Herbicides are an effective tool for control of most weeds on athletic fields. But the turfgrass management environment is unique compared to row crop agriculture in that our goal is to maintain high-quality, aesthetically-pleasing and functional surfaces over a period of many years. This being the case, some of the weed control tools used in row crop agriculture aren’t practical (or possible) for athletic field management, for example, regular tillage. Given the way that turfgrass is managed, herbicides are our primary and usually only option.

Herbicide resistance, long a problem in production agriculture, is an issue that’s becoming more frequently reported in turfgrass. While currently not widespread, it has the potential to become one of the significant challenges athletic field managers will face in coming years.

A brief history

Some of the herbicides used today have been around since the 1940s. Currently, there are relatively few new herbicide chemistries being developed for the turfgrass market. The development of resistance to herbicides by weeds isn’t a new phenomenon; cases have been reported as early as 1970. The severity of this issue in turfgrass varies depending on the target weed species and location. Herbicide resistance issues in turfgrass (in the U.S.) were first reported in the South. It has more recently been reported in cool-season turfgrass but isn’t yet as serious of a problem. By taking steps now to understand the issue, we can perhaps help to prevent and/or delay herbicide resistance becoming a serious issue in cool-season turfgrass.

What and where

The first reported instance of herbicide resistance was in 1970, when it was observed that simazine was no longer controlling groundsel when used in nurseries in Washington. By the first part of this decade, the number of weed species that have been found to be resistant to at least one family of herbicides exceeded 200 – several of these species are important weeds in athletic field management (see Figure 1).

Figure 1. Herbicide resistance is either suspected or confirmed with these weeds that are common on cool-season turfgrass athletic fields.

Goosegrass (Eleusine indica) was one of the first turfgrass weeds in which herbicide resistance was documented, when it was found that the dinitroanaline herbicides were no longer providing control.

Annual bluegrass has shown resistance to the warm-season turf herbicides foramsulfuron, trifloxysulfuron, imazaquin, simazine, atrazine; the nonselective herbicide glyphosate; and the cool-season turf herbicides bispyribac, benefin, pendimethalin and prodiamine. Resistance in these species has been recognized for a long time and management recommendations for herbicides have been altered accordingly. For example, oxadiazon is now more often recommended as a preemergence control for goosegrass.

Aaron Patton, a weed scientist at Purdue University, has documented and is studying resistance development in smooth crabgrass (to quinclorac), buckhorn plantain (to 2,4-D) and ground ivy (to synthetic auxin herbicides). These occurrences aren’t yet widespread, but we’re seeing cases of suspected weed resistance reported with increasing frequency.

A common misconception

It’s generally assumed that weeds acquire resistance to a particular herbicide. But this isn’t the correct way of describing the phenomenon. As with all living things, there’s genetic variability within a population. Some species have more genetic variability than others, but all have it.

Because of this, a population that’s normally susceptible to a particular type of herbicide has individuals that can tolerate and therefore can’t be killed by the herbicide. If this resistant individual is allowed to complete its life cycle and reproduce, then its offspring will also have the trait that allows it to resist that particular type of herbicide.

If the same herbicide that’s no longer effective is used repeatedly, then the population of resistant weeds will grow and can eventually become the majority. The resistant weeds can then be spread by wind, equipment and all the other normal ways that weeds are spread.

Reduce the risk of resistance

In cases of documented resistance to herbicides, rate very quickly becomes a nonfactor – if you have a weed that’s resistant, doubling or tripling the rate of the product and reapplying tends to be ineffective. Where resistance is documented, resistance to up to 80-times rates has been reported by turfgrass scientists.

The best and most effective method to reduce the risk of a weed population acquiring resistance to a class of herbicides is to not use the same product year after year. Instead, rotate among the different herbicidal modes of action that are available to you. The reason for this is that if a weed population develops resistance to a specific herbicide, there’s a good chance that it will resist all other herbicides that work the same way.

This strategy presents some practical challenges for athletic field managers who maintain cool-season turfgrass fields. One of these challenges is mental – many turfgrass managers use certain herbicides to control certain weeds because they know they work, or have worked. But it gets complicated when a turfgrass manager wants to rotate away from one of these desired products, either because of economics or performance. In these cases, switching to another product to help reduce the potential development of a problem that’s real, but may not be obvious yet, can be a hard argument to win. But just like with fungicides, athletic field managers need to learn and pay attention to the different modes of action of available herbicides. (There’s an extension bulletin (SS-AGR-394) available from the University of Florida, that categorizes all herbicides available for both cool- and warm- season turfgrass.)

