How does a weed become resistant to a herbicide?
There are three major ways in which resistance may arise within a weed population:
- Pre-existing resistance: Within any weed population there may be some plants that already contain a rare change in a gene (or genes) that enable them to survive the application of a particular herbicide that would normally kill this species. Genetic variation may alter physiological traits that enable herbicide uptake, translocation and activation at the site of action. Alternatively, changes may influence the plant's ability to detoxify herbicides, or enable transport to a site within the plant where the herbicide is not lethal. Each time the herbicide is applied, susceptible plants die and those with resistance survive.
- Importation of resistance: It is possible that resistance may not be present in the population initially, but is introduced as a weed contaminant in crop seed or fodder, on machinery or on/in animals. This is particularly important for ‘rarer’ forms of resistance such as glyphosate resistance.
- Natural dispersal: Weed seeds can also be spread by wind and water. Pollen can also be dispersed great distances although the percentage remaining viable at distances greater than 10m is low. Floodwater also has the potential to move a wide range of weed seeds over large distances.