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Herbicide Resistance: Where Are We?

April 13, 2001
Herbicide-resistant weed biotypes have plagued Illinois growers for well over 20 years. Over the last decade there has been a dramatic increase in the appearance of herbicide-resistant weed biotypes, in Illinois and worldwide. Currently, there are nine confirmed herbicide-resistant weed species biotypes in Illinois, and more than 200 confirmed resistant weed biotypes worldwide. An extensive list of confirmed herbicide-resistant weed biotypes in Illinois is presented in Table 5.

What factors have contributed to the increase in herbicide-resistant weed biotypes?

Over the years, various characteristics of weeds, herbicides, and cultural practices have been identified as contributing factors to the development of herbicide-resistant weed populations. Many of the herbicide-resistant weed populations encountered over the years have possessed similar characteristics. These include an annual life cycle, high seed production, very little seed dormancy, more than one reproductive generation per year, and a high susceptibility to the herbicide to which the population evolved resistance. Also, there are a number of characteristics that have been associated with the herbicides to which resistance has developed. These herbicides usually act at a single site of action in the plant, are often highly effective on the weed population that has developed resistance, are usually persistent in the soil, and have a high frequency of use. Cultural practices contributing to the development of herbicide-resistant weeds include lack of a complex crop rotation, little or no mechanical weed control, and, most importantly, the increased use of herbicides with the same site of action. Increased use of herbicides sharing a common site of action includes multiple applications of the same herbicide in a growing season or lack of rotating herbicides with different sites of action in consecutive growing seasons. Either way, these practices increase the selection pressure for herbicide-resistant weed biotypes.

What about resistance to glyphosate?

We are often asked about the potential development of weed biotypes resistant to glyphosate, the active ingredient in Roundup UltraMax, Touchdown, and many other products listed in Table 3 of issue no. 1 of the Bulletin. Last year we indicated two weed species had developed resistance to glyphosate: a goosegrass population in Malaysia and annual ryegrass in Australia and California. These two weed species aren't of much concern for Illinois producers since they are not problematic in agronomic production systems in Illinois. However, this year we need to add horseweed (Conyza canadensis) (often called marestail) to the list of glyphosate-resistant weeds. Glyphosate resistance in horseweed was first suspected in a few Delaware fields in 2000 when some horseweed plants were not controlled with glyphosate. Seeds from these suspected glyphosate-resistant horseweed plants were collected and taken to the greenhouse for further testing. Dr. Mark VanGessel, a weed scientist at the University of Delaware, confirmed these populations were resistant to glyphosate, demonstrating 6- to 10-fold resistance to glyphosate, compared with susceptible horseweed populations. This is the first case of glyphosate resistance where soybean was part of the cropping system. In all cases, glyphosate was used for burndown and postemergence weed control in no-till Roundup Ready soybeans for at least 3 years.

While there are only three confirmed glyphosate-resistant weed species biotypes, there have been reports in Illinois, Iowa, and Missouri of waterhemp not being effectively controlled with glyphosate. Some of these escapes may have been due to environmental conditions; however, there are a number of populations that don't seem to fit this explanation. In all three states, weed scientists are working with waterhemp populations that were not effectively controlled with glyphosate. Dr. Pat Tranel, University of Illinois, has screened a number of Illinois waterhemp collections and found three populations that have shown increased tolerance to glyphosate. While there are no confirmed glyphosate-resistant waterhemp populations, there is still some concern that this could be a potential problem in the future.

What can be done to prevent herbicide resistance?

One thing we know is that continuous use of herbicides with similar sites of action increases the selection pressure for herbicide-resistant weed biotypes. The following is a list of management strategies that can be implemented to help delay the onset or development of herbicide-resistant biotypes. In most instances, incorporating a number of these strategies will prove to be more beneficial than using only one.

1. Scout your fields to know what weed spectrum you're dealing with. If you have been relying on one particular herbicide for several years and notice that some weed species that were effectively controlled in past seasons are now abundant, or some species are now present that you haven't ever dealt with before in a particular field, this could be an indication that a herbicide-resistant biotype or a shift in weed species has developed.

2. Rotate herbicides with different sites of action. Do not make more than two consecutive applications of herbicides with the same site of action against the same weed unless other effective control practices are included in the management system. Consecutive applications can be single applications in 2 years or two split applications in 1 year.

3. Apply herbicides in tank-mixed, prepackaged, or sequential mixtures that include multiple sites of action. Both herbicides in the mixture must have substantial activity against potentially resistant weeds as well as similar persistence if they possess soil activity. For example, if one is concerned about potentially ALS-resistant pigweed, a tank mixture of Basagran with an ALS inhibitor would be a poor choice since Basagran has very little activity on pigweed. A couple of guidelines may help with tank-mix or premix selection: (a) when applied alone at the rate that will be used in the tank mixture, does the tank-mix or premix partner control the weed species that I am concerned may develop resistance? and (b) if I apply the tank-mix or premix partner alone at the rate that will be used in the tank mix, will it have residual activity similar to the other component?

4. As new herbicide-tolerant/-resistant crops become available, their use should still not result in more than two consecutive applications of herbicides with the same site of action against the same weed species unless other effective practices are included in the management system.

5. Combine mechanical control practices, such as rotary hoeing and cultivation, with herbicide treatments whenever possible.

6. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those fields that are not infested. This may not always be practical, but it can help prevent the spread of resistant weed seed that is present in soil, which adheres to equipment.

Implementing several of these management strategies will help delay the development of herbicide-resistant weed biotypes, but if you are dealing with a field that already has a substantial population of a resistant biotype, strategies such as herbicide rotation and using tank mixtures may need to be emphasized for a longer time.

For additional information concerning weed resistance to herbicides, consult Chapter 14 of the 2001 Illinois Agricultural Pest Management Handbook.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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