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Pacific Northwest Conservation Tillage Handbook Revised Series No. 18
Chapter 5 - Weed Control Strategies, February 2000 Also Published as PNW Extension Bulletin 437

Herbicide-Resistant Weeds and Their Management

Revised PNW Bulletin PNW 437 and PNW Conservation Tillage Handbook Series 18, Chap. 5

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Editors Note: The following news release announces the revision of Pacific Northwest Extension Bulletin PNW 437 “Herbicide Resistant Weeds and Their Management. This publication is also published as Pacific Northwest Conservation Tillage Handbook Series 18, Chapter 5 -- Weed Control Strategies (see the back page of the publication). As explained below, you can purchase print copies of the publication or you can access it in PDF format --- view and print in published form -- on the Internet (http://info.ag.uidaho.edu/pdf/PNW/PNW0437.pdf). For those who have PNW Conservation Tillage Handbook, please update it with this new publication.

Northwest Scientists Revise Primer on Herbicide-Resistant Weed Control

1/28/03 News Release – University of Idaho College of Agriculture and Life Sciences, Bill Loftus, (208) 885-7694, bloftus@uidaho.edu.

MOSCOW – A newly revised publication by Pacific Northwest Extension experts about herbicide-resistant weeds offers farmers a tool to help avoid major problems in the future.

Wheat growers know firsthand that there is reason for concern, said Donn Thill, a UI weed scientist who co-authored the guide with colleagues at UI and Oregon State University.

Thill, president-elect of the Weed Science Society of America, said the problems posed by weeds developing resistance occur worldwide and affect many crops.

Infestations of herbicide-resistant annual ryegrass in Australia have become so severe that farmers there have simply abandoned efforts to grow wheat in some fields, Thill said. Millions of acres are affected there.

Italian ryegrass threatens Pacific Northwest wheat growers, Thill said, “unless we slow the development of resistance similar to that causing problems in Australia with annual ryegrass.”

Some common Northwest weeds that show herbicide resistance include wild oat, annual bluegrass, common lambsquarters, kochia, yellow starthistle, downy brome and common groundsel.

The best defense for growers is a coordinated herbicide program, Thill said. “What it comes down to is good record keeping.”

“Italian ryegrass is a big one we’re dealing with right now,” Thill said. It is already resistant to some of the herbicides most commonly used to control weeds in wheat.

The publication’s focus is on helping growers monitor herbicide use and rotate the herbicides they use to prevent resistance.

The ability of weeds to adapt and develop resistance to herbicides is no surprise to most growers, Thill said. The first herbicide resistant weed was discovered in Hawaii in 1957. The Northwest’s first herbicide-resistant weed was found in a western Washington nursery in 1968.

Prickly lettuce resistant to sulfonylurea herbicides, which are a mainstay of grain farmers, was Idaho’s first resistant weed. It was found near Lewiston in 1987.

“That really got the ball rolling,” Thill said. “We knew we weren’t in a situation like Australia, and we knew we didn’t want to be.”

“We knew that education to prevent the development of more resistant weeds would be a lot more effective, and cost effective, than trying to fight resistant biotypes,” Thill said.

In the case of annual ryegrass in Australia, some biotypes – a group of plants sharing distinctive genetic traits – are now resistant to eight different groups of herbicides. That means chemical control is no longer an option.

“Herbicide-Resistant Weeds and Their Management” explains how weeds lose their vulnerability to common herbicides. The six-page publication features an easy-to-use chart for agricultural chemical applicators.

The publication was issued by the Extension services of the University of Idaho, Oregon State University and Washington State University, the region’s land-grant universities.

First issued in 1993, the latest edition contains a section about managing herbicide-resistant crops.

The publication was possible because the Northwest holds a skilled group of weed scientists at the three universities, Thill said. The authors are Carol Mallory-Smith and Jed Colquhoun of OSU, and Don Morishita and Thill of UI.

After researchers first confirmed herbicide-resistant weeds in the 1950s, there was a long lag time before the number of resistant species grew. Now about nine new resistant weeds are added to the list each year and the total worldwide is about 260 different biotypes, Thill said.

“That doesn’t seem like a lot on a global basis, but when you consider it on an acreage basis it involves huge areas,” Thill said.

The publication, which costs $2, groups herbicides by how they kill weeds, and lists Pacific Northwest weeds resistant to each herbicide group. Growers can alternate different herbicide groups to delay resistance from developing, Thill said.

The latest addition to the publication is the section on herbicide-resistant crops. In Idaho, only herbicide-resistant canola and winter wheat have been grown commercially, and those on limited acreage, Thill noted.

Herbicide-resistant winter wheat acreage is expected to increase greatly during the next couple of years. The publication offers growers advice to avoid transferring genetic-based herbicide resistance to weeds.

The publication can be ordered by phone at (208) 885-7982, fax at (208) 885-4648 or mail at Agricultural Publications, UI, P.O. Box 442240, Moscow, ID 83844-2240. Internet users can order it online at http://info.ag.uidaho.edu/ or view the publication directly at http://info.ag.uidaho.edu/pdf/PNW/PNW0437.pdf.

More information about herbicide-resistant weeds worldwide is available on the web at www.weedscience.org/in.asp


Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
Redesigned by Leila Styer, CAHE Computer Resource Unit; Maintained by Debbie Marsh, Dept. of Crop & Soil Sciences, WSU