How to Manage Herbicide Resistant Weeds

The rate at which resistant weed populations appear occurs exponentially (Figure 7). It is not a gradual one that you can see. Once you do see it, it is already too late to prevent. The first observation is usually weed control failure, such as a large number of surviving plants (escapes) found in the middle of a group of dead plants). To illustrate, if the field is left uncontrolled after the initial appearance of R-biotypes, the ratio of R-biotypes to S-biotypes can increase the following year. When the same single mode of action herbicide program is used again, R-biotypes will survive, produce seeds, and increase population size in the future. In the third year, the percentage of R-biotypes that grow and produce seeds can increase again. The longer the field is treated with the same single mode of action program, the greater the number of R-biotypes in the field. The problem continues to grow and the herbicide continues to remain ineffective on the R-biotype. With each passing year, as number of seed from resistant weeds builds up in the seedbank the percentage of R-biotypes in the field becomes greater. If the single mode of action program continues, a field can contain enough R-biotypes to result in weed control failures.

Sometimes when a weed control failure is observed, the presence of a herbicide resistant weed population is not always to blame. The following conditions could also result in reduced weed control:

1. Unfavorable environmental conditions at the time of application.
2. Inadequate spray coverage (spray skips).
3. Large weeds at the time of application.
4. Weeds that emerged after application.

The best solution for the problem of weed resistance is prevention in the first place. The key to resistance management is to reduce the intensity of selection pressure by using a combination of the following techniques:

1. Scout fields on a regular basis throughout the growing season to identify weeds and observe escapes. Use a decision-support tool, such as WeedSOFT, to determine whether a weed control practice is necessary based on economics, not for cosmetic or aesthetic reasons.
2. Rotate herbicides with different modes of action.
3. Apply herbicides in tank-mixed, pre-packaged or sequential mixtures that include multiple modes of action.
4. Combine mechanical control practices such as rotary hoeing and cultivation with herbicide treatments whenever possible.
5. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those fields that are not infested.
6. Attempt to keep total weed population numbers down, especially post-harvest and in fallow years.
7. Use crop rotations that allow for different herbicide and/or cultivation techniques and that may also provide different competitive environments.

An effective weed control program is the key here, however the repeated use of any herbicide with the same mode of action over a number of years can select for naturally resistant weed biotypes. Growers will often use the same herbicide year after year because they have been effective, economical, and environmentally sound, even though they may not control a particular R-biotype. Over time, naturally occurring biotypes that are resistant to the herbicide increase in number because the herbicide’s mode of action does not control the biotype, and an effective weed management program has not been used.