Herbicide Resistance in Weeds - Frequently Asked Questions

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 What is herbicide resistance?
Herbicide resistance occurs when a previously controlled weed species is no longer killed with an application of the same herbicide, even at elevated rates. Resistance can occur in other organisms with other pesticide products such as fungicides and insecticides.

Why does weed resistance develop?
There is genetic variability in all weed species. Therefore a population of weeds contain plants with varying sensitivities to particular herbicides. When you frequently seed the same crop and spray with the same herbicide, you quickly select weeds with greater tolerance to the herbicide. Short or limited crop rotations and/or reliance on one particular herbicide group aids in developing herbicide resistance. It is not just repeatedly applying a particular herbicide brand; this applies to all herbicides using that particular mode of action. Repeated applications of the same herbicide, mode of action, creates selection pressure that favours plants with the resistant genes over other weed biotypes. Within a few generations, the majority of those weeds will have the resistant genes. This is why it is so important to know the mode of action of the herbicides you use. See Herbicide Group Classification By Mode of Action for more information.

How do you identify herbicide resistance?
When checking fields after spraying, several telltale signs indicate possible herbicide resistance development. If all other reasons to explain the lack of weed control are ruled out, then weed resistance is the most likely culprit. Some telltale signs are the following:
· Are other weed species listed on the product label controlled satisfactorily? It is unlikely for multiple weed species to develop resistance at the same time.
· Is the herbicide failure patchy with no reasonable explanation?
· Do field records show the same herbicide or herbicide group being used on this land recently and was any reduced effectiveness on a particular weed species noted in this area of the field in the previous year?
· Do individual plants of a specific weed species show herbicide injury symptoms next to other weed of the same species, with no visible symptoms?
· Do field histories show frequent use of the same herbicide or herbicide group year after year?
· Do herbicide resistant weeds already exist in your farming area?
If you suspect weed resistance, collect and send weed seed samples to a lab for testing and confirmation.

How bad is the problem?
As an example, weed surveys, conducted every 5 – 10 years, are finding an alarming increase in the percent of fields in Alberta with herbicide resistant wild oats. The majority of our wild oat herbicides are in the group 1 mode of action. In 2001, 11% of surveyed fields had group 1 resistant wild oat. By 2007 the percentage increased to 39% of surveyed fields. Currently, over half the fields in Alberta are suspected to have group 1 resistant wild oats. There are also populations of Group 2 and group 8 resistant wild oats in Alberta. What’s left? Almost all grassy weed herbicides are Group 1, 2 or 8. Glyphosate (group 9) is effective but overuse will select for resistance to it too. We don’t have an unlimited number of herbicides effective on wild oats, or for that matter, on any other weed species.

How can you prevent it?
Weed resistance can be avoided or at least delayed by utilizing the following practices.
· Rotate herbicide groups and crops. Using different herbicide modes of action on a problem weed prevents selecting for the resistant biotypes. Using different crop types provides different competition levels to the weed from the crops. Crop rotation also changes crop- weed interactions and weed control systems, preventing the buildup of resistant weeds.
· Keep and use detailed field crop histories to prevent a buildup of resistant weeds by stopping the overuse of one group of herbicides.
· Using a herbicide mixture with more than one mode of action on problem weeds can help delay the onset of resistance.
· Apply an integrated weed management approach to weed control. This includes the use of herbicides but also crop rotation, use of perennial crops, competitive cropping, higher seeding rates, mechanical control, biological control, adjusted seeding dates and other tools.
· Measure results and prevent the movement of resistant weed seeds to other parts of the farm or elsewhere through equipment sanitation.
· Use herbicides on weed problems if they exceed the economic threshold for the crop.
· Limit the use of herbicides with long soil residual time. Residual herbicides create enormous selection pressure over extended periods for the resistant plants.

Why should you care?
Herbicides are an important tool to control weeds. However, sole reliance of this tool for weed control can decrease or even eliminate its effectiveness. An integrated weed management plan ensures herbicides remain effective on problem weeds well into the future.

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For more information about the content of this document, contact Harry Brook.
This document is maintained by Brenda McLellan.
This information published to the web on May 2, 2014.