Herbicide-resistant weeds increasingly common in Estonia

The first documented cases of herbicide resistance have been recorded in Estonia. Similar to the rest of the world, the excessive use of broad-spectrum herbicides is driving the development of resistance, which is becoming an increasingly serious problem.
Silvia Pihu, a researcher at the Estonian Crop Research Institute at the Center of Estonian Rural Research and Knowledge, co-authored a scientific article with colleagues describing herbicide resistance in common chickweed (Stellaria media.)
The study shows resistance primarily occurs against sulfonylurea-based herbicides. These are widely used because they are safe for many cultivated plants and require relatively low doses.
The research by Pihu and her colleagues suggests that resistant plants likely emerged locally rather than arriving in Estonia through seed dispersal.
"In terms of quantity, herbicides are the most widely used pesticides both in Estonia and across Europe. Since resistance has already become a significant problem elsewhere in the world, I examined whether it might also be present in our fields. We tested 11–12 different species, and a couple of them showed resistance," she explained.

Pihu said experiments take a lot of time as the plants must be grown from seeds and then sprayed with different solutions and doses. Herbicide-resistant common chickweed was discovered on three test fields.
Although the article only studied chickweed, researchers also found cleavers (Galium aparine) was resistant. Both weeds are difficult to control using mechanical methods and are widespread across Estonia.
Plants can develop resistance if the same herbicides are frequently used against them as they learn how to adapt and evolve, Pihu explained. The same principle applies to antibiotic resistance in bacteria.
"If the same herbicide is used repeatedly for several years in a row, only those plants that happen to have a survival-enhancing gene will remain. Over time, only individuals with the beneficial mutation will produce seeds. This is how resistant plant populations eventually emerge," the researcher said.

According to the scientific article, sulfonylureas are one of the most widely used herbicide groups in the world. However, their effectiveness and widespread use have led to the development of resistance in several weed species, including common chickweed (Stellaria media.)
"Herbicides have the widest variety of active ingredients among all pesticides. However, farmers tend to choose them based on lower cost, using relatively broad-spectrum herbicides year after year. Sulfonylureas, for example, are particularly common active ingredients. This is why resistance to them develops frequently," Pihu noted.
The study's results highlight the need for broader herbicide resistance monitoring and research in Estonia. Pihu said the situation in Estonia is still relatively good compared to Western Europe, and resistance is not yet widespread. Among nearby countries, resistance is more common in Denmark and Sweden, with isolated cases also recorded in Lithuania.
"It is essential to use herbicides with different active ingredients and to rotate them regularly," the researcher advised.
Pihu's study "The first recorded case of herbicide resistance in Estonia: common chickweed (Stellaria media) resistant to sulfonylureas" can be read here.
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Editor: Helen Wright