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When Herbicides Don’t Work

You have probably heard about antibiotic resistance in humans.  Particularly with antibiotics that were used for many years, surviving bacteria adapt and develop a resistance to the antibiotic.  (CDC on antibiotic resistance)  The same thing can happen with herbicides and weeds. The first herbicide resistant weed was reported in1957 against 2,4-D in Hawaii. Today the problem is increasing exponentially with over 300 resistant species worldwide. Of those resistant weeds, 129 are in the United States.

How does this happen? Herbicides typically target a single biochemical process in a weed such as an enzyme or protein that is essential for plant growth and development.  A class of herbicides will usually target the same biochemical process.  A resistant weed has a mutation in this process. Research has shown that even before herbicide exposure there are a few resistant plants are in a weed population.  Application of the herbicide selects for the plants with this mutation. Normal plants will be killed off and plants with the mutation will survive to reproduce. Each class of herbicides usually targets the same process. So if the plant is resistant to one herbicide in a class, it is often resistant to other herbicides in that class. Resistance to multiple classes of herbicides is not as prevalent.

What can be done about it? First only use herbicides when absolutely necessary.  Some of the worst herbicide resistance problems occur where there is a total weed control program such as along railroads and highways. Manually remove weeds by hand or by cultivation or by mulching. When you do use herbicides, rotate between different classes of herbicides. In a farming situation, rotate between crops with different life cycles. Use herbicide mixes where the herbicides target different processes.

Herbicides can be a useful tool but they are only one of many tools you should be using to control weeds. If you are only using herbicides to control weeds, especially a single herbicide, resistance will develop more rapidly. In other words, eventually you will make your problem. Think through your options. Contact your county extension office for more information.

For more information, check the International Survey of Herbicide Resistant Weeds on  the Weed Science Society of America web site. It has a wonderful database and several graphs of herbicide resistant weeds.  There is also a list of resources on the subject. In the United States, your county extension office probably has information on the subject.  While it can seem like an easy option, it isn’t always the best option.

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