Midwest: Herbicide Resistance and Weed Management – New Online Course

Young waterhemp seedling. Photo: Iowa State University

Young waterhemp seedling. Photo: Iowa State University

Plan ahead. Dealing with herbicide resistance can be expensive. The United States Department of Agriculture estimates the cost of dealing with herbicide resistance once it occurs to be $20 to $60 per acre.



Therefore, implementing a long-term weed management strategy that reduces the chances of resistance developing will maximize long-term profitability.

This online, interactive, and self-paced course contains narrated presentations, lesson activities, and resources to provide farmers and agribusiness professionals the tools to develop successful long-term weed management plans that will maximize long-term profitability.

Course sections

  • Herbicide Resistance.
  • Weed Identification.
  • Using Multiple and Effective Herbicide Groups.
  • Calculating Effective Herbicide Rates.
  • Virtual Tour of Herbicide Plots.
  • Planning for Long-Term Weed Management.

Concepts covered

Weed identification 

A successful weed management plan requires the proper identification of the weeds in the field so that the weed management plan can be tailored to the weeds of concern.

Selecting effective products 

Not all herbicides are effective against the same weed spectrum. A successful weed management strategy must utilize herbicides that are effective against the weeds present in the field.

Using effective rates 

It is tempting to use less than full rates of herbicides. Many pre-mixes use partial rates of active ingredients, compared to the single-active-ingredient herbicides, to keep the final cost palatable.



Partial rates promote the development of herbicide resistance. This section will provide the skills necessary to analyze and adjust various herbicide alternatives to ensure that rates that are effective against the weeds in the field will be used.

Using multiple sites of action 

Repeatedly using the same mechanism of weed management selects for weeds that can overcome that mechanism. It is important to not rely on just one mechanism of weed management in the short term and to “mix up” the mechanisms long-term.

Participants will learn how to create both short-term and long-term strategies to keep Mother Nature “off balance,” thereby minimizing the potential of herbicide resistance developing.

Proper timing 

Weeds can grow quickly and can easily become too large for effective herbicidal activity. In an effort to be sure that weed seed germination is complete prior to a post-emergence application, herbicide applications are sometimes delayed, which can easily backfire with poor control of larger weeds.

Participants will be able to see the effects of proper and improper timing of herbicide applications.

Developing the long-term strategy 

The “Planning for Long-Term Weed Management” section pulls together the concepts in the previous sections and adds non-chemical concepts to consider as improvements to weed management in the field.



  • Those who complete the course are eligible for 3.0 integrated pest management Certified Crop Advisor continuing education units. In order to receive CCA credits, the entire course must be completed.
  • The materials for this online course were originally developed by Iowa State University and Outreach field agronomists along with weed scientists Bob Hartzler and Mike Owen for a series of face-to-face Weeds Week workshops. 
  • Course questions can be sent to weeds@iastate.edu.
  • This is a self-paced online course delivered on the Moodle e-learing platform. Internet access is required to view the course materials. There is a $50 registration fee. Those interested can register here.

Source URL: http://www.aep.iastate.edu/weeds/