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Daily News Archive
From August 16, 2005

UC Scientists 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 of yards.

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 prevalent.

"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.