Look Out for Herbicide Resistant Weeds in Turf

Herbicide resistance can be defined as the acquired ability of a weed population to survive an herbicide application that previously was known to control the population. The number of herbicide resistant weeds has been rapidly increasing in agriculture in recent years. Currently, 459 unique cases of herbicide resistant weeds exist globally, with 246 species (143 dicots and 103 monocots) and with resistance to 22 of the 25 known herbicide sites of action including resistance to 157 different herbicides are known (weedscience.org).

In a recent article by Scott McElroy in Golf Course Management where he reviewed herbicide resistance among turf weeds, he stated that “the development of herbicide-resistant weeds will be one of the great challenges for golf course superintendents and other turf managers in the 21st century.” Further he added that herbicide resistance among turf weeds is not a “potential problem” but a current problem. This is best illustrated by the recent identification of an annual bluegrass (Poa annua L.) population in Alabama that was found to be resistant to Revolver (foramsulfuron), Monument (trifloxysulfuron), Image (imazaquin) and Velocity (bispyribac)(McElroy, 2013). We also have glyphosate, dinitroaniline (preemergence herbicides), and triazine (simazine and atrazine) herbicide resistance among other annual bluegrass populations.

While annual bluegrass is obviously a wide-spread and problematic weed in turf, herbicide resistance is not limited to just annual bluegrass and not just limited to golf courses. Goosegrass, marestail (horseweed), and smooth crabgrass are among other weeds that were identified as herbicide resistant and still other weeds like ground ivy are known to be “tolerant” of herbicides with as much as 3x rates being needed to achieve control of some populations.

Do we have any of these resistant weeds in turf in Indiana? Yes, but fortunately not very many at this time.

• Quinclorac (Drive XLR8 and others) resistance exists in smooth crabgrass found on a golf course driving range tee in central Indiana. The quinclorac resistant smooth crabgrass in Indiana was unresponsive to quinclorac applications by the golf course superintendent at label recommended rates (Drive 75DF at 0.75 lb ai/A) as well as a 3x label rate of quinclorac (Drive 75DF at 2.25 lb ai/A). We quantified the resistance of this population in the greenhouse to be resistant to quinclorac rates of 80x or more.  
• A population of buckhorn plantain was discovered in central Indiana at a location that had received more than 30 years of herbicide applications with 2,4-D containing products. We assessed the level of resistance of that population and found that it required about 16x the label rate of 2,4-D to produce the same amount of injury as a susceptible population. We were able to find an alternative herbicide that worked to control the buckhorn plantain at this site, but at a cost of $100/A rather than the $15/A it previously cost to control buckhorn plantain with a 2,4-D containing herbicide at this site.
• We are also currently evaluating a population of suspected herbicide tolerant/resistant ground ivy. 

2,4-D resistant buckhorn plantain.

If you suspect you have a herbicide resistant weed, please contact me (ajpatton@purdue.edu)

10 things YOU should know about herbicide resistant weeds

1. Herbicide resistant weeds are in turf and not just crop fields
2. It is the tough to control weeds that are often developing resistance
3. Few new herbicides coming to market to help control resistant weeds
4. We aren’t currently doing a good job rotating the mode of action for herbicides/PGRs
5. Preemergence herbicides are a tool to help control resistant weeds but they may not be labeled for sites with resistant weeds
6. Preemergence herbicides don’t always help to control the weed (example: perennial weeds)
7. Cultural practices only help so much to reduce weed populations
8. Resistant weeds will spread by mowing, aerification, pollination, etc.
9. Low herbicide rates – to save $ - increase resistance development rate
10. Turf injury from herbicides may need to be tolerated to control resistant weeds

Bottom line is this:

• Resistant weeds are a problem now, but luckily we don’t have too many in Midwest turf yet.
• Turf managers will need to increase their knowledge of herbicide mode of action and increase their rotation of these various herbicide modes of action in the future to prevent weed resistance from developing just as they currently rotate fungicide mode of action in order to reduce the likelihood of developing fungicide resistance.
• For more information on this topic, I suggest you read a helpful publication on preventing and managing herbicide resistant weeds by my colleague Dr. Jay McCurdy at Mississippi State University linked here.
• If you suspect you have an herbicide resistant weed, please contact me (ajpatton@purdue.edu).

Dr. Aaron Patton
Turfgrass Extension Specialist

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