Roundup resistance hits California (23/7/2005)

Glyphosate is the active ingredient in Monsanto's Roundup herbicide which is used in combination with Monsanto's herbicide-resistant GM crops. University of California scientists have just reported finding glyphosate-resistant horseweed in California. As the article below notes, "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."

The scientists who found it advise, "If resistant horseweed turns up on a farm, the grower will want to avoid glyphosate-resistant crops [ie Monsanto's GM Roundup Ready crops] and vigilantly monitor horseweed until it is under control."

And it's not just horseweed that's proving a problem: "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."

Here are more examples of a problem fast becoming rampant in US agriculture.


"Tropical spiderwort, Commelina benghalensis, is now the most troublesome weed in Georgia cotton and the second most problematic weed in peanut. The weed competes with crops for water and nutrients, and smothers the crops at the same time. One reason for the surge in the weed's growth is its resistance to the commonly used herbicide glyphosate." (Little-known weed causing big trouble in Southeast)


"Mare's tail, a familiar nemesis for Missouri farmers, has reappeared with a new resistance to glyphosate-based herbicides, a University of Missouri weed scientist said. Mare's tail, also known as horseweed in the Delta region, 'isn't a new problem, but until recently, glyphosate controlled it' said Andy Kendig, weed science specialist at MU Delta Research Center in Portageville. 'Now, we see fields where everything is burnt down except horseweed. It's really erupted over the past two years.'" (Glyphosate-resistant mare's tail infests Southeast Missouri fields)


"For some 30 years... the herbicide glyphosate has kept morning glories quite effectively out of farm fields. Now, for the first time, however, researchers at the University of Georgia have identified morning glory families that are tolerant to glyphosate – noxious vines that could cause problems for the country's farmers.

"'This finding, along with an analysis suggesting a critical evolutionary threshold has been crossed, will be of broad interest to scientists and policymakers... The new evidence for genetic variation of tolerance in morning glories, however, points toward a potential problem with no easy solutions.'" (Morning glories creeping their way around popular herbicide, new UGA research reports)

UC scientists find herbicide-resistant horseweed in California

Weed ecologist Anil Shrestha, left, and farm advisor Kurt Hembree look over their horseweed project

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 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 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-resistan

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