| December 15,
2004, Mike Holmberg, Successful Farming--CropChoice.com:
Missouri has the first known case of a summer annual
weed resistant to the popular glyphosate line of herbicides.
The resistant weeds, a biotype of common ragweed, were
found in a 20-acre field in central Missouri. Plants
from the field, examined by University of Missouri weed
scientist Reid Smeda and by officials of Monsanto Company,
were found to be resistant to 10 times the rate of glyphosate
herbicide that normally controls common ragweed. Glyphosate
is the active ingredient in Monsanto Roundup herbicides
as well as other brands.
"While our yearlong investigation has led us to
conclude that resistance has occurred in this particular
field, there have been no other complaints," Smeda
says. Common ragweed is the third biotype in the United
States to develop resistance to glyphosate, which has
been used commercially for more than 30 years. Resistance
also has been discovered within biotypes of the weeds
marestail and ryegrass.
However, "This case of ragweed is significant
in that it is the first time a summer annual has developed
resistance to glyphosate," Smeda says. Marestail
and ryegrass are winter annuals.
The development of these resistant weeds, as with other
cases of weed resistance throughout history, is chiefly
due to continual use of one weed-control strategy, Smeda
says. It was found in a field with a history of continual
soybean production and the repeated use of glyphosate
in glyphosate-resistant soybean varieties.
Smeda will discuss proper weed control strategies to
manage resistance with farmers and pesticide applicators
at meetings throughout the winter. |
