It started out as a fairly routine farm call. A giant foxtail problem in corn and soybean fields and the "What do I do now?" question. The curious thing about these fields was that the corn had been sprayed with Accent and the soybeans had been sprayed with Raptor [both products having an amino acid synthesis (ALS) inhibitor mode of action to kill weeds]. Both were timely applications and it didn’t appear that the problem was sprayer related. As we walked the first corn field, there were small patches of foxtail throughout the field. What didn’t make sense was that this field had been rotated into alfalfa over past years ----- not a situation where you would expect a possible herbicide resistance problem. We continued to walk. Another corn field --- same problem, but again a field that was in an alfalfa rotation. The journey continued to the "low" ground. We looked at a soybean field loaded with foxtail. Next, we viewed an adjacent corn field. The grower explained that this field had been corn "forever". Some years it would flood-out. A typical low marsh field that is present on many Fond du Lac County farms. The herbicide history on this field was "Accent since it came out". We had struck the mother load. Foxtail samples were sent to the University of Wisconsin Agronomy Department to test for herbicide resistance. The results were as expected. These plants survived a 32X rate of Accent and an 8X rate of Pursuit. This producer’s farm was virtually covered with ALS-resistant foxtail that will likely be present for many years.
How does herbicide resistance develop? Herbicide resistance involves a shift in weed biotypes within a particular species. A biotype is an individual within a species that has characteristics not common to the population as a whole. In this case, the selection was for giant foxtail plants with the ability to survive an application of ALS herbicide. Over a period of five or six years, repeated use of a chemical with the same mode of action effectively turned the minority population into a majority population. The development of herbicide resistant weeds is not new. The first case of herbicide resistance was documented in 1957. As of 1998, the International Survey of Herbicide Resistant Weeds Internet site reports 218 resistant biotypes worldwide. There are likely two more known cases of ALS resistant foxtail in Fond du Lac County but confirmation tests have not been performed. Should I quit using ALS herbicides? ABSOLUTELY NOT! ALS inhibitor herbicides represent a high percentage of products currently being marketed. Taking steps to avoid herbicide resistance insures that these products will be effective weed control tools for many years to come. However, if you’ve been using ALS herbicides on the same field for several years, it’s time to rotate your herbicide program to a product with a different mode of action. This doesn’t mean you can’t return to an ALS inhibitor product in future years. If you have a field(s) in a situation where ALS herbicides have been applied for four or five years, take time this winter to sit down with your ag chemical dealer and explore some different herbicide options for 2000. A list of common ALS herbicide products currently being marketed is presented in Table 1.
How do I avoid herbicide resistance? Aside from rotating herbicide modes of action, there are several things producers can do to help prevent herbicide resistant weeds from taking hold. They are as follows:
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