Moscow, Idaho
February 20, 2008
New strategies are needed to
combat many of the most damaging weeds in farmers' fields that
all show a common trait: resistance to one or more herbicides.
Careful selection of herbicides can keep weeds in control and
lessen the weed threat, said Donn Thill, a
University of Idaho weed
scientist who recently revised a popular guide for growers, "Herbicide-Resistant
Weeds and Their Management."
"It was a major overhaul of the publication this time," said
Thill, who co-authored it with four fellow weed scientists at
Idaho and Oregon State University.
New herbicides and new information about resistant weeds
required the overhaul, he said. In addition to a basic primer on
herbicide resistance and ways to avoid it, the publication also
contains a large color-coded chart.
The chart divides more than 100 commercially available
herbicides into 19 groups according to the way they control
weeds. The chart helps growers plan which herbicides to use
against particular weeds and choose a spectrum of herbicides to
lessen the chance of resistance developing.
Resistance to at least one herbicide exists in populations of
all eight major weeds that cut yields in dryland and irrigated
crops across Idaho, Oregon and Washington.
Prickly lettuce, kochia, Russian thistle, wild oat, Powell
amaranth, common lambsquarters, yellow starthistle and Italian
ryegrass are among the weeds that have developed resistance to
particular herbicides in some areas.
Particularly worrisome, Thill said, is that some of the most
damaging weeds have developed resistance to several herbicides.
Italian ryegrass supplies a clear example of the threat of
multiple resistance, Thill said, posing a constant threat to
yields across most of the rich dryland wheat areas of eastern
Washington and northern Idaho.
Australian wheat growers lost control of Italian ryegrass when
it developed resistance to herbicides in some of their key
production areas. Their wheat yields fell below profitability as
the weed outcompeted the crop for water and nutrients.
Now combines pull chaff wagons to collect the lighter ryegrass
seed in a last-ditch effort to reduce the competition. Thill
said some farmers simply abandoned wheat as a crop.
Seth Gersdorf, Thill's graduate student, recently tested Italian
ryegrass from 75 fields across the region and found that 95
percent showed herbicide resistance.
"The level of resistance is far more than I had anticipated,"
Thill said. "The path we're going down is not unlike the path
the Western Austalian wheat farmers already went down."
The most worrisome development on the Italian ryegrass front,
Thill added, is there was one case of the weed developing
resistance to the herbicide Roundup or glyphosate one of the
most universal weed killers.
With 90 percent control of Italian ryegrass, growers can harvest
75 bushels of wheat an acre. With the weed uncontrolled, yields
drop to five to 10 bushels an acre and a large proportion of
that is ryegrass seed.
"Herbicide-Resistant Weeds and Their Management" by Thill, Carol
Mallory-Smith of Oregon State University, Andy Hulting of Oregon
State; Don Morishita of Idaho and Jen Krenz of Oregon State was
published as PNW 437 a joint publication by Idaho, Oregon State
and Washington State Universities.
The publication costs $2.50 plus shipping and handling and can
be ordered from Educational Publications Warehouse, College of
Agricultural and Life Sciences, P.O. Box 442240, Moscow, ID
83844-2240.
More information is available by phone at (208) 885-7982, by
e-mail at
calspubs@uidaho.edu or online at
http://info.ag.uidaho.edu/catalog.
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