it,” he said. “Marestail
control was random throughout the
field, and we were able to rule out
sprayer problems or application
error.”
VanGessel explains that about
half of all soybeans planted in the
U.S. have been genetically modified
to be resistant to glyphosate.
The first signs of trouble with
marestail date back to the summer
of 1999 when VanGessel received
calls about poor performance from
some growers. He received addi
tional growei calls in 2000 and calls
from Monsanto and Syngenta (the
newly formed company combining
Novartis agribusiness and Astra-
Zeneca agrochemicals business).
“The companies wanted me to go
with them to investigate certain
fields,” he said. “However, we could
see no pattern that would explain
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the problem.”
VanGessel first considered that
the drought in 1999 might be the
answer to the non-performance be
cause some herbicides don’t per
form well under hot, dry conditions,
he said. The 2000 season, however,
was typical for amount of rainfall in
Delaware.
“We applied 10 times the recom
mended rate of herbicide on the un
controlled marestail and did not kill
the weeds,” said VanGessel. “We
injured them, but didn’t control
them.”
“We have since raised a second
Capitol Region
Agronomy Team
(Continued
from Page 5)
but die before the
growing point
reaches the soil sur
face.
Depth control at
seeding is critical.
Loose soil conditions
can cause problems
1 jpGteritial
generation of marestail plants and
find that these also can tolerate 10
times the recommended rates of
glyphosate.”
A major concern, said the special
ist, is that this particular weed
grows tall and its seeds are as light
as dandelion seeds, and easily car
ried by wind. Worse yet, if the weeds
are resistant, they could spread even
more quickly.
VanGessel points out that certain
species of plants, including mares
tail, develop resistance more readily
than others. He is exploring the pos
(Turn to Page 11)
with depth control.
No-tilling can pro
vide effective stands,
but seeding depth is
also critical.
Be sure to cali
brate your seeder.
Settings that come
with the tool are
only guidelines and
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seed size variations
will certainly affect
seeding rates. Refer
to manuals for cali
bration procedures
and to seed pro
ducer recommenda
tions for seeding
rates.
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