Herbicide Resistance

The Basics of Herbicide Resistance

Over the past two decades, herbicide-resistant weeds have increased globally, creating major economic consequences for farmers and pushing both the private and public sectors to find innovative solutions to this crisis. Before widespread resistance, farmers could control problematic weeds relatively easily with herbicides. Roundup Ready and other herbicide-resistant crop technologies helped drive this success. By the mid-2000’s, these simple herbicide solutions were quickly becoming ineffective, with several problematic herbicide-resistant weeds spreading onto more farms and into new regions. (Examples of common herbicide resistant weeds across the U.S. are provided in Figure 1.) Multiple approaches to managing resistant weeds were becoming necessary. This led to more herbicide use, the need for tillage or other mechanical options, and a search for more innovative technologies.

In the U.S., most farmers continue to rely on herbicides as the primary approach to managing weeds in major field crops. Not only does this approach cost more per acre than ever before, but each year, successful weed control with herbicides alone becomes less reliable and likely. It is not uncommon for farmers in some crops and regions to make at least three or more herbicide applications per growing season, with multiple products applied at each application timing. Current expert recommendations call for using overlapping residual products in addition to over-the-top or POST applications in corn, soybean, and cotton. The latest herbicide-resistant technologies, which allow POST applications of certain Group 4 herbicides such as 2,4-D and dicamba to soybean and cotton, are already beginning to fail because of regional herbicide-resistance problems emerging.

Cross-resistant and multiple-resistant Palmer amaranth, waterhemp, Italian ryegrass, barnyardgrass, and others are on the rise, and a more ominous resistance mechanism called “metabolic resistance” has been identified in several weed species and for several herbicide groups. In this first education module, we will cover the Basics of Herbicide Resistance where you will learn:

Section 1. How do we define herbicide resistance, susceptibility, and tolerance?
Section 2. How does herbicide resistance happen?
Section 3. Can you reverse herbicide resistance?
Section 4. What does it cost to manage herbicide resistance?
Section 5. How do you identify herbicide resistance?
Section 6. How are herbicides classified based on modes of action (MOA)?
Section 7. What are cross and multiple resistance?
Section 8. What are the mechanisms for herbicide resistance?
Section 9. Are all weeds and herbicides vulnerable to resistance?
Section 10. What can we expect from new herbicide modes-of-action or other technologies?

A future education module will follow that presents management options using Integrated Weed Management (IWM) approaches including prevention, non-chemical strategies, and best management practices for herbicides.


William Curran, Penn State University (emeritus)

Claudio Rubione, GROW


Mark VanGessel, University of Delaware

Emily Unglesbee, GROW 


Patrick Tranel, University of Illinois 

Franck Dayan, Colorado State University


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