Weed resistance is becoming an issue across much of Western Canada. The list of herbicide resistant weeds grows every year. New resistant weeds are being discovered, and the resistant weeds are spreading throughout the prairies.
Resistance develops over time and is more likely to occur when herbicides with the same active group are used repeatedly on the same target weeds in the field.
In general, weed populations have varying levels of sensitivity to herbicides. When a herbicide is applied and the target weed species is not killed, these weeds will continue to grow. These escapes will mature and reproduce, creating offspring that also have limited sensitivity to that herbicide group. If the same herbicide or a herbicide in the same group is used in the following growing season the process will repeat itself and so on. Soon the weed population is such that the resistant weeds will dominate the population. The herbicide group will no longer have effective control in that field.
Some management practices that growers can use to help minimize their risks of developing herbicide resistance include;
- Correctly identify the weeds present in your fields and their growth stage. It is easier and more effective to select an appropriate herbicide if the weeds are identified correctly and if the growth stage of the weed is known.
- Keep the fields as clean as possible. A pre-seed burn off in the spring will help control early season weeds. Most herbicides have a higher efficacy on weeds that are in the earlier stages of growth, this is true for both pre-seed and in-crop herbicide applications.
- Herbicides should be applied at the recommended rates, cutting the rate will provide less efficacy on the target weeds.
- When selecting new seed, ensure it is cleaned and from a reputable source.
- Use a longer crop rotation (ideally 3-4 years). A short rotation often means that herbicides of the same group will be used every 2 years. If a short rotation is being used because of economics, be sure to rotate the herbicide groups that are being used, and that the pre-seed herbicide groups are different from the in-crop herbicide groups
- Add another component to the glyphosate for pre-seed herbicide application. This additional component should be from a herbicide group that will not be used for in-crop applications. This will give a second or even third mode of action to the herbicide mix acting on the early season weeds.
- Using herbicides that have multiple modes of action , this will allow the weeds that are less sensitive to a particular herbicide group to still be acted upon by the other mode of action.
- When purchasing land, attempt to gain as much information on those fields as possible.
- Have a resistance test done on any suspect weeds. Early identification is beneficial in controlling the problem immediately and in years to come.
Weed resistance can be managed with careful planning of a complete crop rotation and specific herbicide selection. Often more detailed attention is needed when selecting herbicides in order to have effective control of problem weeds. It often means that herbicides from particular groups are only used minimally or even not at all. If you have resistant weeds, suspect that you have resistant weeds, or if you require assistance formulating a crop rotation or herbicide plan, talk to your local Cavalier Agrow Agronomist.