Russell R. Hahn, Department of Crop and Soil Sciences, Cornell University
According to the International Survey of Herbicide Resistant Weeds (http://www.weedscience.org), 372 resistant weed biotypes representing 116 dicot (broadleaf) and 85 monocot (grass and sedge) species have been confirmed as herbicide resistant as of February 1, 2012. Historically, there was great concern about triazine resistant (Photosystem II inhibitors – Group 5 herbicides) weed biotypes, which now number 69 (Table 1). In recent years, concern has shifted to ALS (Acetolactate Synthase inhibitors – Group 2 herbicides) and to glyphosate (EPSP Synthase inhibitors – Group 9 herbicides) resistant weeds. In the case of ALS resistance, this heightened concern is due to the rapid increase in the number of weed biotypes (116) that are resistant to these Group 2 herbicides and due to the fact that there are numerous
herbicides with this mechanism of action that are used on multiple crops. Finally, although there are only 21confirmed cases of glyphosate resistant weeds, the widespread adoption of glyphosate resistant crops and the increased used of glyphosate herbicide has placed a spotlight on weed populations resistant to this mechanism of action.
The continued increase in the number of herbicide resistant weeds along with the fact that no new herbicide mechanisms of action have been introduced in recent years prompted the Weed Science Society of America (WSSA) to develop a lesson module on herbicide resistant weeds. The intended audiences for these lessons are ag professionals, including Certified Crop Advisors, extension educators, dealers, custom applicators, and others who interact with or advise growers on weed management practices. The lessons can be accessed on the WSSA website at http://www.wssa.net under “WSSA News”. The modules are available for download as PowerPoint slides or as a Flash File. According to the Training Module Agreement, “WSSA grants you a limited license to use these materials for training and educational purposes. Slides may be used individually, and their order of use may be changed; however, the content of each slide and the associated narrative may not be altered.” A brief description of the five lessons in the WSSA Lesson Module on Herbicide Resistant Weeds follows:
Lesson 1 – Current Status of Herbicide Resistance in Weeds, helps understand the need for herbicide resistance management and provides information on the status of herbicide resistance by mechanism of action, on the increase in weeds with resistance to multiple mechanisms of action, and on the global distribution of herbicide resistance.
Lesson 2 – How Herbicides Work, provides information on terminology associated with herbicide use along with an understanding of how herbicides are grouped according to their mechanism of action.
Lesson 3 – What is Herbicide Resistance? A definition of herbicide resistance, an understanding of the different types of resistance, and information on how resistant weed populations develop are included in this lesson.
Lesson 4 – Scouting After a Herbicide Application and Confirming Herbicide Resistance, provides information on the importance of scouting for herbicide resistant weeds, on reasons why weeds can be present after herbicide application, on knowing how to identify herbicide resistance in the field, and on procedures for confirming resistance in the field or greenhouse.
Lesson 5 – Principles of Managing Herbicide Resistance, provides an understanding that diversity is an important concept in resistance management, identifies broad strategies and specific tactics for managing herbicide resistance, and compares the value of proactive and reactive management practices.
Although New York State has been fortunate to have limited problems with herbicide resistant weeds, these lessons provide an in-depth understanding of this growing problem and information on how farmers can avoid or deal with herbicide resistance.
