Identifying Herbicide Resistance in the Field

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Herbicide resistance is one of the most serious problems in Kansas and across global agriculture. Since the first report of resistance to 2,4-D in wild carrot in 1957 (Switzer), the problem has ballooned to include 273 species worldwide involving 21 modes of action, including numerous species in Kansas, which were described in a recent eUpdate article. When weed control fails, it is essential to determine the cause. If herbicide resistance is the cause, herbicide programs need to be adjusted accordingly. Herbicide resistance occurs with the repeated use of herbicides with the same or similar mode of action (MOA)at the same site for several years. This leads to selection pressure favoring the survival of resistant individuals within a previously susceptible population. Timely and accurate diagnosis, regular field scouting, and early detection not only help reduce yield losses but also allow for tracking resistance across larger regions.

What does herbicide resistance look like in the field?

According to the Weed Science Society of America (WSSA) and Take Action on Weeds (2015), there are three broad indicators of possible herbicide resistance that could be used to evaluate the potential case of resistance:

1. Species-specific survival. If a particular individual weed survives an herbicide treatment while other adjacent individuals are controlled well by the same herbicide.
2. Spreading patch. Resistant weeds appear as clustered patches within a field. These patches may spread next season after seed production with tillage equipment or wind.
3. Mixed injury response. A single weed species may show a mixed response to the herbicide, where some plants are either completely dead, moderately affected, or some remain totally healthy without any damage.

This heterogeneous response within a single weed population suggests a genetic diversity within the species, which could allow the evolution of multiple herbicide resistances in the future.

However, not all weed control failures can be attributed to herbicide resistance. Nine out of ten herbicide failures are due to factors other than resistance. Other factors, such as unfavorable weather conditions, incorrect rates, poor coverage, or application timing, skips, weed size and density, soil moisture, and equipment malfunctions, can also cause poor weed control or allow weeds to escape.

Normally, growers rely on pre-emergence (PRE), post-emergence (POST), or residual herbicides - either individually or in combination to achieve broad-spectrum weed control. Despite having a proper herbicide program in place, weeds may still escape due to the evolution of herbicide resistance. In addition to the three indicators discussed earlier, accurate identification in the field also requires an understanding of how each herbicide MOA behaves and what symptoms, or the lack thereof, indicate a failure. For instance, POST herbicides should produce visible symptoms, such as yellowing, burning, drooping, or complete plant death. On the other hand, residual herbicides are expected to suppress the new flushes of emergence weeks after treatment. When weeds continue to emerge and survive despite proper and timely spraying with no other factors to blame, it could suggest resistance and should be promptly investigated.
 

Table 1: Weed species and field indicators of resistance to specific herbicide groups

Application Timing	Herbicide Group	Herbicide Example	Weed Species	Field Symptoms of Resistance	Reference
POST	Group 2	Imazamox, Chlorsulfuron	Kochia	Kochia patches survive ALS inhibitors; no chlorosis or stunting observed	Burgos et al. (2013)
PRE	Group 3	Trifluralin	Rigid ryegrass	Ryegrass emerges uniformly despite PRE trifluralin; normal root development	Holt (1992)
POST	Group 9	Glyphosate	Horseweed	Survivors after POST application; no injury while adjacent weeds were controlled	Heap (2025)
POST	Group 14	Fomesafen	Waterhemp	Waterhemp survives POST fomesafen with regrowth and no necrosis	Burgos et al. (2013)
Residual	Group 15	S-metolachlor	Waterhemp	Late-emerging flushes 3–4 weeks after PRE S-metolachlor under moist conditions	Heap (n.d.)

 

These observations may not be definitive for identifying resistance, but they serve as the initial red flag for further scrutiny. Contacting the local extension agent or an expert about the issue may help in evaluating the problem more scientifically if further investigation is needed.

Target Site vs Non-Target Site Resistance in the Field

Identification of herbicide resistance at the field level may also provide early insights into management. Generally, weeds have two mechanisms to evolve resistance: target site resistance (TSR) and non-target site resistance (NTSR). In target site resistance, a single change in the gene encoding an herbicide target protein prevents the binding of the herbicide, thus preserving the normal enzymatic function. Non-target site resistance involves changes in the physiological mechanisms aimed at reducing the amount of herbicide reaching the target site, either by altered uptake or enhanced metabolism.

Weeds with confirmed TSR often appear as uniform survivors across a population, with plants showing little to no herbicide injury and a reduced chance of cross-resistance to more than one herbicide group. However, weeds that exhibit patchy, irregular survival patterns with partial injury symptoms most likely have NTSR. NTSR is usually more challenging to identify and can be more daunting to manage due to cross-resistance across multiple groups of herbicides. These distinctions form the basis for field diagnosis prior to greenhouse or molecular confirmation in the laboratory.

What is next if herbicide resistance is suggested?

It is imperative to take immediate action to manage a population suspected of herbicide resistance. Some of the recommendations that may help mitigate the spread of herbicide resistance include:

• Post-spray scouting. Soon after the initial spraying, walk the field to observe weed responses. It is important to look for survivors and compare injury across the weed species.
• Mix multiple effective herbicides. When possible, use different herbicide modes of action for re-sprays.
• A second application may not be effective if weeds are too large. In this case, cultivation or hand-weeding may be necessary.
• Scout for weed escapes during and after harvest. Adjust herbicide and/or crop rotations for long-term resistance management.
• Contact your extension agent, input supplier, or commercial applicator if you suspect herbicide resistance in your field.

Take-home message

Herbicide resistance is a growing challenge that poses a serious threat to effective weed management on farms. Although herbicide failure is a key sign, other agronomic or environmental factors must first be ruled out. Knowing the mode of action of different herbicides and then identifying when weeds do not respond as they should is crucial for early detection of resistance. 

Watch closely for warning signs in your field, as early detection is key to preventing the spread of herbicide resistance. Some key signs that may indicate herbicide resistance include:

• Dead plants next to live plants after an herbicide application
• Weeds that were controlled in previous years are surviving and increasing in density.
• Patches or trails of living plants following an application.

Should resistance be suspected, use alternate methods to control those surviving plants and prevent reproduction. Report any cases that may be concerning to your local area agents, regional Extension Agronomists, or Weed Scientists at K-State.

References

Switzer, C. M. 1957. The existence of 2,4-D–resistant strains of wild carrot. Proc. Northeast. Weed Control Conference. 11:315-318.

Weed Science Society of America (WSSA), & Take Action on Weeds. (2015). Field identification and control of suspected herbicide-resistant weeds. United Soybean Board. Retrieved from https://iwilltakeaction.com/uploads/files/Field-Identification-of-Suspected-Herbicide-Resistant-Weeds.pdf

 

Yasir Parrey, Assistant Scientist, Weed Extension Lab
yasir1@ksu.edu

Sarah Lancaster, Extension Weed Management Specialist
slancaster@ksu.edu

Patrick Geier, Weed Scientist, Garden City
pgeier@ksu.edu

Tags:  weeds weed control herbicide resistance