By Tripp Rogers and Travis Gannon

Herbicide-resistant weeds are a leading problem in the turfgrass industry and may compromise the functionality and aesthetic quality of turfgrass systems.

What is herbicide resistance? Herbicide-resistant weeds are not a new problem in turfgrass systems (or any agricultural system). Herbicide resistance is a selection process that develops through the repeated use of the same herbicide or herbicides with the same mechanism of action (MOA). Typically, herbicide resistance traits are present within a population at very low levels and the continuous application of the same herbicide (or herbicide with same MOA) over time removes susceptible plants allowing resistant individuals to survive, grow, reproduce, and proliferate.

What has contributed to herbicide resistance in turfgrass systems? There are a number of factors that contributed; however, overreliance on herbicides over time is a primary contributor coupled with not rotating MOAs. It’s also important to note, herbicides with new and novel MOAs have been very limited in recent years (which limits turfgrass managers ability to rotate MOAs), particularly in certain turfgrass species and/or systems.

What should turfgrass managers do to mitigate or delay resistance?

1. Promote a healthy, thick turfgrass sward. Common methods for promoting healthy turfgrass systems include selecting an adapted turfgrass species, scouting for disease activity, proper fertilization and irrigation as well as ensuring that stands are mowed at the appropriate height and frequency.
2. Implement integrated weed management (IWM) strategies. It’s important to employ various integrated (preventative, mechanical, chemical, cultural, etc.) weed management techniques and not rely solely on herbicides.
3. When using herbicides, rotate MOAs. Less expensive herbicides may be effective initially, but overreliance on these products significantly increases the likelihood of resistance (and greater associated costs) in the long-term. Refer to this for a complete list of herbicides and respective MOAs.
4. Use multiple MOAs with PRE and POST activity within a season. An example of this for annual bluegrass in non-overseeded bermudagrass is using a PRE herbicide (examples include indaziflam, prodiamine, pendimethalin, simazine, among others) at optimum PRE timing for your location with a POST herbicide (examples include foramsulfuron, flazasulfuron, rimsulfuron, sulfosulfuron, trifloxysulfuron, among others) applied POST. Turfgrass managers can also delay the initial application and combine PRE and POST herbicides into a single application (tank-mix). When feasible (non-overseeded, dormant bermudagrass), well-timed applications of a non-selective herbicide such as glyphosate or glufosinate may also enhance efficacy while including an additional MOA. It’s also important to note POST herbicides should be applied to small, actively growing weeds to obtain optimal control.
5. Use the appropriate herbicide rate to maximize efficacy (and minimize escapes). Sub-lethal herbicide rates may expedite resistance evolution.
6. Ensure environmental and edaphic (soil) conditions are optimal. Conditions should be monitored prior to, at, and following application.

Identification of resistant populations. Herbicide resistance must be confirmed through a comparison of resistant and susceptible plants in a replicated scientific trial, which may not be practical for professional turfgrass managers to complete themselves. There are several university laboratories that can confirm if your weed is resistant to a given herbicide or MOA.

What resulted in failed or reduced efficacy (if I don’t have resistance)? Just because you did not obtain control after an application does not confirm herbicide resistance as there are multiple reasons why an herbicide application may not have been efficacious. A partial checklist:

Was the herbicide applied at the correct time? This is especially important for preemergent (PRE) herbicide applications. PRE herbicides should be applied based on environmental conditions (i.e. soil temperature and moisture content), not calendar dates.

Were environmental conditions conducive for efficacy? If foliar-absorbed postemergent (POST) herbicides are applied in winter and plants are not actively growing at application, the herbicide may not have been adequately absorbed and / or translocated.

Was there a rainfall or irrigation event that potentially moved the herbicide? Herbicides have varying rates of aqueous solubility that affect sorption to soil and organic matter. While PRE herbicides need irrigation or rainfall, if a product is highly water soluble and has low binding affinity, excessive rainfall or irrigation may move the herbicide off-target, potentially compromising efficacy.

Was the herbicide applied to a saturated soil or did the soil become saturated after application? In anaerobic soils (saturated), some herbicides may break down much faster than in aerobic conditions.

If you rule out the aforementioned factors that could influence herbicide efficacy, it’s time to ask a few more questions. Has the herbicide historically controlled the target weed at this location? Has control declined after years of continuous use? Are dying plants intermingled with unaffected plants in the treated area? Are other weed species in the treated area that are controlled? If you answered yes, resistance may be present and it’s likely worth additional investigation.

What can turfgrass managers do if resistance is confirmed? If an herbicide-resistant biotype is identified, it is important to act quickly. If left unchecked, a resistant population can further contribute to the soil seed bank. This means you could be dealing with resistance issues to that MOA (at this site) for the next 7-10 years, depending on multiple factors including species and seed bank dynamics. If practical, switching to an herbicide with a different efficacious MOA is the easiest way to manage a resistant biotype. If switching MOAs is not a viable option, other control means including hand removal, mechanical, cultural controls, or renovation may be required.

In summary, herbicide-resistant weeds are present in turfgrass systems and will likely become more problematic in the future. To mitigate the potential for resistance development, professional turfgrass managers are encouraged to maintain a healthy turfgrass sward, employ various aspects of integrated weed management, and rotate herbicides with differing MOAs frequently as well as use multiple MOAs within each season (when practical).

Tripp Rogers is a Ph.D. candidate under the direction of Dr. Travis Gannon in the Department of Crop and Soil Sciences at North Carolina State University.