How do you identify herbicide resistance?

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With few exceptions, resistant and susceptible weeds of the same species look the same (Figure 6). Resistant weeds are often identified by patterns in the field rather than by any differences in appearance. If weeds survive a herbicide application, investigate and rule out other factors affecting herbicide performance before concluding that resistance is responsible.

Good questions to ask are:

  • Did only one species survive or are there multiple weed species that were not controlled?
  • Do the plants display herbicide injury?
  • Are there areas in the field that the herbicide application missed?

The Weed Science Society of America (WSSA) recommends exploring several other factors, including:

  • The field history
  • The weed’s biology
  • The environmental conditions before and after the herbicide was applied
  • Any potential application problems
  • The field’s cultural practices
  • The potential for herbicide resistance.

Figure 6. Whether herbicide-susceptible or resistant, these individuals look the same. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

How do you confirm herbicide resistance? There are three primary ways to confirm herbicide resistance. These include field testing, greenhouse testing and more limited genetic testing for some types of resistance. Once you suspect herbicide resistance, quickly contact your local county Extension agent or ag consultant and seek advice.

Field testing, which involves treating some surviving plants with the same herbicide mode of action, is not very practical, but it may be the first reaction when weeds escape a soil or foliar-applied herbicide that should have been effective. More importantly, if the escaped weed(s) are suspected to be resistant, attempts should be made to prevent them from producing seed and furthering the problem. This will likely involve applying alternative effective herbicide groups or using mechanical control options.

A more accurate method to test for herbicide resistance is through greenhouse confirmation (Figure 7). This requires working closely with your local Extension service and weed science expert to allow some plants to mature for seed collection for a greenhouse assay (test) after the field season ends. Genetic testing for some types of resistance may also be available which can confirm resistance and provide the mechanism(s) at play. Because these types of tests are specialized and limited in availability, they are generally only sought and used for more unique types of resistance.

Figure 7. Herbicide-susceptible horseweed (left) and resistant horseweed (right) are identified in a greenhouse glyphosate screening test. (Photo credit: Mark VanGessel, University Delaware)

Plants surviving a herbicide application is not uncommon, since many issues can compromise weed control. Consider several factors when determining if resistance is the cause for lack of control (Figure 8).

Figure 8. These factors can all influence herbicide effectiveness and potential for weed escapes. Be sure to explore all potential causes for reduced weed control before concluding that herbicide resistance is to blame. (Image adapted from Lesson 4, Scouting After a Herbicide Application and Confirming Herbicide Resistance, WSSA)

  1. Review the field’s history of weed management practices. What herbicides have been used and how often? What is the cropping and tillage history? What non-chemical weed management tactics are used? What are the problem weeds, and has resistance been identified with these species before?
  2. Know the biology of the escaped weed(s). When did they emerge relative to the herbicide application? For a POST application, how big and old were the weedy plants at application?
  3. Examine the environmental conditions before and after herbicide application. Did it rain? What were daytime and nighttime air temperatures? Were the weeds drought stressed? What were the soil characteristics (wet, dry, soil texture, pH, organic matter, etc.)?
  4. Scrutinize the application history. Were there any problems related to equipment, herbicide rate and calibration, herbicide delivery, or wind?
  5. Look carefully at the field and the pattern of weed escapes. Are there any identified patterns in the field such as multiple weed species present, or is only a single species present, and should the herbicide(s) control it? Is there little or no injury on the surviving species while other species are controlled? Is the spatial pattern of surviving weeds random or can a sprayer skip or malfunction be identified?
  6. If you can eliminate potential causes and narrow the list, herbicide resistance may be a factor for poor control and escaped weeds.

After you thoroughly examine the factors that can compromise weed control, this should help answer whether herbicide resistance may indeed be the problem.

How do you confirm herbicide resistance? Greenhouse testing often requires assistance from an institution that has a research greenhouse facility and a great deal of herbicide knowledge. Greenhouse testing often takes a few months to obtain results, particularly if low-level resistance is involved. It requires collecting seeds from mature suspect plants, as well as some seeds of the same species collected from fields where resistance is not a concern (to create a control group). Some knowledge about how to culture the weed species is necessary. The seeds of many species require exposure to a cold (and perhaps moist) environment for up to several months, either after or during the process of maturing, in order to germinate. Some seeds even require “scarification,” a process of physically or chemically treating the seed to create openings in the seed coat to spur germination. Once the plants germinate, emerge, and have developed three or four leaves, they should be sprayed very precisely with several rates of the suspect herbicide (Figure 9). This also requires specialized equipment and trained technical staff. This type of test can confirm or refute resistance, but it does not provide the mechanism of resistance (see Section 8). Most university weed science programs are equipped with specialized equipment and the experience to conduct this type of greenhouse assay. As mentioned previously, you would generally only seek out these services for more unique types of resistance, not for herbicide-resistance cases that are common in your area. Contact your local county Extension service or state Extension weed specialist for more information. Criteria for confirming herbicide resistance according to International Herbicide-Resistant Weed Database can be accessed at: http://weedscience.org/Pages/ResistanceCriterion.pdf.

Figure 9. A greenhouse experiment screening kochia for glyphosate resistance. (Photo credit: Het Desai, Montana State University)

Genetic testing uses leaves or seeds from the suspected plant. It is very accurate and fast, but it is not widely available in the U.S. The downside of genetic testing is that it only tests for known resistance mechanisms. Consequently, it is possible that a plant could test “negative” for resistance but actually be resistant due to a different mechanism. Because of this, genetic testing is generally used only for common, well-known resistance cases. The University of Illinois Plant Clinic has a molecular assay for glyphosate (Group 9) and PPO (Group 14) resistance in pigweeds. Contact the University of Illinois Plant Clinic for more information. In Canada, several public and private labs across the country test plants and seeds submitted by Canadian farmers or ag professionals for specific types of resistance. In Australia, the Australian Herbicide Resistance Initiative (AHRI) is actively testing both plants and seeds from Australian farms, as is as a lab associated with the University of Adelaide. Other testing opportunities may be available in other American states and in other countries.