What is herbicide resistance?
It is important to differentiate between herbicide resistance and herbicide tolerance.
Herbicide resistance is the inherited ability of an individual plant to survive a herbicide application that would kill a normal population of the same species. Whereas, herbicide tolerance is the inherent ability of a species to survive and reproduce after herbicide treatment at a normal use rate. There is no selection involved (through herbicide application) because the species is naturally tolerant.
Over 25 weed species in Australia currently have populations that are resistant to at least one herbicide 'mode of action' (MOA) group.
Herbicide resistance is normally present at very low frequencies in weed populations before the herbicide is first applied. Variation exists within every population, with some individuals having the ability to survive the herbicide application.
A weed population is defined as resistant when a herbicide that once controlled the population is no longer effective (sometimes an arbitrary figure of 20% survival is used). The proportion of herbicide resistant individuals will rise due to selection pressure in situations where one herbicide MOA group is applied repeatedly.
Herbicide resistance is permanent in weeds and their progeny with dominant 'target site' resistance. With cessation of the use of that herbicide MOA group, the ratio of dominant target site resistant to susceptible individuals will remain the same – only the total number of weeds present can be reduced. Weeds with this type of resistance do not exhibit a fitness penalty.