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Maverick marestail won't be rounded up
By Mike Holmberg
Farm Chemicals
Editor
Successful Farming
A report out of Delaware indicates that marestail (horseweed) that is
resistant to glyphosate has been confirmed in three separate fields -- and
may be present in as many as six fields in Delaware and others in New
Jersey and Maryland. Glyphosate is the active ingredient in Roundup and
Touchdown herbicides.
The three fields have similar herbicide
histories, according to Mark VanGessel, a weed scientist at the University
of Delaware. Roundup had been used in combination with other herbicides as
a burndown over a number of years. Over the last three years, each field
has had a burndown and an over-the-top application of Roundup or Touchdown
with no other herbicides in the rotation.
The three fields are at
least 60 miles apart, VanGessel says. In one field sprayed with the
recommended rate of glyphosate last season, VanGessel saw dead marestail
right next to marestail that looked like nothing had happened to
it.
"It certainly looked like resistance," VanGessel says.
"Marestail control was random throughout the field, which ruled out
sprayer problems or applicator error. With almost ideal weather conditions
early in 2000, we also ruled out environmental or stress
factors."
VanGessel says they applied 10 times the recommended rate
of Roundup and Touchdown on the uncontrolled marestail and only stunted
the weeds.
Scientists from Syngenta, the manufacturer of Touchdown,
are collaborating with VanGessel on additional tests. Seeds from these
surviving plants have been grown out in a greenhouse and treated with 10X
rates of glyphosate. The second generation appears to have the tolerance
as well. 02/15/2001 07:38 a.m.CDT
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