China said on Friday it is slapping anti-dumping duties on U.S. distillers' dried grains (DDGS) shipped by some of the biggest suppliers of the animal feed ingredient, including Louis Dreyfus and Archer Daniels Midland.

The duties of 33.8 percent are effective immediately, the Ministry of Commerce said in a preliminary ruling. The move comes after a months-long probe following complaints by China's ethanol producers that the U.S. industry was unfairly benefiting from subsidies.

It did not give a timing for a final decision.

China is the world's top buyer of DDGS, a by-product of corn ethanol that is used by feed mills as a substitute for corn and soymeal. China imports almost all of its needs from the United States, the largest exporter.

The move could intensify a spat between the world's two-largest economies over agricultural trade policy, and also comes as the countries are embroiled in disputes over China's exports of steel and aluminium.

In the near term, the impact on trade may be limited as exporters have already curbed shipments into China since the investigation started in January to avoid any retroactive penalties, traders said.

Some traders said they had feared duties would be higher, between 40 percent and 60 percent.

Still the long-awaited decision is a major blow to some of the biggest players in the U.S. ethanol industry, including Poet LLC, Patriot Renewable Fuels LLC, ADM, Louis Dreyfus, Valero Energy Corp and Andersons Inc.

"The problem is this is just the preliminary result. People are worried that the final one could be even higher," said one Shanghai-based trader.

China previously launched an anti-dumping investigation into DDGS imports from the United States in late 2010, later extending the probe before dropping it in mid-2012.

The earlier investigation slowed China's imports of the feed ingredient but did not stop them entirely.

U.S. farm cooperative CHS Inc. was excluded from the ruling because the information submitted by the company to the government was inconclusive, the ministry said.

Other companies included in the ruling are: Big River Resources LLC, Marquis Energy LLC; Absolute Energy LLC; Ace Ethanol LLC; Elkhorn Valley Ethanol; Flint Hills Resources LP; Goldan Grain Energy LLC; Illinois River Energy LLC; Louis Dreyfus Commodities Grand Junction LLC.