* U.S. soybean typically dominates market in fourth quarter
* U.S. soy, grains imports down sharply amid trade war (Adds U.S. grains import data, background)
BEIJING, Nov 1 (Reuters) - China's purchases of Brazilian soybeans in September jumped 28 percent from the prior year, data from the General Administration of Customs showed on Thursday, as buyers stocked up ahead of an expected shortfall in the fourth quarter.
This is the first time that China has provided data on the country of origin for its commodity imports since the month of March.
China typically buys most of its soybeans in the fourth quarter from the United States but has sharply reduced its purchases of American beans amid an ongoing trade war.
Beijing hit U.S. products including soybeans and grains with a 25 percent duty on July 6 in response to similar trade measures levied on Chinese goods.
China is the world's top importer of the oilseed which it crushes to turn into soymeal to feed its huge herd of livestock.
Chinese buyers imported 7.59 million tonnes of Brazilian soybeans in September, up from 5.94 million tonnes a year ago, according to data released on Thursday.
Brazil accounted for 95 percent of the total 8.01 million tonnes imported in September, compared with 73 percent at the same time last year.
Customs had previously provided data on the origins of commodity imports for a fee but suspended this service in April.
On Thursday, Customs said on its website that it was making the data available to the public in a move aimed at promoting trade.
Soybean imports from the U.S. were 132,248 tonnes, compared with 937,000 tonnes in September last year.
Imports from Argentina were also sharply down to 153,510 tonnes compared with 738,472 tonnes last year.
China's grains shipments from the U.S. in September also slid significantly as the tariffs curbed buying, the data showed.
China's September corn imports from the U.S. fell to only 515.7 tonnes, down from last year's 174,965 tonnes, according to the data.
China brought in 59,455 tonnes of U.S. sorghum in September, down more than 80 percent from a year earlier.
China's total imports of corn and sorghum in September also fell because of higher prices in the global market.
Customs data showing a breakdown of origins of imports for last year is available here. (Reporting by Dominique Patton and Hallie Gu; Editing by Christian Schmollinger)