Managing Herbicide Resistance in Sod Production

Patrick McCullough, Ph.D.

Extension Specialist, Crop and Soil Sciences

• Planning a Resistance Management Program
• Cross-Resistance Management

Weed control is a critical component to successful sod production. The presence of weeds may reduce aesthetics,
marketability and strength of turfgrass sod. Chemical control is often warranted. Weed management
strategies with preemergence and postemergence herbicides are often modified in sod production to meet
harvesting requirements and client demands. Herbicide selection may be limited due to economics or turfgrass
tolerance; however, repeated use of a single herbicide chemistry may lead to problems with herbicide-resistant
weeds.

Weed populations resistant to herbicides may present long-term challenges in commercial sod production. In
order to prevent or manage resistance, sod growers must understand herbicide mode of action and the potential
influence on weed population dynamics. Prior to the development of resistance, initial weed populations are
susceptible to herbicides and controlled by labeled rates when appropriately applied. Susceptibility to herbicides
is a natural response to toxic effects of the active ingredient that influence the potential for successful weed
control. Susceptible weeds may be controlled from certain herbicide chemistries due to the inability to detoxify
or withstand the effects of an applied herbicide. For example, common dandelion (Taraxacum officinale) is
generally controlled by 2,4-D. Turfgrasses are not affected since they are able to inactivate or metabolize the
herbicide prior to toxic effects occurring in cells. Therefore, practitioners may selectively control dandelion in
most turfgrasses.

Growers are most concerned about herbicide tolerance in desirable turfgrasses but weeds may also be tolerant of
certain herbicide chemistries. Herbicide tolerance is the natural ability of a turfgrass or weed species to withstand
the effects of an applied herbicide. For example, most turfgrasses are tolerant of herbicides such as 2,4-D
or dicamba that selectively control broadleaf weeds. On the other hand, broadleaf weeds (e.g., dandelions) may
be tolerant of herbicides such as fenoxaprop that selectively control grassy weeds. A broadleaf species that was
once controlled by 2,4-D but is no longer affected by the herbicide may suggest resistance is occurring throughout
the population.

Resistance is an inherited ability of a weed biotype to survive herbicide applications to which the original population
was susceptible. Resistance is encouraged when the same herbicide or different herbicides with a similar
mode of action are applied to control a weed species over a period of years. Resistant weeds may survive the
herbicide application, reproduce and become the dominant biotype present in the population. As susceptible
biotypes are controlled by the herbicide, resistant plants survive treatments in subsequent years and begin to
dominate the population through reproduction and establishment.

Planning a Resistance Management Program

Resistance management begins with investigating potential application parameters that may result in herbicide
failure for controlling weeds. Preemergence herbicides applied at low rates or after weed seedling emergence
may not be as effective for controlling annual species compared to labeled recommendations and appropriate
timings. Efficacy of many postemergence herbicides may be reduced on mature annual weeds compared to
seedlings. For example, quinclorac is highly efficacious on crabgrass at the multi-leaf to one tiller growth stage
but two applications may be required to control multi-tillered plants. Environmental effects on plant growth
such as rainfall, drought or high humidity may also influence herbicide absorption and overall weed control
activity and be unrelated to herbicide resistance.

If the herbicide, application rate and techniques, and environmental factors have been ruled out as potential
causes for poor weed control, herbicide resistant biotypes may be present in the field. Practitioners should then
evaluate products applied for controlling the potentially herbicide-resistant weeds and note the mode of action
for the active ingredients. Rotating chemistries will likely be needed. Applying a different herbicide with the
same mode of action will be ineffective for controlling resistant biotypes.

In Georgia, diclofop is a popular postemergence herbicide for goosegrass (Eleusine indica) control on bermudagrass
(Cynodon dactylon) golf courses. Diclofop is ordinarily highly efficacious for selective postemergence
goosegrass control but repeated use over many years may lead to resistant biotypes making up the majority of
the population in certain areas. Switching the herbicide mode of action to a branch chain amino acid synthesis
inhibitor like foramsulfuron (Revolver) or a chlorophyll synthesis inhibitor such as sulfentrazone (Dismiss) will
help control a goosegrass biotype potentially resistant to diclofop in bermudagrass.

Cross-Resistance Management

Weed populations may show resistance to one herbicide or mode of action. Potential for cross-resistance is also
often significant. Weed populations may have cross-resistance when the biotype is resistant to more than one
herbicide with the same mode of action (e.g., annual bluegrass (Poa annua) that is resistant to postemergence
applications of atrazine and simazine). Both of these herbicides are triazines that inhibit photosynthesis, and
thus, the annual bluegrass population has cross-resistance to triazines. Another example would be annual bluegrass
that is tolerant of the preemergence herbicides prodiamine and dithiopyr. Although these herbicides are
from different families, both are mitotic inhibitors with the same mode of action; therefore, replacing one with
the other will not be an effective option for control.

Herbicide rotation programs may be complicated by the presence of biotypes resistant to multiple modes of action.
Weed biotypes exhibiting multiple-resistance may tolerate more than one herbicide with different modes
of action (e.g., triazine-resistant weed biotypes that are not controlled by sulfonylurea herbicides). Although
multiple-resistance may be less common than cross-resistance, triazines and sulfonylureas are the two most
common herbicide chemistries to which weed-resistant biotypes have been reported. Selecting a safe and effective
herbicide with a different mode of action may be difficult, and practitioners may need to incorporate new
cultural techniques or management plans to combat biotypes with resistance to multiple modes of action.

Herbicide resistance in annual weeds is primarily due to the potential for biotypes to survive an herbicide application
and spread throughout a field via seed dispersal. In Georgia, herbicide resistance has been reported in
populations of crabgrass (Digitaria spp.), goosegrass, annual bluegrass and other annual species in both agronomic
crops and turfgrass sod production fields. These annual weeds are prolific seed producers and can spread
rapidly throughout a given location. Table 1 shows annual weeds in Georgia with reports of potential herbicide
resistance issues and alternative chemistries for use in management programs. Sod growers must consider herbicide
rotation as a fundamental aspect of weed control programs in order to prevent or manage herbicide-resistant
weeds. Tables 2 and 3 list the mode of action and chemical family of herbicides currently registered for use
in sod production. Selection and rotation of herbicides with different modes of action could assist sod growers
with controlling herbicide-resistant weed populations and promote sustainable, long-term sod production.

Table 1. Examples of weed resistance reported in Georgia with potential herbicide options for controlling
tolerant biotypes.

Table 2. Preemergence herbicides used in sod production listed by mode of action and chemical family.
See product labels for rates, weeds controlled and turfgrass tolerance.

Table 3. Postemergence herbicides used in sod production listed by mode of action and chemical family.
See product labels for rates, weeds controlled and turfgrass tolerance.