By Kim Stonehouse, PAg, Regional Crops Specialist, Tisdale
For some time now, producers have been aware of the potential for weeds to develop resistance to specific groups of herbicides. 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 resistance to other herbicide groups such as cleavers, kochia and wild mustard with Group 2 herbicides. In addition, there is now evidence that there are biotypes of each of these weeds that have developed resistance to multiple modes of action.
Resistance develops as a result of repeated use of the same herbicide groups over extended periods. There may be a small number of naturally occurring plants in the initial population that have some resistance, generally showing up as small patches of weeds that were not controlled by herbicide application. As these small patches go unnoticed, the size of this resistant population will increase over time with continued use of the same non-controlling herbicide group. Once a weed species becomes resistant to a particular herbicide group, it is resistant to that group forever. There is no going back.
It is important to recognize that, of all the herbicides available, there are only 15 different groups or modes of action. As well, only seven 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 years due to the extreme cost associated with research and development. Because of this, it is more cost effective to deal with herbicide resistance through preventative practices.
A number of practices can be used to prevent the development of resistant populations. For example, increasing crop diversity by rotating three or more types of crops (such as cereals, oilseeds, pulses and forages) will reduce the risk of developing herbicide resistance over tight rotations.
Herbicide resistance risks can be further reduced by rotating groups of herbicides and using tank mixes of different groups for control of the same weeds.
Alternatively, when weeds highly prone to resistance are known to exist, herbicide layering may be a good option. Herbicide layering involves the use of multiple active ingredients and modes of action to control the same weed, in the same field, in the same year.
If it suspected that there are some patches of weeds that are herbicide resistant, producers should ensure they prevent those plants from setting seed by either herbicide or mechanical means. If seeds are already present, producers may wish to collect some seeds and have them tested to confirm these suspicions.
For more information on prevention or management of herbicide resistant weed populations, please contact Kim Stonehouse at 306-878-8807, the Agriculture Knowledge Centre at 1-866-457-2377 or visit our website.