The present document is a tool for turfgrass professionals, sod growers, landscape managers, and extension specialists to develop herbicide programs that reduce the risk of herbicide resistance (HR) evolution in turfgrass systems.
Herbicides are the most common and effective tool for weed control in turfgrass. However, weed species are capable of adapting, and now almost all herbicide groups have confirmed cases of weed species with resistance. Although HR is always a concern, this problem is particularly serious in turfgrass because typical HR management practices such as crop rotation, tillage and cultivation, cover cropping, and fallow periods are generally not an option in turfgrass production. More importantly, there are considerably fewer products that can be used for herbicide rotation in turfgrass than there are for other agricultural systems.
Reducing the Risk of HR
There are several things that can delay and perhaps prevent HR. The most practical and effective strategy is to rotate herbicides with different mechanisms of action (MOA). The MOA is the way the herbicide disrupts the metabolism of the weed, ultimately causing the weed's death. Table 1 provides a comprehensive list of herbicides that are registered for use in turfgrass and their respective MOA and classification according to the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC). Many herbicide labels include the herbicide group number (Figure 1) to help the user design herbicide rotations. Simply put, if two herbicides have the same MOA number or code, regardless of whether they have different names or active ingredients, they affect weeds in the same way. Thus, the frequent and repeated use of herbicides with the same MOA will increase the risk of weeds becoming resistant. Conversely, using a diverse herbicide program that either rotates or combines herbicides with different MOAs will help delay the appearance of resistant weeds.
MOA rotation helps delay HR because changing the MOA reduces the chances of survival and reproduction of HR weeds. In simple terms, if a weed survives an herbicide application with one MOA because it is resistant, the problem can be controlled if the surviving weed is treated with a herbicide with a different MOA to which it has not become resistant.
As shown in Table 1, most herbicides for preemergence (PRE) control are mitosis inhibitors (Groups 3 and 15), while postemergence herbicides are predominantly ALS-inhibitors (Group 2). Although many turfgrass professionals base their herbicide programs solely on Group 3 and Group 2, it is critical that herbicides from other MOAs are included in herbicide programs. To ensure that the most frequently used herbicides in turfgrass will continue being effective for a long time, herbicides with different MOAs should be included in weed management programs even if they are not as effective or require repeat applications to provide the desired level of control.
Groups that are useful for MOA rotation in PRE programs in turfgrass:
Cellulose inhibitors (Group 21)
Fatty acid and lipid biosynthesis inhibitors (Groups 8 and 16)
PPO inhibitors (Group 14)
Groups that are useful for MOA rotation in POST programs in turfgrass:
Carotenoid biosynthesis inhibitors (Group 28)
Lipid biosynthesis inhibitors (Group 16)
Photosystem II inhibitors (Group 5)
PPO inhibitors (Group 14)
Synthetic auxins (Group 4)
There are two ways to rotate herbicides in turfgrass. The first way is to change the MOA from year to year (Figure 2). For example, one could use a Group 3 herbicide in the fall of year 1 and change to Group 21 in the fall of year 2. The second way is to rotate herbicides within a season. In this case, MOAs are rotated to control escapes from the previous application. For example, if a Group 16 herbicide was used as PRE or POST and some plants escaped, a Group 14 herbicide can be applied to kill the escapes. This is called the "double-knock down" strategy because weed control is based on two consecutive actions. Ideally, both approaches should be used within the same weed management program.
Another strategy to delay HR is to apply herbicides with different MOA simultaneously. This can be done by tank-mixing herbicides with different MOAs or by using pre-mixed products with two or more herbicides with different MOAs. For this option to work for HR management, the herbicides with different MOA have to be effective when applied alone to control the target weed species.
Mechanism of action (MOA) classification according to the Weed Science Society of America (WSSA) and the Herbicide Resistance Action Committee (HRAC) for preemergence (PRE) and postemergence (POST) herbicides registered for use in turfgrass.