Posted August 5, 2005, as reported by CropChoice:
A weed that five years ago was seen only occasionally
in California is now growing prolifically on irrigation
canal banks, vacant lots, orchard and vineyard floors,
roadsides and gardens. One reason, University of California
scientists can now confirm, is that biotypes of horseweed
have evolved that are unaffected by the most commonly
used herbicide glyphosate.
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
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 of yards.
UC Integrated Pest Management weed ecologist Anil Shrestha
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 prevalent.
"You see it everywhere now," 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," Shrestha said. "This continuous
use was, in effect, selecting for horseweed that was
resistant to the chemical."
The scientists collected horseweed seed from the Dinuba
site to compare with horseweed seed collected in western
Fresno where glyphosate had seldom been used. The weed
seeds were planted in pots in a greenhouse at KREC and
treated with three rates of glyphosate at five different
growth stages. Generally, the weeds from west Fresno
died when exposed to the herbicide. The plants from
Dinuba grew robustly, even when sprayed with four times
the recommended amount of glyphosate.
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, Hembree and Shrestha believe
that glyphosate-resistant horseweed may exist in other
areas as well. They have heard from farm advisors, farmers,
pest control advisors and other land managers from several
parts of the south Central Valley that glyphosate isn't
killing horseweed like it used to.
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,
Hembree said. Cultivation, hand pulling and pre-emergent
herbicides will control the pest.
Crop rotation will also be a valuable tool. The glyphosate-resistant
horseweed can be a problem when farmers grow Roundup
Ready crops. In this growing system, farmers plant seed
that has been genetically modified to be resistant to
glyphosate. Then the herbicide may be sprayed over the
top of the crop, leaving the desired plants unaffected
and killing the weeds. However, now that a glyphosate-resistant
weed is known in California, farmers must watch for
weeds that are surviving the herbicide treatment.
"We are lucky we can grow so many crops in California.
Crop rotation is a factor in our favor that they don’t
have in the Midwest," Hembree said. "If resistant
horseweed turns up on a farm, the grower will want to
avoid glyphosate-resistant crops and vigilantly monitor
horseweed until it is under control."