Herbicide-Resistant Weeds: Where Are We in Illinois?

April 14, 2000
Each year we receive several phone calls reporting suspicions of herbicide-resistant weed biotypes. In fact, this year alone we collected or received seed samples of three "new" weed biotypes that were suspected to be resistant to ALS-inhibiting herbicides. In Illinois previously, these three weed species, shattercane, eastern black nightshade, and giant foxtail, have not been confirmed resistant to ALS-inhibiting herbicides. After conducting resistance studies in the greenhouse, we were able to determine that two separate populations of shattercane and one population of eastern black nightshade were highly resistant to certain herbicides in the ALS-inhibitor family. We strongly suspect that the giant foxtail population is also resistant to some ALS inhibitors. These three weed species add to the list of confirmed herbicide-resistant biotypes in Illinois. An extensive list of confirmed herbicide-resistant weed biotypes in Illinois is presented in Table 5.

So what's next? While we keep adding to the list of weed biotypes that are resistant to ALS-inhibiting herbicides, does this render these herbicides ineffective for use in weed management programs? No, this is not the case. Although there are a number of weed biotypes that are resistant to ALS-inhibiting herbicides, many of these resistant biotypes are isolated cases and may not occur on large numbers of acres. Even in areas where resistance to ALS-inhibiting herbicides is a problem, a number of these herbicides are still used effectively in weed-management programs by combining other viable means for controlling the resistant biotypes. Options for controlling herbicide-resistant weed biotypes include tank mixtures or sequential applications of herbicides with different modes of action that have activity on a particular resistant biotype. Another option is to combine mechanical control practices, such as rotary hoeing and cultivation, with herbicide treatments to help control the resistant populations.

These different techniques have been used quite effectively to help manage current herbicide-resistant weed problems, including ALS-resistant waterhemp and triazine-resistant common lambsquarters and pigweed. However, as with many other management considerations in production agriculture, identification of the problem is the first step. Knowing that a particular weed biotype is resistant to a family of herbicides helps you plan a strategy to deal with that species. Most important is to incorporate the use of herbicides with different modes of action to control herbicide-resistant biotypes. Listings of triazine- and ALS-inhibiting herbicides are presented in Table 6 and Table 7. Since weed biotypes resistant to these herbicide families have been identified in Illinois, producers may find it beneficial to know which products on the market today have the same mode of action.

We are frequently asked about the potential for weed biotypes to develop resistance to glyphosate, the active ingredient in Roundup Ultra, Touchdown, and many other products that were listed in last week's Bulletin. With the widespread adoption of Roundup Ready crops, the selection pressure for glyphosate-resistant biotypes has increased dramatically. However, after many years of continuous use of these products, there are only a few confirmed cases of weed species that have developed resistance to glyphosate. These cases include populations of annual ryegrass in Australia and California and a population of goosegrass in Malaysia. Even though the potential to select for biotypes resistant to glyphosate may be less than for other herbicide families, widespread utilization of one weed control technique carries the potential to cause a shift in the weed spectrum to one that is not easily controlled. From an applied viewpoint, weeds that are difficult to control due to resistance or simply due to a shift in spectrum translate into many of the same problems for the producer.

One thing we do know with near certainty is that continuous use of herbicides with a similar mode 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 utilizing only one.

1. Scout your fields to know what weed spectrum your dealing with. If you're have been relying on one particular herbicide for several years and notice that some weed species that was effectively controlled in past seasons is 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 an herbicide-resistant biotype or a shift in weed species has developed.

2. Rotate herbicides with different modes of action. Do not make more than two consecutive applications of herbicides with the same mode 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 modes 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, because 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? (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 similar residual activity 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 mode 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 the 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 tank mixtures may need to be emphasized for a longer period of time.

For additional information concerning weed resistance to herbicides, consult Chapter 19 of the 2000 Illinois Agricultural Pest Management Handbook.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague