MANHATTAN, Kan. — For decades, herbicide resistance was often tied to changes at a single target site in a weed plant.
Today, Kansas State University experts say many weeds are developing non-target-site resistance — a broader, more complex form of resistance that allows plants to survive multiple herbicide groups and complicates management decisions for producers.
K-State weed scientist Pat Geier recently marked the 50th anniversary of the first documented case of herbicide resistance in the state, tracing how weed resistance has evolved and what it means for modern weed management in Kansas agricultural systems.
“Herbicide resistance occurs when a weed population survives applications of a chemical that would normally control it, often driven by repeated use of herbicides with the same mode of action over many seasons,” Geier said. “Over time, naturally occurring genetic variations allow some weeds to survive and reproduce, leading to resistance becoming more widespread within fields and across regions.”
“The first herbicide-resistant weed confirmed in Kansas was kochia in 1976, resistant to atrazine,” he said.
Since then, resistance has spread to a wide range of herbicide groups and weed species. Today, at least 16 weed species in Kansas have confirmed resistance to one or more herbicide modes of action, including widely problematic species such as Palmer amaranth and waterhemp.
“These pigweeds are particularly concerning because they not only survive common herbicide treatments, but also cross-pollinate, accelerating the spread of resistance traits,” Geier said.
To help growers address this issue, K-State weed scientists emphasize diversified weed management strategies that go beyond reliance on a single herbicide or chemistry class. These best practices include:
- Rotating herbicide modes of action to reduce selection pressure on weed populations.
- Integrating non-chemical tools and cultural practices (such as crop rotation and timely tillage where appropriate).
- Regular scouting and early detection to identify resistant weed escapes before they seed and add to the seed bank.
- Leveraging diagnostic and extension resources to make informed decisions about herbicide choice and resistance management.
Experts also note that understanding the mechanisms of resistance — whether target site changes that prevent herbicide binding or non-target site mechanisms that limit herbicide movement or increase its detoxification — is critical for designing effective control programs.
As resistance continues to challenge producers, researchers are working to provide updated tools and information, including diagnostic methods and local resistance confirmation maps, to support adaptive management strategies tailored to individual fields and weed species.
For more information on herbicide resistance and weed management, contact the K-State Extension Agronomy team or reach out to your local extension office.
