Herbicide-resistant weeds are a growing problem throughout the world. Monitoring herbicide resistance in Switzerland allows us to understand the mechanisms behind it and to better manage the use of herbicides.
Herbicide resistance has currently been identified at global level in 267 weed species and affects 165 herbicides in 72 countries. Since 2011, the ‘Herbology in Field Crops’ research group at Agroscope Changins has been testing weed populations with suspected resistances on a national scale in Switzerland. The plants in question have survived a herbicide treatment in the field, which should normally have destroyed them.
Greenhouse and molecular tests
To detect resistance, greenhouse tests are conducted. For several years now, these tests have been supplemented with molecular tests. Leaves of plants that have survived herbicide treatments in greenhouse trials undergo genotyping and are examined for mutations in relevant target genes. The detection of point mutations in genes encoding protein targets of active ingredients results in the loss of herbicide efficacy. The weed becomes resistant.
Weed populations with a survival rate of at least 50% in the greenhouse tests are declared resistant to the tested herbicide. Since 2011, resistances have been confirmed in 131 populations of six weed species in Switzerland. The most affected species is blackgrass, followed by loose silky-bent and Italian ryegrass.
Herbicide-resistant weeds have been detected throughout the Swiss Central Plateau and in Valais, mainly in field crops but also in vineyards. Although the number of resistant populations is steadily increasing, the level of herbicide resistance in weeds detected in Switzerland is low, affecting only six weed species.
Different types of resistance
Molecular tests allow us to differentiate between target site resistance (TSR) and non-target site resistance (NTSR). Most Swiss weed populations exhibit resistance to one mode of action. Herbicide multiple resistance refers to weeds that have evolved resistance to two or three different modes of action.
Multiple resistances conferred by several mutations on different genes are more difficult to manage with herbicides because the choice of herbicide options is limited. It is then necessary to consider non-chemical weed control measures such as mechanical weeding, sowing of grassland, cover crops or regular ploughing.
- A sound knowledge of herbicide-resistant weed populations and the underlying mutations can help mitigated reduced herbicide efficacy and avoid unnecessary herbicide applications.
- In Switzerland, the level of herbicide resistance is relatively low, and resistant weed populations in field crops are generally well controlled.
- Given the increasingly restricted availability of herbicides, Swiss farmers increasingly make use of sustainable weed-management strategies (diversified rotations, undersowing, cover crops, mechanical weeding, etc.)