Herbicide Resistance Action Committee (HRAC)
Herbicides are the primary economic means to control weeds and the evolution of herbicide resistant weeds is a problem in modern agriculture. Herbicide resistance is the evolved capacity of a susceptible weed population to withstand a herbicide application and complete its lifecycle when the herbicide is used at normal rates in an agricultural situation. The Herbicide Resistance Action Committee (HRAC) is an international body founded by the agrochemical industry to supporting a cooperative approach to the management of herbicide resistance.
Resistance Management Strategies
Resistance often becomes a problem because of high selection pressure exerted on a weed population over several years. This may be a result of repeated use of the same herbicide, or several herbicides with the same site of action and is often associated with crop monoculture as well as reduced cultivation practices. Therefore the key to resistance management is to reduce selection pressure by using a combination of the following techniques:
Mixtures or sequences of herbicides with differing sites of action are important especially to prevent or overcome resistance based on target site differences. To be effective the herbicides used in mixtures or sequences must have similar efficacy against the target weed. If the resistance is based on enhanced metabolism, this technique may also be useful, as the metabolic processes may be specific to certain types of molecule, but an empirical approach is needed to determine the best herbicide combinations.
Crop rotations may allow different herbicides or cultivation techniques to be used and may also provide different competitive environments to shift the weed flora. Set-aside programmes also allow new opportunities to manage populations of resistant weeds.
Cultivation practices may be adjusted if this fits to general agronomic needs. Measures such as stale seedbeds, ploughing or stubble burning (where permitted) can be very effective in reducing weed populations. In some husbandry systems, the grazing off of weeds (including the resistant ones) by sheep or cattle may be possible. In other cropping systems it is possible to use mechanical methods of weed control.
Economic control levels should be the aim, not higher cosmetic levels which increase selection pressure without providing a financial return to the farmer.
Generally, the best approach to resistance management is Integrated Weed Management. This means to utilise all available control methods in an economic and sustainable manner.