University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

Photo of marestail killed by herbicide and marestail which appears to be unaffected, standing side by side in the same field.
The eastern Nebraska soybean field where researchers suspected the development of glyphosate-resistant marestail plants last summer. Live plants stand beside dead plants which were controlled by a regular application rate of glyphosate. (IANR Photo)

October 27, 2006

Glyphosate-resistant marestail confirmed in Nebraska

UNL researchers have confirmed the first glyphosate-resistant weed species in Nebraska — marestail, also know as horseweed. While Nebraska researchers first suspected resistance had developed almost a year ago, it took tests and growing plants from last year's seeds to confirm it.

Weed resistance usually results from repeated use of the same herbicide. Widespread use of glyphosate-tolerant crops and repeated use of glyphosate herbicide has resulted in selection pressure on weed populations in recent years. Prior to the introduction of glyphosate-tolerant crops only a few weed species (eg. ryegrass and goosegrass) had developed resistance to glyphosate worldwide. However, the number of glyphosate-resistant weeds tripled in just over eight years of repeated glyphosate use over a large land area (over 50 million acres) due to introduction of Roundup-Ready crops. Current examples of glyphosate-resistant weeds in the United States include: waterhemp, lambsquarters, giant ragweed, common ragweed, palmer amaranth and marestail (horseweed).

Identifying resistance in Nebraska

Seeds of three suspect marestail populations were collected in fall 2005 in eastern Nebraska. Greenhouse bioassays were conducted this summer, and dose response curves for glyphosate were defined for each marestail population. Curve comparisons clearly showed glyphosate resistance requiring three to six times the rate needed in a susceptible population. (Resistance level varied with the marestail population. ) For example, 90% control of a susceptible population was achieved with 32 oz of a glyphosate herbicide (3 lb/gal acid equivalent) (1X rate), while the resistant populations needed about 100 oz/acre (3X rate) and 200 oz (6X rate) in order to achieve the same level of control.

Using management to counter potential resistance development

We believe that glyphosate- and herbicide-tolerant crops, including those based on glyphosate herbicide, can remain useful components of crop production systems only with proper management. It is easy to fall into a trap of overusing glyphosate, versus combinations of pre-emergence herbicides or tank mix partners, when one Roundup-Ready crop is grown after another. Therefore, proper use of herbicide tolerant technology, as a component of an integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many concerns about its use or misuse. More details about glyphosate-resistant marestail will be provided during this year’s UNL Extension Crop Protection Clinics and other UNL Extension programs.

Stevan Knezevic
Extension Specialist, Integrated Weed Management
Alex Martin
Extension Weeds Specialist

Integrated Weed Management Resources

Several resources are available from UNL Extension on implementing an integrated weed management strategy to reduce the risk of developing herbicide-resistant weeds.

The UNL Extension Guide for Weed Management in Nebraska outlines attributes and weed responses for herbicides labeled for Nebraska and covers how to plan and implement an integrated weed management strategy. The 2006 edition is available on the Web and the 2007 edition will be available in early January 2007. The 2007 edition incorporates glyphosate-resistant marestail to ratings of weed responses to herbicides.

Biology and Management of Horseweed, GWC9, part of The Glyphosate, Weeds and Crop Series, addresses identification, distribution, emergence, herbicide tolerance and control of horseweed. This publication series is written by a team of weed scientists representing 15 universities from major corn- and soybean- producing states. Get a print copy from UNL Extension offices or read it online at the Purdue University Extension Web site.

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Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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