June 30 (Reuters) - Bayer on Tuesday asked Washington to impose duties on glyphosate, the chemical used in the company's weedkiller Roundup, imported from China, saying it was being sold at artificially low prices.

The move angered U.S. farmers, many of whom have lost money for years, because duties would increase the cost of the herbicide and add to their financial woes.

The company scored a major legal victory last week when the U.S. Supreme Court blocked thousands of state-court lawsuits that accuse Bayer of failing to warn users that glyphosate causes cancer.

Bayer sought duties through a petition by its subsidiary Monsanto to the U.S. Department of Commerce and the U.S. International Trade Commission.

"Monsanto filed antidumping and countervailing duties petitions to address predatory trade practices and subsidized imports of glyphosate," the company said in a statement.

"The domestic glyphosate business as it stands today is not sustainable."

Bayer is the only U.S. maker of glyphosate and asked the agencies for countervailing duties on cheaper Chinese shipments to offset the difference between those products and domestically produced glyphosate, according to a copy of the petition.

"They are taking this step purely for the benefit of the company and its shareholders, once again at the expense of the American farmer and at a time when the ag economy is facing one of its most difficult periods in decades," said Jed Bower, president of the National Corn Growers Association, in a statement. 

Grain and soybean growers have struggled financially for the past four years partly due to high costs for inputs including chemicals, seed and fertilizer.

"Actions to impose import taxes on those products limit market competition, threaten cost spikes, and ultimately hurt U.S. farmers," the American Soybean Association said in a statement.

On June 25, the Supreme Court overturned a jury verdict in Missouri awarding $1.25 million to a man named John Durnell who said he was diagnosed with non-Hodgkin lymphoma after years of exposure to glyphosate ‌in Roundup.

    Bayer previously told U.S. lawmakers it might stop selling Roundup unless they could strengthen legal protections against product liability litigation. Bayer has paid about $10 billion to settle disputed claims that Roundup causes cancer.