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What To Do When The Weeds Don’t Die

Living, growing weeds in a field you’ve already sprayed is not something you ever want to see. More and more these days, weeds that survive spraying are not the result of misses or poor product but because of resistance to a growing list of products. Classifying a weed as truly herbicide resistant isn’t as cut and dried as it surviving one spray, however if the weed truly is herbicide resistant the question becomes what to do about now and in years to come.


The first and most important step to controlling resistant weeds is to first positively and accurately determine if the weeds in your field are herbicide resistant. It could be that the weeds are more “tolerant” to the chemical or formulation being used, or that survival was more a result of conditions than actual resistance.

The first thing to do is to call the representative from the chemical company. This way they can come out to the field and have a discussion with you about the weeds in question as well as take a sample of the weeds if need be. There are several factors that can inhibit a herbicide from properly working against certain weeds. These include the actual application, soil conditions, climate and the weeds themselves.

If the representative thinks that there may be a chance of actual herbicide resistance there are some signs that they will look for. If all other targeted weeds have been controlled very well, and only one species is left there is a chance of resistance. If only some of a particular weed type die, that can also be a sign of resistance developing in the weed population. That said, it’s important to determine if the weeds (of the same type) were all at the same stage when sprayed; partial survival of the population could be because of herbicide efficacy differences on weed stage rather than the weeds.

Field history of that particular species and chemical used can tell the story of a resistant weed. Suspect resistance if the same method and chemical has been used for years. Take note of what has been reported in your area; other confirmed cases of resistant weeds increases the likelihood that escapes in your field are also resistant.


When all signs indicate herbicide resistance, it’s time to take seed and plant samples to the lab for testing. If seeds are not available, then whole plant samples will have to be sent in. This is typically done by the chemical company.

There are a couple of different types of lab tests that can be used to test herbicide resistance. One of the most commonly used is a called a plant pot assay. This is when many offspring from the collected weeds are grown in a lab environment. When the weed reaches a certain stage, the pots are sprayed with a single dose or variety of doses of the chemical in question. Plants are then watched for plant mortality and the vigour effects of the chemical on the weeds. It’s important that when these tests occur there is a known susceptible weed species grown with the resistant ones.

Another type of lab test that is used to determine resistance is known as dose response experiments. In this test a lab will give a range of doses to known susceptible and suspected resistant weeds. By doing this, a lab can determine a more exact amount for dose and the effect it has on both types of weed species. The lab takes this information and graphs it creating a resistance index giving a numerical value to the weed and its resistance to a certain chemical at specific dose amounts.

A quicker and more effective test is a single dose assay. This is just when one dose is given to the set of control weeds and the effects are monitored. A single dose assay is quicker and more effective because only a single dose needs to be monitored and fewer weeds are needed for this type of test. This also works very well in cases where the outcome is believed to be absolute.

There are other types of tests used on weeds gathered in the field. These include: petri dish assays, chlorophyll fluorescence tests and enzyme sensitivity assays. Different tests come with different pros and cons, however in the end any of these done by a proper lab can give a positive or negative result on the resistance to a herbicide.


Herbicide resistant weeds can be difficult to control within the year of discovery, because once suspected weeds are tested and confirmed resistant, it’s too late to do too much about it. There may be time late in the season to spray that field with a strong dose of a non selective herbicide that will kill everything in the field.

The first line of attack will be in the following spring. The goal must be to first eliminate the resistant weeds, but reducing the risk of more weeds become resistant should also be part of the plan. It’s important to flush those resistant weeds with a strong dose of non selective herbicide and get rid of them, however neighbouring fields may also be infested with resistant weeds. Plan to treat all surrounding land as if it also has that resistant weed. This means that in the spring it will be important to get the best burn off job as possible. It also means changing the chemistry of the chemical you were using in-crop –and that means changing mode of action, not just using another chemical name from the same herbicide group.

There are some other chemical tips that can help a farmer from developing resistant weeds. It is important to not use the same chemical in application on the same field year after year. It’s also important to not apply a chemical with the same mode of action more than once in a year. A farmer can also use a tank mix of chemicals that have different modes of action but target the same weeds. The use of non selective herbicides for burn off cannot be underestimated, either.

For more information on this, visit’s a great website with lots of information and links.

Jay Peterson farms near Frontier, Sask.

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Jay Peterson farms near Frontier, Sask.

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