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The invincibility of herbicides
by Bob Hartzler

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February 10, 1997 - Dr. Stephen Powles was made an Honorary Member of the Weed Science Society of America at the society's recent annual meeting in Orlando. Dr. Powles is a weed scientist at the University of Adelaide in South Australia. He gave an excellent presentation during the general session titled "Super resistant weeds have helped Aussie weed science" in which he discussed changes brought about by the spread of herbicide resistance in Australia.

The primary focus of his talk was annual ryegrass (Lolium rigidum), and how resistance in this species has forced farmers and scientists to change their view of weed management. Dr. Powles stated that annual ryegrass has shattered the view held by many that modern herbicides were invincible and could handle any weed problem that might come along.

Annual ryegrass is the first species to possess multiple resistant mechanisms to a wide variety of herbicides. Annual ryegrass is a cross-pollinated species native to the Mediterranean and now a major weed in many regions of the world with Mediterranean type climates. Resistance was first reported in annual ryegrass in 1982. Biotypes of ryegrass have been identified which have resistance to:

ALS-inhibitors (e.g. Glean, Pursuit, etc)
ACC-ase inhibitors (e.g. Poast, Fusion, etc)
chloroacetamides (e.g. Dual, Harness, etc.)
dinitroanilines (e.g. Treflan, Prowl, etc.)
photosystem II inhibitors (e.g. atrazine, Lorox, etc.)

Resistance to these herbicides is achieved via modified target sites and/or enhanced metabolism. Many different populations of ryegrass have been identified, some having resistance to only one class of herbicides, whereas other populations have varying levels of multiple resistance. An annual ryegrass biotype with resistance to glyphosate has recently been confirmed.

As Dr. Powles stated, annual ryegrass is resistant to herbicides that haven't even been discovered!! Powles and Preston prepared a review article on cross resistance and multiple resistance that provides in-depth information on this topic.

Currently, no weed in the U.S. has displayed the type of multiple resistance found in annual ryegrass. In Iowa and surrounding states, waterhemp is the closest thing we have to an annual ryegrass. Waterhemp biotypes have been identified with resistance to the ALS-inhibitors and the triazine herbicides. Nebraska has confirmed populations with resistance to both herbicide classes, we suspect we may have populations with multiple resistance in Iowa, but this has not been confirmed to date. With the continued heavy reliance on herbicides in the corn-soybean system utilized in the midwest, it seems inevitable that herbicide resistance will continue to increase as a problem. The development and implementation of integrated weed management systems is the only means of minimizing problems associated with herbicide resistance.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.