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There has been an increase in herbicide-resistant weeds across US farmland as they develop faster than new weedkillers can be developed, Reuters reported.

In many cases weeds are developing resistance against multiple herbicides, according to scientists quoted in the 16 January report.

As part of its report, Reuters interviewed two dozen farmers, scientists, weed specialists and company executives and reviewed eight academic papers published since 2021 which described how kochia, waterhemp, giant ragweed and other weeds were pushing out crops in North Dakota, Iowa, Wisconsin and Minnesota as chemicals lost their effectiveness.

Over the last two decades, chemical companies have reduced investment in research and development and were introducing less products, according to AgbioInvestor, a UK-based firm that analyses the crop protection sector.

Farmers have said they are losing their battle with weeds that threaten grain and oilseed harvests at a time when they are also dealing with inflation and extreme weather linked to climate change.

“We're in for big problems over the next 10 years,” said Ian Heap, director of the International Survey of Herbicide Resistant Weeds, a group of scientists in over 80 countries that maintains a global database.

The database has recorded reduced effectiveness for glyphosate, one of the most common herbicides, against 361 weed species, including 180 in the USA, affecting corn, soyabeans, sugar beets and other crops.

Globally, around 21 weed species had shown resistance to dicamba, the most recent major US chemical, which was launched in 2017, the report said.

According to environmental groups, farmers should use natural weed-control methods instead of chemicals.

Kochia, which spreads as many as 30,000 seeds/plant, can cut yields by up to 70% if left unchecked, according to the United Soybean Board’s farmer resource programme Take Action.

Five of North Dakota's 53 counties had confirmed populations of dicamba-resistant kochia, a year after it was first reported in the state, North Dakota State University weed specialist Joe Ikley said.

Although the development of more robust seeds had pushed overall global crop yields higher, scientists expected weed problems to worsen, with some weeds showing resistance to chemicals from the first exposure, Reuters wrote.

According to chemical producers Bayer, Corteva and FMC, longer development and regulatory processes have constrained new products to combat weed resistance while industry sources say regulators have become more stringent about environmental and health impacts.

Meanwhile, the US Environmental Protection Agency (EPA) has said standards for approving new herbicides had not substantially changed since 1996.

However, the EPA said recent efforts to assess the impact of new active ingredients on threatened plants and wildlife had delayed some decisions.