China's surprise currency devaluation is the latest challenge for U.S. soybean exporters already facing their slowest sales pace in seven years as huge South American supplies and a strong dollar have been crimping demand for U.S. shipments for months.

China's central bank on Tuesday set its official guidance rate down nearly 2 percent to its lowest point in almost three years, making imports costlier for the world's top commodities buyer and No. 1 soybean importer.

Concerns that the move by Beijing could be the first of several devaluations, or that other export-reliant countries could follow suit, created further uncertainty for U.S. exporters struggling to compete as the dollar hits highs against several currencies and global stocks are projected at record levels.

So far, Indonesia, Japan and South Korea, all top-10 buyers of U.S. soybeans, have said they see no reason for tit-for-tat trade-war policies.

U.S. export sales for the 2015/16 marketing year, which begins in September, were 47 percent behind a year ago through late July, according to U.S. Department of Agriculture data. Chinese purchases of new-crop U.S. soybeans of just 3.27 million tonnes by the end of July are 67 percent behind a year ago.

And the USDA is expected to increase its forecast for South American soybean exports and may trim its view of the upcoming season's U.S. exports - valued at more than $14 billion last year - in a report due on Wednesday.

"All this does is create additional headwinds for U.S. product trying to find its way out," said Joe Lardy, analyst at CHS Hedging, a division of CHS Inc, the largest U.S. farm cooperative and a large exporter of agricultural goods.

A trader with a major multinational grain export company said the phones went quiet on Tuesday after the devaluation eroded soy processing margins overnight by about $9 per tonne.

Traders also said business slowed as buyers moved to the sidelines ahead of Wednesday's report.

Many importers lock in margins by using soybean, soymeal and soyoil futures markets both in China and the United States, but some do not hedge their currency risk, a trader said. "If they didn't touch the forex, they got hurt there."

Traders contacted by Reuters said they had not adjusted hedging strategies as it remains unclear if the devaluation would significantly reduce China's overall import volumes.

Importers cautious

China has already imported record volumes from South America this summer, according to Chinese customs data. Imports this season are expected to hit an all-time high of 76 million tonnes.

Although strong demand for livestock feed from China's swine sector suggests its appetite for soybeans will persist, uncertainty over further currency moves could keep importers cautious in the near term.

"Any time you have a move like this it just makes people nervous. I certainly think it's going to temper some enthusiasm for imports," said the trader at the multinational grain export company, asking not to be named.

Many Chinese importers have already booked purchases from Brazil for shipment in the fourth quarter, a period traditionally dominated by newly harvested U.S. soybeans, as Brazilian farmers took advantage of a tumbling real, down 30 percent against the dollar this year.

Competition from cheap Argentine soybeans is expected to continue into November, traders said.

But now the narrower crush margins could have Chinese importers buying on a more hand-to-mouth basis, rather than months in advance like they normally would. Persistent competition from South America will make it difficult for lagging U.S. export sales to catch up, traders said.

"China will continue to oscillate and buy seasonally from the U.S. and South America. But since South America is flooded with beans right now it really shortens the window for U.S. exports," said Terry Reilly, senior commodities analyst with Futures International.