Spread of glyphosate-resistant kochia appears limited

Sugar beets grow in an Eastern Oregon field earlier this year. Weed scientists believe the distribution of kochia weeds resistant to glyphosate is limited in the Treasure Valley region and they say growers can limit their spread by using the right resistance management strategies.

BOISE — Weed scientists believe the number of glyphosate-resistant kochia weeds in the Treasure Valley area is small enough that growers can prevent their spread with the right management strategies.

Oregon State University and University of Idaho weed scientists last year confirmed the presence of kochia weeds that are resistant to glyphosate, the active ingredient in Roundup, the popular weed killer produced by Monsanto Co.

The weeds were found in sugar beet fields in Southwestern Idaho and Eastern Oregon. Virtually all of the 180,000 acres of sugar beets grown in this region are genetically engineered by Monsanto to resist glyphosate.

“If we can convince growers who use Roundup Ready sugar beets and corn to implement some resistance management strategies, our feeling is we can keep these herbicide-resistant weeds at bay,” said UI weed scientist Don Morishita.

OSU weed scientist Joel Felix said weeds don’t develop resistance to an herbicide. Rather, a tiny population of weeds in each plant variety is naturally resistant and the herbicide kills off its competitors and allows them to flourish.

The best way to prevent the spread of glyphosate-resistant kochia, he said, is to use other herbicides along with Roundup.

“We need more than one mode of action so weeds not killed by Roundup will not spread,” he said.

This can be accomplished by using different herbicides either throughout the season or on other crops included in a field’s rotation in following years and also through “tank mixing,” which is mixing one or more modes of action in with glyphosate and applying them at the same time.

Felix said studies, including a recent one by University of Illinois, have shown tank mixing to be more effective than alternating herbicides but some growers are reluctant to do that because they believe it will injure their beets.

Felix and Morishita have conducted studies on tank mixing and Felix said growers can talk to either of them to find the proper rates to use.

According to Felix, glyphosate-resistant kochia has been found in 10 states: Idaho, Oregon, Wyoming, Montana, Kansas, North Dakota, South Dakota, Nebraska, Colorado and Oklahoma.

In the Treasure Valley, it has been found near Ontario, Vale and the Oregon Slope area in Eastern Oregon and near Wilder in Southwestern Idaho.

Morishita and Felix conducted drive-by surveys of sugar beet fields in the region this year and collected seed from any kochia plants that survived initial and follow-up applications of Roundup.

Morishita said they did not detect any spread of resistant weeds beyond the areas where they have already been confirmed. Agronomists thought they found some near sugar beet pilings in Idaho’s Magic Valley area but tests determined they were not resistant.

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