Another challenge for cool-season turfgrass managers is that, compared to production agriculture or even warm-season turfgrass, there aren’t a lot of practical rotation options available.

Table 1. Classification of preemergence herbicides registered for use in cool-season turfgrass, according to both the Herbicide Resistance Action Committee (HRAC) and the Mechanisms of action codes according to the Weed Science Society of America (WSSA). A proper herbicide rotation involves changing between not just products or chemical classes, but also modes of action.

  • Table 1 lists the preemergence herbicides available for cool-season turfgrass managers.
  • Table 2 lists herbicides for postemergence control of grassy weeds and sedges, and
  • Table 3 lists the herbicides available for cool-season turfgrass for postemergence control of broadleaf weeds.

The tables are organized according to how the herbicides work to control weeds, often referred to as their Mode of Action (or MOA). Two different classification systems have been developed to categorize herbicides by their MOA. One was developed by the Herbicide Resistance Action Committee (HRAC) and the other by the Weed Science Society of America (WSSA).

The tables report the MOA of the herbicides according to both classification systems. For example, the herbicide siduron at the top of Table 1 was categorized as a C(category)2 by the HRAC and C7 by the WSSA.

The fact that there are two different classification systems used may cause some confusion. But with both systems, there’s a general agreement on how herbicides should be categorized according to their MOA. For example, siduron is a substituted urea class herbicide that works in plants by inhibiting photosystem II. It has a different MOA than the dinitroanaline herbicide benefin, which inhibits mitosis. Most pesticide manufacturers now include on their label a box with the WSSA code to help turfgrass managers easily identify what type of herbicide they’re using.

Table 2. Classification of postemergence grassy and sedge herbicides registered for use in cool-season turfgrass, according to both the Herbicide Resistance Action Committee (HRAC) and the Mechanisms of action codes according to the Weed Science Society of America (WSSA). A proper herbicide rotation involves changing between not just products or chemical classes, but also modes of action.

Proper rotation involves using herbicides with a different MOA for each application. For example, if the objective is to avoid resistance development in crabgrass, then rotating between benefin and prodiamine isn’t effective because both are dinitroanaline herbicides.

It’s also not effective to rotate between a dinitroanaline herbicide and the herbicide dithiopyr, which is in a different chemical class (pyridine). However, the pyridines and the dinitroanalines have the same MOA. Looking at the other preemergence herbicides in Table 1 that are effective for crabgrass control reveals that our options for an herbicide to rotate with the mitosis inhibitors include just siduron (which is used more at seeding time) or bensulide.

A strategy to help deal with this is lack of options to incorporate the use of postemergence herbicides into the management plan and to also rotate among these chemistries. For example, the use of fenoxaprop in year one followed by quinclorac in year two and then either topramazone or mesotrione (different chemical classes but same mode of action) in year three.

It was previously mentioned that if a weed population acquires resistance to one herbicide, then it probably will resist all herbicides that work the same. There’s an exception to this however, in that with goosegrass, dimethenamid-p is labelled for control even though it’s a mitosis inhibitor. But dimethenamid-p is a different type of mitosis inhibitor, according to both classification systems.

For the broadleaf herbicides, if the target is an annual weed, a preemergence herbicide may be useful. Pendimethalin, prodiamine or dithiopyr in year one followed by bensulide in year two may effectively control many of our summer annual broadleaf weeds.

Another option may be to rotate to isoxaben, depending on the target weed. When rotating, make sure that the target weed is on the label.

Some closing thoughts

While the overall problem of herbicide resistance in cool-season turfgrass is still relatively minor, it does have the potential to become one of the more significant management issues the sports turf industry will face in the future. So, you should do your part and adopt a proper herbicide rotation program.

The other thing you should do is remain observant. Whenever an herbicide application fails, it’s easy to rationalize the failure by saying:

“It must have been mixed wrong…” or;

“It was applied incorrectly…” or;

“The weather must have been bad after I applied it.”

Yes, these things can and unfortunately do happen. But when noticing herbicide failure (if it targets more than one weed), observe whether all the weeds kept growing or whether there was one specific species that wasn’t controlled adequately. If this is the case, then certainly follow up.

It could also be that you have found an herbicide-resistant population. Report suspected cases to your state extension specialist, as they may be able to test to verify the resistance and then alter management recommendations for control of that weed.