Modes of Action - Recommendations - Prevention
The Herbicide Resistance Action Committee (HRAC) classifies herbicides into groups according to their specific biochemical modes of action, with a specific designation for each group. This classification enables the establishment of strategies for preventing the development of specific resistances to each mode of action. In 2020 this classification was revised, and is currently based on a numerical system.
Herbicides generally act by blocking functions that are essential to the development and survival of the target plant. In Switzerland, resistances to four biochemical modes of action have been observed: ACCase inhibitors (HRAC 1), ALS inhibitors (HRAC 2), Photosystem II-photosynthesis inhibitors (HRAC 5) and EPSP synthase inhibitors (HRAC 9).
Resistances can be of two types:
- Target-site resistance (TSR) is due to a mutation in a single gene in the herbicide target in the weed. This leads to increased expression of the target protein or a change in the herbicide binding site. There are molecular tests to detect this type of resistance.
- Non-target-site resistance (NTSR) includes all other mechanisms such as e.g. reduced herbicide penetration in the plant.
From generation to generation, the plants that survive treatment with the herbicide can produce seeds and occupy their ecological niche so completely that this resistance trait is found in almost all individuals of the species in an agricultural plot. The seeds of these plants as well as their pollen can transmit this resistance to neighbouring plots.
Single or multiple resistances
If a weed population is resistant to just one mode of action (or HRAC group) we are dealing with a single resistance. If the population is resistant to two or three different modes of action, we then speak of double and triple resistances.
To reduce the risks of developing herbicide resistance, several agricultural practices must be combined. The ideal prevention strategy hinges on good growing practices and appropriate use of herbicides. To increase their efficacy, these measures should be coordinated at regional level.
Agronomic recommendations
- A varied rotation with spring crops allows differentiated sowing dates and/or intercropping.
- It is very important to diversify weeding practices. In addition to mechanical weeding, tillage (ploughing, stubble ploughing, false seed-bed) helps to control weeds.
- Staggered sowing dates inhibit the emergence of weeds that are specific to the crop in question.
- Planting competitive varieties and increasing sowing density enables the crop to compete more successfully with the weeds.
- Mowing the weeds before heading prevents pollen or seed production and stops the spread of resistance.
- Prevent the spread of weed seed between fields and regions by using crop seed that is free from undesirable seeds, and ensure that farm machinery does not act as a vector between plots.
Chemical control recommendations
- The most important thing is to alternate herbicides with different modes of action (refer to the HRAC groups instead of the trade names) and/or use mixtures of herbicides with different modes of action.
- It is also essential to apply the manufacturer’s recommended dosage and use the herbicides at the optimum stage of weed susceptibility and during the best possible weather conditions.
- Chemical weed control must achieve a minimum efficacy rate of 95%.
- Monitor the relative efficacy of the herbicides based on past experience. This could give indications of the development of any new resistance in the treated plot.