Herbicide resistance

There are 3 major ways that resistance may arise in a weed population:

Novel or pre-existing resistance

In any weed population, there may be some plants that develop or contain a rare change in a gene (or genes) that enable them to survive the application of a particular herbicide that would normally kill the species.

Genetic variation may alter physiological traits that enable herbicide uptake, translocation (movement within the plant), and activation (when the herbicide takes effect) at the site of action. Alternatively, changes may influence the plant's ability to detoxify herbicides, or enable transport to a site in the plant where the herbicide is not lethal. Each time the herbicide is applied, susceptible plants die and those with resistance survive.

Human-assisted spread 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 a particularly important mechanism for the spread of ‘rarer’ forms of resistance such as glyphosate resistance. 

Natural dispersal of resistance

Weed seeds that carry resistance genes can also be spread by wind and water. Floodwater has the potential to move a wide range of weed seeds over large distances. Pollen that carries resistance genes can be dispersed great distances on wind although the percentage remaining viable at distances greater than 10 m is low. 

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 in groups, based on their MOA, and this information 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 the herbicides. Groups 1 and 2 are high risk while the remainder are moderate risk.

MOA subgroup chemical classes

Within an herbicide MOA group, there may be multiple chemical classes that inhibit the same enzyme. Group 0 contains the herbicides that do not fit elsewhere.

Selection pressure

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.

Resistance mechanisms

Resistance mechanisms are 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 for the herbicide. The alteration occurs at the normal herbicide site of action in the plant and is in the form of either a structural or biochemical change. This means the herbicide will no longer be able to bind to its site of action, allowing the plant to survive herbicide treatments. 

Non-target site resistance

Non-target site resistance, which is also referred to as metabolic resistance, is used to describe mechanisms other than changes at the target site that enable an individual plant to survive an 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

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 being exposed to one of the herbicides. There are 2 types of cross-resistance: 

1. Across herbicide subgroups 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 1 'fops' (aryloxyphenoxy-propionates) may also be resistant to Group 1 ‘dims’ (cyclohaxanediones), even though they have not been exposed to an herbicide from the ‘dim’ subgroup. This is usually target-site resistance. 

1. Across herbicide MOA groups occurs when a weed population is resistant to herbicides from more than one MOA group. For example, a population of annual ryegrass selected only by Group 1 herbicides may become resistant to Group 1 and Group 2 herbicides. This is usually non-target site resistance. 

Multiple 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 to more than one mode of action (MOA).

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 specific herbicide. Weed populations are normally classified by testing services as resistant when more than 20% of the population survives the standard application rate of herbicide. 

There are several 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.

In-situ testing

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.

To conduct the test, herbicide should be applied to the crop with the weed infestation in test strips. Herbicide should be applied using rates appropriate to the current crop growth stage and weed size, plus a double rate – that is, herbicide applied at twice the recommended rate that would normally kill the weed. The test strips should only be applied if the weeds are stress free and actively growing. To accurately assess the level of control, conduct weed plant counts before and after application, and calculate green or dry plant weights.

Herbicide resistance seed tests

Seed tests require collection of suspect weed seed from the paddock at the end of the season and submitting it to a commercial testing service. 

There are 2 commercial seed testing services in Australia: 

1. Peter Boutsalis, Plant Science Consulting 

1. John Broster, Charles Sturt University, email: jbroster@csu.edu.au , or call +61 2 6933 4001 

Approximately 3000 seeds of each weed (an A4-sized envelope full of good seed heads) are required for a multiple resistance test. This equates to about one cup of annual ryegrass seed and 6 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 issue of seed dormancy and enabling a more rapid turnaround time. The tests are conducted during the growing season. A resistance status result for a weed sample is possible within 4 to 6 weeks.

For each herbicide to be tested, 50 plants are required. To reduce postage costs, plants can be trimmed to remove excess roots and shoots. On arrival at the testing service, plants are carefully trimmed to produce cuttings and transplanted into pots. After the appearance of new leaves (normally 5 to 7 days), plants are treated with herbicide in a spray cabinet.  

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 less critical to the testing procedure. Trimming the plants prior to herbicide application means herbicides are applied to actively growing leaves, mimicking chemical application to young seedlings. The Quick-Test has been used to test resistance in both grass and broadleaf weed species. Testing includes both known sensitive and resistant biotypes for comparison.

The only service provider for Quick-Tests is Peter Boutsalis, Plant Science Consulting .