The WSU Herbicide Resistance Screening Program continues to provide farmers with a useful tool for determining whether poor herbicide performance is related to herbicide resistance. By testing weed populations collected from farms across Washington, the program helps identify resistant biotypes and provides information that can be used to develop more effective, integrated weed management strategies.
Since the program began, grass weeds have consistently accounted for a large portion of the resistant populations submitted for testing. Recent results from 2024 and 2025 continue this trend, with downy brome, wild oat, and Italian ryegrass making up most of the grass weed samples received. These results reinforce a continuing concern for wheat growers: many of the most troublesome weeds in wheat belong to the same grass family as the crop, leaving growers with relatively few selective herbicide options. Repeated reliance on the same herbicide groups can increase selection pressure for resistance.
What did we see in 2024?
In 2024, the program received 20 samples. The samples included downy brome, wild oat, prickly lettuce, Italian ryegrass, mustard, and windgrass. As in previous years, the majority of resistance identified was in grass weeds.
Downy brome
In downy brome, Resistance to several Group 2 (ALS-inhibiting) herbicides was detected, including Beyond, Olympus, Osprey, and Osprey Xtra. Of the downy brome samples tested, seven showed resistance to Beyond, seven to Olympus, and 10 to Osprey. Resistance to the Group 1 herbicide Assure was also detected in two samples. In addition, one downy brome sample was confirmed resistant to Roundup PowerMAX.
Wild oat
The 2024 samples exhibited resistance to multiple herbicides. Two samples were resistant to Axial Bold, while resistance was also detected to Olympus, Osprey, Beyond, PowerFlex, and Osprey Xtra.
Other weeds
Resistance was also detected in less frequently submitted species. The windgrass sample tested was resistant to Olympus, Osprey, Beyond, PowerFlex, and Osprey Xtra. One tansy mustard sample was resistant to Beyond. In contrast, prickly lettuce was generally sensitive to most of the herbicides tested.
What did we see in 2025?
The program received six samples in 2025, including Italian ryegrass, downy brome, wild oat, and cereal rye. Although the number of samples was smaller than in 2024, the results again highlighted the importance of resistance in grass weeds.
Cereal rye
The cereal rye sample was resistant to Beyond, Osprey, and PowerFlex. The wild oat sample was resistant to Assure, Tacoma, Osprey, and Axial Bold.
Italian ryegrass
Italian ryegrass was resistant to Osprey and PowerFlex. The plants also showed injury from Beyond and Olympus but subsequently recovered and regrew. Because recovery after herbicide injury does not necessarily meet the criteria for confirmed resistance, these responses should be considered tolerance or reduced sensitivity rather than confirmed resistance.
Downy brome
Downy brome again accounted for several of the resistance findings. Two samples were resistant to Beyond, three were resistant to Osprey, three were resistant to PowerFlex, two were resistant to Roundup PowerMAX, one was resistant to Osprey Xtra, and two were resistant to Olympus.
Grass weeds remain the biggest concern
The 2024 and 2025 results are consistent with what the WSU Herbicide Resistance Screening Program has observed over the past several years: grass weeds continue to dominate the resistance problem in eastern Washington wheat production systems.
Downy brome is particularly concerning because resistance is occurring across several herbicides that have historically been important tools for its control.
Downy brome showed the broadest range of resistance in both years. Resistance was detected to multiple Group 2 herbicides, as well as Group 1 herbicides and glyphosate. Wild oat also showed resistance to multiple Group 1 and Group 2 herbicides.
Submit a sample
If a weed population survives an herbicide treatment that has previously provided good control, consider submitting a seed sample to the WSU Herbicide Resistance Testing Program. The WSU Herbicide Resistance Testing Program can help determine whether poor control is associated with a heritable resistance trait.
Samples should be collected broadly from the area of interest rather than from a single plant or small spot, and enough mature seed should be collected to allow for testing.
2024-2025 WSU Herbicide Resistance Program Samples
| Weed species | 2024 Herbicide Resistance (cases) | 2025 Herbicide Resistance (cases) |
|---|---|---|
| Downy brome | Beyond (7), Olympus (7), Osprey (10), Assure (2), Roundup PowerMAX (1) | Beyond (2), Osprey (3), PowerFlex (3), Roundup PowerMAX (2), Osprey Xtra (1), Olympus (2) |
| Wild oat | Axial Bold (2), Olympus, Osprey, Beyond (2), PowerFlex (2), Osprey Xtra | Assure, Tacoma, Osprey, Axial Bold |
| Italian ryegrass | No confirmed resistance reported | Osprey, PowerFlex |
| Windgrass | Olympus, Osprey, Beyond, PowerFlex, Osprey Xtra | No samples received. |
| Cereal rye | No samples received. | Beyond, Osprey, PowerFlex |
| Mustard | Beyond | No samples received. |
| Prickly lettuce | Sensitive to most herbicides tested | No samples received. |
Contributed by Marija Savic Associate in Research, Weed Science, Washington State University.