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.
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.
Commonly used herbicide resistance terms
Herbicide mode of action groups
Herbicides act by targeting specific plant processes. This process-specific activity is termed 'mode of action' (MOA). In Australia all herbicides are classified into groups based on their MOA. MOA group classifications can be found on all herbicide labels, to identify the group to which a herbicide belongs.
MOA groups are ranked according to the risk of weed populations becoming resistant to those herbicides. Groups A and B are high risk while the remainder are moderate risk.
MOA subgroup chemical classes
Within a herbicide MOA group there may be two or more subgroups. Subgroups are usually based on the different chemical classes that inhibit the same enzyme. Group Z contains those herbicides that do not fit elsewhere.
Selection pressure is a term used to describe the amount of selection for resistance applied by the herbicide application. Every time herbicide is used, susceptible individuals are killed and resistant individuals survive. The greater the number of susceptible individuals killed by the herbicide, the higher the selection pressure.
This term is used to describe the specific processes that enable the plant to survive an application of herbicide. Resistant populations of weeds may have either 'target site' or 'non-target site' resistance.
Target site resistance
Target site resistance occurs when there is an alteration at the target site. The alteration occurs at the normal herbicide site of action within the plant and is in the form of either a structural or biochemical change. This means that the herbicide will no longer be able to bind to its site of action, allowing the plant to survive the herbicide treatments.
Non-target site resistance
Non-target site resistance (also referred to as metabolic resistance) is used to describe mechanisms other than changes at the target site which enable an individual plant to survive a herbicide application. The potential mechanisms include reduced herbicide uptake, reduced translocation, reduced herbicide activation, enhanced herbicide detoxification, changes in intra or inter-cellular compartmentalisation and enhanced repair of herbicide-induced damage.
Cross-resistance is defined as the ability of a weed population to express resistance to more than one herbicide. It may arise without the weed population ever being exposed to one of the herbicides. There are two types of cross-resistance:
- Across herbicide subgroups. This occurs when a weed population is resistant to more than one herbicide subgroup within a specific MOA group. For example, populations of wild oats that are resistant to group A 'fops' may also be resistant to group A ‘dims’, even though they have not been exposed to a herbicide from the ‘dim’ subgroup. This is usually target-site resistance.
- Across herbicide mode of action groups. This occurs when a weed population is resistant to herbicides from within more than one MOA group. For example, a population of annual ryegrass selected only by group A herbicides may become resistant to both group A and group B herbicides. This is usually non-target site resistance.
Multiple resistance is a term used to describe weed populations that exhibit more than one resistance mechanism, allowing the plant to withstand herbicides from different subgroups. Some populations of resistant annual ryegrass possess both target and non-target site resistance. For example, one population of annual ryegrass present in Australia exhibits confirmed resistance to five different herbicide MOA groups.
Partial resistance or developing resistance
Partial resistance and developing resistance are terms used to describe a situation where only a small proportion (often less than 20%) of the population survives the standard application rate of the herbicide in question. Weed populations are normally classified by testing services as resistant when more than 20% of the population survives the standard application rate of herbicide.
Options for herbicide resistance testing
There are a number of different methods of testing for herbicide resistance. Tests can be performed in situ (in the paddock during the growing season), on seed collected from the suspect area or by sending live plant samples to a testing service.
This can be conducted on-farm or by a commercial resistance testing service.
An in-situ test can be performed following herbicide failure in a paddock. The test should be done at the earliest opportunity, remembering that the weeds will be larger than when the initial herbicide was applied. Test strips should be applied using herbicide rates appropriate to the current crop growth stage and weed size, plus a double rate. The test strips should only be applied if the weeds are stress free and actively growing. To more accurately assess the level of control, conduct weed plant counts before and after application. Green or dry plant weights can be calculated for more accurate results.
Herbicide resistance seed tests
Seed tests require collection of suspect weed seed from the paddock at the end of the season. This seed is generally submitted to a commercial testing service.
There are two commercial seed testing services in Australia
Approximately 3000 seeds of each weed (an A4-sized envelope full of good seed heads) is required for a multiple resistance test. This equates to about one cup of annual ryegrass seed and six cups of wild radish pods.
Syngenta herbicide resistance Quick-Test™
The Syngenta herbicide resistance Quick-Test™ (QT) uses whole plants collected from a paddock rather than seeds, eliminating the problem of seed dormancy and enabling a far more rapid turnaround time. In addition, the tests are conducted during the growing season rather than out of season over the summer. A resistance status result for a weed sample is possible within four to six weeks. The QT, which was developed by Dr Peter Boutsalis while working for Syngenta in Switzerland, is patented in Australia.
For each herbicide to be tested, 50 plants are required. To reduce postage costs, plants can be trimmed to remove excess roots and shoots. Upon arrival at the testing service, plants are carefully trimmed to produce cuttings and transplanted into pots. After appearance of new leaves (normally 5-7 days), plants are treated with herbicide in a spray cabinet. The entire procedure, from paddock sampling to reporting results, takes between 4-6 weeks, depending on postage time and the herbicides being tested. Unlike paddock tests, the QT is performed under controlled conditions, so it is not affected by adverse weather conditions. The age of the plants is also less critical to the testing procedure. Trimming the plants prior to herbicide application means that herbicides are applied to actively growing leaves, thus mimicking chemical application to young seedlings. The Quick-Test™ has been used to test resistance in both grass and broadleaf weed species. During testing, both known sensitive and resistant biotypes are included for comparison.
Quick-Tests can be done with Peter Boutsalis, Plant Science Consulting.