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Herbicide resistant weeds

The 2017 planting season will soon be upon us. Producers are making final decisions about the types of crops to be grown this year.

The 2017 planting season will soon be upon us. Producers are making final decisions about the types of crops to be grown this year. Although spraying season is a little further away, this is also a good time to think about the weed control measures that can be used to prevent the build up of herbicide resistant weeds.

For some time now producers have been aware of the potential for weeds to develop resistance to specific herbicide groups/modes of action.  The first documented cases in western Canada were wild oat and green foxtail that exhibited resistance to Group 1 herbicides.  More recently we have seen the development of many more resistant weeds to other herbicide groups such as wild oat, kochia and wild mustard with Group 2 herbicides.

Resistance develops as a result of repeated use of the same herbicide groups over extended periods. There may only be one plant in the initial population that has resistance. This plant will increase with repeated use of herbicides of the same group and after several years show up as a small patch of plants that were not controlled by herbicide application.  By the time it is visible as a patch in the field, it could be a little as three years before the whole field is infested.

It is important to recognize that, of all the herbicides available, there are only 30 groups or modes of action and only 18 of these are available in Saskatchewan (many are used on crops we don’t grow here). As well, roughly 8 of these modes of action dominate the majority of applications made by Saskatchewan crop producers. Breakthroughs with new modes of action have been few and far between in recent decades due to the lack of additional metabolic pathways that can be interrupted in a plant to result in its death. Because of this, producers have few options other than to deal with herbicide resistance through preventative practices.

There are a number of practices that can be used to prevent the development of resistant populations. For example; increasing crop diversity by rotating 3 or more types of crops (such as: cereals, oilseeds and pulses) will reduce the risk of developing herbicide resistance over less diversified rotations.

As well, tank mixes of different groups for control of the same weeds can reduce herbicide resistance risks. While rotation through various modes of action or groups of herbicides has been promoted as a way to slow resistance from developing, recent research has found that resistance evolution will continue under rotation strategies, just at a much slower pace.

If you suspect that you have some patches that are resistant you should ensure that you prevent those plants from setting seed by either herbicide or mechanical means.  Mark these patches since weed seed dormancy will mean that they will be back in the future.  You may also wish to collect some seeds from those areas and have them tested to confirm your suspicions.  

For more information on management of herbicide resistant weed populations, please contact your local Saskatchewan Agriculture regional office.

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