As harvest season quickly approaches, it is a good time to think about a different kind of harvest as well. Weed scientists at the University of Nebraska–Lincoln (UNL) are collecting seed samples from suspected herbicide-resistant weed populations to better understand the distribution of resistance across Nebraska and identify potentially emerging resistance traits. Growers can contribute by collecting seed heads from suspected herbicide-resistant weeds, including populations that showed reduced sensitivity to herbicides or increased tolerance during the 2026 growing season, and delivering them to their local extension office or mailing them directly to UNL.
Why Is It Important to Screen for Resistance?
If weeds in one of your fields survived an herbicide application because of suspected resistance or reduced sensitivity, submitting a seed sample can help confirm the issue and support more informed weed management decisions in future growing seasons. These samples help researchers identify where resistant populations are occurring, and the level of resistance present. The resulting information can support site- and region-specific weed management decisions as well as statewide herbicide-resistance monitoring efforts.
What Types of Fields Should Be Sampled?
In Nebraska, herbicide resistance is most commonly associated with pigweeds, with waterhemp more prevalent in the east and Palmer amaranth more common in the west. These species are especially concerning because a single plant can produce hundreds of thousands of seeds. Repeated use of herbicides with the same mode of action can select for resistant biotypes, allowing resistant populations to spread over time.
Fields with weed escapes following postemergence herbicide applications are good candidates for sampling. Survival of waterhemp or Palmer amaranth after treatment may indicate reduced sensitivity or resistance, particularly where multiple postemergence applications were made and substantial populations remain.
Resistance to certain herbicides is of particular interest. Resistance to glyphosate and acetolactate synthase (ALS)-inhibiting herbicides is already widespread and well-documented in Nebraska. However, researchers are especially interested in samples of waterhemp or Palmer amaranth that survived applications of 2,4-D, including low-volatile ester and choline formulations, dicamba products such as Clarity, DiFlexx, Tavium, Engenia and Stryax, or glufosinate products such as Liberty and Interline. These samples will help researchers determine whether resistance to these herbicides may be emerging.
How to Collect and Submit Seed Samples
This video provides an overview of how to collect weed seed heads for resistance screening.
1. Print and fill out the resistance screening form
The information on this form provides valuable background about the sampled field and weed population. Place the completed form with the sample when mailing or delivering it.
2. Collect seed heads
Take a paper bag to the field where the weed escapes — weeds that remain uncontrolled after an herbicide application — are present. Do not use a plastic bag because trapped moisture can promote mold.
Cut mature female seed heads from at least five separate plants. Collecting from multiple plants increases the likelihood of capturing a representative sample of the population. Female plants are identified by their spiky, rough seed heads. Male plants have smoother heads and may release pollen when rubbed.
Seed heads should be mature and ready for harvest. Seeds that readily shatter from the seed head when shaken are a good indication of maturity. Rubbing a mature seed head between your hands should release small black or reddish seeds.
3. Submit the sample
Place the paper bag and completed resistance screening form in a cardboard box and seal it for delivery. Samples may be dropped off at a local Nebraska Extension office or mailed to the appropriate UNL laboratory listed below and shown in Figure 2.
Western Nebraska: Samples collected in Keya Paha, Rock, Garfield, Valley, Sherman, Buffalo, Kearney or Franklin counties — or any county west of them — should be mailed to:
Milos Zaric
402 West State Farm Rd.
North Platte, NE 69101
Eastern Nebraska: Samples collected in all remaining Nebraska counties should be mailed to:
Amit Jhala
279 Plant Science Hall, East Campus
University of Nebraska Lincoln,
Lincoln, NE 68583-0915
What Will UNL Do With the Samples?
Researchers at UNL will plant the collected seeds in greenhouse facilities and conduct an initial resistance screening using dicamba, 2,4-D, mesotrione, and/or glufosinate. Plants will be treated at the standard field rate and three times that rate, then compared with plants known to be susceptible to the herbicides. Populations showing reduced sensitivity or potential resistance during this preliminary screening will advance to full dose-response testing to better characterize the level of resistance present.
If new cases of herbicide resistance are identified through full testing, the findings will be shared through Nebraska Extension crop protection clinics, publications and other outreach channels to help inform growers and agricultural professionals.
Final Thoughts
Weed escapes do not always indicate herbicide resistance, as factors such as extended emergence, weather conditions, application quality and application timing can reduce herbicide performance. When resistance is suspected, submitting a seed sample can help confirm whether it is present and support more informed management decisions.
Growers can also help delay resistance by using herbicides with different modes of action and integrating cultural practices such as narrow row spacing, cover crops and other integrated weed management strategies where applicable. Together, these efforts can improve researchers’ and growers’ understanding of herbicide resistance in Nebraska and help preserve effective weed control options for the future.
For questions or additional information, contact Milos Zaric, UNL pesticide application technology specialist, or Amit Jhala, Nebraska Extension weed management specialist.
