Confirming Resistance

To be classified “herbicide resistant,” weeds must meet established criteria. Weed scientists use these guidelines when evaluating weed populations.

The International Survey of Herbicide-Resistant Weeds documents weed resistance to herbicides. Guidelines to confirm weed resistance to herbicides have been established.

Following these guidelines will ensure accuracy, objectivity, transparency and consistency in herbicide resistance determination. It’s critical that the scientific method be applied. Their use should also be timely, as information on herbicide-resistant weeds helps farmers, herbicide manufacturers and academics develop in-field weed management systems and stewardship programs for herbicides.

Criteria required for weeds to be listed:

1. Fulfillment of the Weed Science Society of America (WSSA) definition of resistance and the survey’s definition of a herbicide-resistant weed
2. Resistance confirmation using accepted procedures
3. The resistance must be heritable
4. Demonstration of practical field impact
5. Identification of weed to species level
6. Plants that demonstrate resistance or tolerance to herbicides derived from deliberate or artificial selection should not be considered

1. Fulfillment of the WSSA definition of resistance and the survey’s definition of a herbicide-resistant weed

Plants may be naturally tolerant to herbicides, or tolerance may be intentionally introduced in herbicide-tolerant crops. The survey definition involves weed populations that were originally susceptible to a herbicide application but have evolved to be resistant to that same herbicide application.

2. Resistance confirmation using accepted procedures

Before declaring a weed “herbicide-resistant,” proper scientific protocols must be followed. We’ve established these protocols in our Criteria for Confirmation of Herbicide-Resistant Weeds document.

The preferred test for initial identification is a dose-response test on growing plants in controlled conditions like a growth chamber or greenhouse. A range of herbicide doses – from sub-lethal to lethal – are used on resistant and susceptible weeds for comparison.

A major challenge is deciding between low-level resistance and natural tolerance in weed populations. A scientific definition and an agricultural field definition are options.

To qualify as resistant by HRAC definition, the weed population would not be controlled in a field situation by a herbicide applied at the recommended field application rate.

3. The resistance must be heritable 

Testing plants grown from collected seeds is required to establish resistance, as opposed to testing plants currently growing in the field. To make the test even more conclusive, second generation seed from greenhouse grown plants should be collected and tested for resistance as well.

4. Demonstration of practical field impact 

To qualify as resistant, the weed must present a problem to the farmer when using the herbicide at recommended rates. If there’s no detectable difference in weed control, the weed won’t be added to the list.

5. Identification of weed to species level

The resistant plant must be a weed and identified to the species level.

6. Plants that demonstrate tolerance to herbicides derived from deliberate or artificial selection should not be considered

For more information, download our Criteria for Confirmation of Herbicide-Resistant Weeds document.