Frequently Asked Questions about
Herbicide Resistant Weeds

Chris Boerboom

The occurrence of herbicide resistant weeds is definitely on the increase in Wisconsin and across the Midwest. The last three issues of the Wisconsin Crop Manager reported specific details on ALS-resistant giant foxtail, eastern black nightshade, and green foxtail. In this issue, I have answered many questions that are frequently asked about herbicide resistant weeds.

Q: What weeds are known to have resistant biotypes in Wisconsin?

A: Documented cases of resistance include: triazine-resistant common lambsquarters, smooth pigweed, velvetleaf, kochia; ACCase-resistant giant foxtail and large crabgrass; and ALS-resistant giant foxtail, green foxtail, eastern black nightshade, waterhemp, and kochia.

Q: What’s a biotype?

A: A biotype refers to a group of plants of the same species that differ in a specific trait. In this case, they differ in being herbicide resistant. For example, the normal biotype of velvetleaf is susceptible to atrazine, but there are a few locations in the state where atrazine-resistant velvetleaf biotypes exist. All of these plants are still velvetleaf, just different biotypes.

Q: Can you tell if a plant is resistance by its appearance?

A: No, a resistant plant looks just like a susceptible plant. The genetic and biochemical changes inside the plant that cause the resistance are not visible.

Q: What clues can help confirm that a weed might be resistant?

A: If you answer "yes" to all of the following questions, be very suspicious that the weed is resistant. 1) Does the herbicide usually control this weed species? 2) Were other weed species killed in the same area where this weed remained healthy? 3) Is this weed the only weed species left in the field? 4) Has the same herbicide or other herbicides of same mode of action been used frequently in the field before? 5) Have you ruled out the potential mimics listed under the next question? If the field needs to be resprayed to control this weed and you strongly believe that it is resistant, use a herbicide with a different mode of action. Don’t waste money respraying with the same herbicide that failed because of resistance. To confirm resistance, hand spray a small patch with the herbicide to which resistance is suspected.

Q: What can mimic herbicide resistance?

A: There are a few situations that may mimic a resistance problem. Consider if the weed is naturally tolerant to the herbicide. For instance, Pinnacle will not control nightshade in soybeans. This is not a case of resistance. Weeds that emerge after a non-residual herbicide application are also not resistant, but they could appear to be resistant if their time of emergence was not considered. Adverse weather like drought could cause weeds to survive a herbicide application, but is also not resistance. Application errors such as spray skips, the wrong herbicide rate, etc. also need to be ruled out as mimics of resistance.

Q: Can I use higher herbicide rates to control a resistant weed?

A: No, the resistant biotypes are often many times more resistant to the herbicide than normal, susceptible biotypes. In greenhouse tests, the Accent rate had to be about 20 times higher to control ALS-resistant giant foxtail at the same level as susceptible giant foxtail. With ALS-resistant eastern black nightshade, the Raptor rate had to be increased 133 times higher to get the same level of control as with susceptible nightshade. Wisconsin’s atrazine-resistant velvetleaf is also about 100 times more resistant than susceptible velvetleaf. We could not afford to spray these herbicides at rates high enough to kill these resistant biotypes, even if it was legal.

Q: Will the resistance disappear if I stop using that herbicide for 10 years?

A: Realistically, once resistance has developed, it becomes permanent because a large percentage of the seeds in the soil carry the genes for resistance. Consider a field where 50% of the seeds in the soil are resistant and 50% are susceptible. Even if a different herbicide was sprayed this year, about 50% of the escaping weeds would still be resistant and will produce more resistant seeds for future years in addition to the resistant seeds still in the soil. Once this genetic shift towards resistance has occurred, it will stay at that level. Triazine-resistance biotypes are one exception because they are slightly less vigorous and produce slightly less seed than the susceptible biotypes. After many years, triazine-resistance may slowly decrease, but will increase rapidly onc  a triazine is used as a solo treatment again.

Q: If a weed is herbicide resistant, how can it be controlled?

A: Frequently, herbicide resistant weeds are cross resistant to other herbicides with the same mode of action. However, you will be able to control a herbicide resistant weed with herbicides that have a different mode of action. For example, dicamba (a growth regulator) will control triazine-resistant common lambsquarters. There are examples of resistant weeds in other parts of the country or world where multiple resistance has developed. With multiple resistance, a biotype may be resistant to two or more different modes of action, such as a biotype that is resistant to both triazines and ALS inhibitors.

Q: How can resistance be prevented?

A: The only way to absolutely prevent resistance is to not use herbicides. Each time a herbicide is used, there is a chance to increase the selection for a resistant biotype. However, we can do several practices to delay resistance: 1) use IPM and only respray fields when necessary, 2) rotate among herbicides with different modes of action, 3) tank mix herbicides with different modes of action (both herbicides must be active on the same weed spectrum for this strategy to be effective), 4) cultivate and rotary hoe, 5) rotate to crops that use herbicides with different modes of action or crops like alfalfa or wheat that are not treated often with herbicides, and 6) avoid spreading resistant weeds from field to field if resistance develops.

Q: Do all herbicides have the same risk of developing resistance?

A: The potential for herbicide resistance seems to be greater with some modes of action than others. The ALS inhibitors and ACCase inhibitors are two groups that have a higher risk than others and should be managed more carefully. The risk for triazine resistance is also higher because of the many years that triazines have been used. Even though other herbicides may have "less-risky" modes of action, care should be taken not to over use or abuse these herbicides. For example, glyphosate would probably fall in a less risky mode of action group. A few years ago, it was even argued that glyphosate resistance would not occur. However, there are now two weed species with glyphosate-resistant biotypes.

Q: Where can I find a current list of herbicide resistant weeds?

A: Ian Heap maintains a web site with current information, lists, and summaries on herbicide resistant weeds. The web site is www.weedscience.com.