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Issue No. 7, Article 10/May 12, 2006

How Shalt Thou Rotate?

The first lecture of many "Weed Control 101" courses outlines why weed control is so important to crop production; namely, weeds are controlled so that more of the finite resources needed for plant growth are available to the crop instead of being used by the weeds. Lectures that fill the remainder of the semester usually provide ideas for how to achieve the goal outlined in lecture 1. Numerous weed control tools are available to farmers, including chemical, mechanical, and cultural methods. Herbicides are perhaps the most integral part of weed management programs in Midwest agronomic crops. However, intense reliance on herbicides for weed control can result in additional problems with which farmers must contend. Of particular concern in recent years has been the continuous selection of various weed biotypes no longer controlled by previously effective herbicides. In other words, repeated use of herbicides that act in a similar manner within the target weed has resulted in the selection of weed biotypes that are resistant to these herbicides.

The development of herbicide resistance in weed populations can result in significant economic losses for farmers. Many weed control practitioners, however, frequently continue to use a successful herbicide program until it no longer provides acceptable weed control instead of proactively implementing herbicide resistance management strategies. Some have suggested that the greatest economic loss farmers encounter due to selection of herbicide-resistant biotypes occurs not in the years following the identification of the herbicide-resistant population but during the first year of poor weed control.

Extension specialists and professionals from private industry have proposed numerous management strategies to retard the selection for herbicide-resistant weed biotypes, including utilizing nonchemical weed management options (such as mechanical cultivation), crop scouting and rotation, herbicide tank mixtures, and rotation of herbicides that act in dissimilar fashions. If weed control practitioners elect to implement herbicide rotation or tank mixtures as a resistance management strategy, information is needed to identify which herbicides act in a similar or dissimilar manner.

In 2001, Extension weed scientists at the University of Illinois developed Utilizing Herbicide Site of Action to Combat Weed Resistance to Herbicides, a bulletin establishing a color-coded herbicide site of action classification system based on 14 sites of action. The publication was intended to enhance the ability of weed control practitioners to rotate herbicides based on site of action (not mode of action) to slow further development of herbicide-resistant weed populations. We are pleased to announce a revised version.

The overall design of the publication has changed little, but important modifications and updates have been made. The front page outlines the importance of using a site-of-action classification for herbicide resistance management rather than a mode-of-action classification. The large inner table has been expanded to separate herbicide sites of action into 16 families, each in a "primary" color. You might notice some changes in the primary colors from the first edition of the publication, as well as some changes to the text descriptions of some site-of-action families. Herbicide chemical families sharing a particular site of action are coded in shades of the respective primary color for site-of-action family. A new column, "WSSA Group," provides a numeric representation for each site-of-action family that corresponds to the numerical grouping established by the Weed Science Society of America. The bulletin also gives common and trade names of herbicides commonly used in agronomic production systems in the Midwest, and the back page provides an updated list of corn and soybean herbicide premixes, with individual premix components coded with the appropriate colors based on their respective sites of action.

Individual copies of this publication are $2, with packages of 25 copies available for $30. You will soon be able to download an order form from the University of Illinois Weed Science Web page (Adobe PDF), or call (217)333-4424 with your ordering information.--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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