Herbicide resistance is the inability of an herbicide to control a weed species that it was previously effective against. Weed scientists will often tell growers that a means for managing resistance is to rotate herbicide chemistries such that you are rotating among mechanisms of action. Sometimes (and I am guilty of this, too) we neglect to describe what a mechanism of action actually is…and why it is so darn important to us. So, back to the basics.
What is an herbicide? An herbicide is defined as a chemical substance that is used to eliminate unwanted plants. This is a very general description and it is important to remember that herbicides differ with respect to their use (for example, pre-emergence or post-emergence), activity (for example, contact or systemic), selectivity (for example, grasses or broadleaves), and their mechanism of action (also known as: mode of action, site of action).
What is a mechanism of action? The mechanism of action (MOA) is the way the herbicide controls susceptible plants. More specifically, it describes the biological processes that are disrupted by the herbicide. These biochemical pathways control the growth and development of plants; when herbicides are applied, these processes cannot be carried out and plant injury and death will occur. For example:glutamine synthetase inhibitors inhibit the activity of glutamine synthetase, the enzyme that converts glutamate and ammonia to glutamine. This leads to an accumulation of ammonia in the plant; ammonia can destroy cells and interfere with photosynthesis, among other things.
Where can I find descriptions of the MOAs? Check out the Weed Science Society of America’s (http://www.wssa.net/Weeds/Resistance/WSSA-Mechanism-of-Action.pdf) and the Herbicide Resistance Action Committee’s (http://www.wssa.net/Weeds/Resistance/HerbicideMOAClassification.pdf) (http://www.hracglobal.com/Publications/ClassificationofHerbicideSiteofAction/tabid/222/Default.aspx) websites.
What does MOA have to do with herbicide resistance? The over-reliance (across acres and time) on one MOA for weed control in an agricultural system can increase the probability of selecting for an herbicide-resistant population. With repeated applications, susceptible individuals of a target weed species will die off while the numbers of resistant plants will continue to grow. With time, the MOA will no longer control that species in that location. The chances of the population reverting back to a susceptible state are low. Several MOAs are comprised of multiple chemical families that are alike with respect to chemical structure and cause similar injury symptoms. Rotating between chemical families within an MOA is not the same as rotating among MOAs.
Where can I find out information about a specific herbicide’s MOA? This information is sometimes, but not always, found on the herbicide label. Some manufacturers will directly specify which WSSA group the herbicide belongs to (hypothetical example: Group 9, Glycines), while others will verbally describe the herbicide’s MOA in the product description section of the label (hypothetical example: this product inhibits the enzyme, EPSP synthase). If you are unsure …ASK!
So, if I just rotate my MOAs I’ll be okay, right? If only it were that easy. Rotating chemicals is a start, but there is more to be done than just that. When/if possible: rotate/alternate crops, use certified seed, cultivate, hand-weed, mulch or inter-crop, prevent weeds from going to seed in fields and orchards, prevent weed seed from being dispersed on farm equipment, etc…And remember to scout. Evaluate weed populations both BEFORE and AFTER weed control strategies are employed; this will allow you to detect potentially resistant populations early and manage them most effectively.
For more detailed information on the subject of herbicide resistance and resistance prevention and management in California, check out this publication by authored by Timothy Prather, Joseph DiTomasso and Jodie Holt http://ucanr.org/freepubs/docs/8012.pdf.