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
"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
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
"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."