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.