Weed resistance to herbicides and how to manage it
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Ελληνικά (Greek)
How herbicide-resistant weeds occur – The mechanisms behind herbicide resistance
Herbicide resistance (HR) is “the selected, heritable ability of some weed biotypes to survive the application of the recommended dose of a herbicide to which the original population of the weed was sensitive.”
HR is divided into:
- simple resistance,
- cross-resistance and
- multiple resistance
Simple HR is “the resistance of a weed to a single herbicide with a specific mode of action”.
Cross-resistance is the “resistance of a weed to more than one herbicide belonging to the same or different chemical groups with the same mode of action or metabolism”.
Multiple HR is “the resistance of a weed to more than one herbicide belonging to chemical groups with different modes of action or metabolism”.
Resistance mechanisms may or may not be due to a modification in the site of action of the herbicide:
- TSR (Target site Resistance) or
- NTSR (Non-Target site Resistance)
First, we should remember that herbicides bind to certain proteins produced in plant cells to impair certain physiological functions of the target weed (e.g., photosynthesis), ultimately leading to its necrosis.
In TSR mechanisms, a mutation occurs in the DNA of the resistant weed, leading to a change in the protein to which the herbicide would normally bind. Since the herbicide cannot bind, it cannot exert its phytotoxic effect.
As for NTSR mechanisms, resistance may be due to a reduction in the herbicide’s absorption rate and its movement in the plant tissue or the way it is distributed in the plant cells. Some species also achieve the isolation or metabolism of herbicides by degrading their molecules.
The increasing global problem of herbicide-resistant weeds
In 2023, 523 cases of HR are reported worldwide:
- 269 resistant weed species (154 grasses and 115 broadleaf weeds),
- in 99 crops
- founds in 72 countries
- concerning 21 of 31 herbicide modes of action, and 167 different herbicides
In Greece, HR (in some cases multiple resistance) is mainly reported in populations of rigid ryegrass (Lolium rigidum), sterile oat (Avena sterilis), hairy fleabane (Conyza bonariensis), corn poppy (Papaver rhoeas), etc.
Picture 1. Herbicide-resistant population of hairy fleabane (Conyza bonariensis) (on the left) and its control with an alternative herbicide (on the right) in a vineyard in Central Greece.
Picture 2. Herbicide-resistant population of rigid ryegrass (Lolium rigidum) (on the left) and its control with an alternative herbicide (on the right) in a citrus orchard in Western Greece.
How to prevent and manage herbicide resistance
The prevention and management of HR is based on Integrated Weed Management (IWM) practices.
Important preventive measures to reduce the spread of herbicide-resistant weeds are the use of certified seedlots that are free of weed seeds or rhizomes and the thorough cleaning of agricultural machinery, especially after use on fields with resistant weeds. In addition, reverse tillage contributes to the management of herbicide-resistant weed species whose seeds accumulate in the uppermost soil layers.
Of particular interest is another cultural practice: the false seedbed. In this practice, the germination of weed seeds is triggered by tillage or/and irrigation, and the weeds are controlled before sowing. This helps reduce the weed seed bank.
Cover crops are another important technique to help in weed management and reduce the development of herbicide-resistant weeds. The use of competitive crop cultivars and practices that increase crop density are additional measures for HR management. Crop rotation also plays a crucial role in HR management because when different crops are grown on the same field in rotation, herbicides with different modes of action are also used in rotation to prevent weeds from adapting and developing resistant biotypes. Finally, using herbicide mixtures with different modes of action and pre-emergence soil herbicides delays the selection of resistant weed biotypes over time.
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