Daily News Archive
August 16, 2005
Find Herbicide-Resistant Horseweed In California
(Beyond Pesticides, August 16, 2005)
University of California researchers have identified strains of mare’s
tail, also known as horseweed, that are resistant to herbicides. New
data shows that clusters of horseweed can grow robustly even when sprayed
with four times the recommended amount of the herbicide glyphosate.
This is the first confirmation of the resistant weed in California.
Scientists believe that glyphosate-resistance is one of the reasons
that horseweed is now growing prolifically on irrigation canal banks,
vacant lots, orchard and vineyard floors, roadsides and gardens in California,
while five years ago it was seen only occasionally. Glyphosate
is the active ingredient in 55 brand-name and generic herbicides registered
for use in California. The most common brand is Roundup. According to
the California Department of Food and Agriculture, 5.7 million pounds
of glyphosate were used by the agricultural industry in 2003.
Horseweed is a particularly sinister vegetative foe. Also known as mare's
tail and by its botanical name Conyza Canadensis, it grows straight
upright on a central stem surrounded by long, thin leaves. Horseweed
is difficult to pull. Mowing makes the problem worse instead of better.
Unabated, it grows 8 to 10 feet tall, competing with agricultural crops
for water, nutrients and sun, and getting in the way of farm equipment
and laborers. In untended yards or vacant lots, horseweed forms a tangled
jungle. And perhaps most ominously, each plant produces 150,000 to 200,000
seeds on yellowish fluffy flowers that a breeze will spread for hundreds
UC Integrated Pest Management weed ecologist Anil Shrestha, Ph.D. and
UC Cooperative Extension weed management farm advisor Kurt Hembree,
both based in Fresno County, began to suspect the herbicide resistance
in horseweed a few years ago when the distinctive plant became more
"You see it everywhere now," Mr. Hembree said. "In 2000,
I had a garlic field with just a few horseweeds. Now it is completely
infested. That is just one example on the west side of the (San Joaquin)
valley. On the east side, it is common especially between the rows in
orchards and vineyards. Large numbers of horseweed are now popping up
from Napa County in the north down through Southern California."
A call from a Dinuba irrigation district manager spurred the research
project at the UC Kearney Research and Extension Center (KREC) near
Parlier. The irrigation district was controlling weeds in a Pest Management
Zone, an area where most herbicides are banned because they threaten
groundwater contamination. Glyphosate is the only herbicide permitted
in these zones since the chemical is considered environmentally benign.
"The irrigation district was using glyphosate year after year,"
Dr. Shrestha said. "This continuous use was, in effect, selecting
for horseweed that was resistant to the chemical."
Glyphosate-resistant horseweed was first reported in 2000 in Delaware.
It has since been found in ten other states. This is the first confirmation
of the resistant weed in California. Even though the study focused on
weeds from the Dinuba site, Mr. Hembree and Dr. Shrestha believe that
glyphosate-resistant horseweed may exist in other areas as well.
The scientists believe that another weed, hairy fleabane, may also be
evolving glyphosate resistance, a phenomenon that has been confirmed
in hairy fleabane in only two other areas worldwide - one in Spain and
the other in South Africa. Hairy fleabane and horseweed look similar
when immature and grow under similar conditions, but hairy fleabane
reaches just three feet in height.
Farmers and other land managers who notice a great number of horseweed
or hairy fleabane should begin using a diversity of methods to bring
them under control. By any means, make sure the weeds do not go to seed,
Mr. Hembree said. Cultivation, hand pulling and pre-emergent herbicides
will control the pest, advised Mr. Hembree. Corn gluten is an example
of a safe and effective pre-emergent herbicide.
Read more about other incidences of genetic contamination and threats
to the environment related to GM crops in Beyond Pesticides Daily
News. In an attempt to monitor this problem Greenpeace and GeneWatch
UK have also released an online
database of GM incidences for more information on the prevalence
of these incidences.
TAKE ACTION: Avoid genetically modified food
and products made with genetically modified crops. Read more about the
hazards of genetically
modified crops and use safer organic
alternatives. Contact your government representatives and let them
know you want tighter controls on GM crops. Encourage you community
to adopt a GM ban like California’s Medocino
county plan or contact Beyond Pesticides for more resources.