American soybean farmers are beginning to harvest what's expected to be their second-biggest crop. At the same time, they're seeing their share of the global export market shrink, reflecting the growing interdependence of Brazil and China in agricultural trading.

Brazilian production is also booming, adding to a worldwide glut. And while the country's economy languishes, a weaker currency is helping it compete internationally, particularly in China, the biggest soybean buyer.

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"There is a huge financial incentive for Brazil to ship as much as possible of its record harvest because of the dollar strength," Bill Tierney, the chief economist at AgResource Co. in Chicago, said Tuesday. "The U.S. window of exclusivity for shipping soybeans is shrinking."

On Friday, the U.S. Department of Agriculture is expected to say domestic production will total 3.841 billion bushels, 20 percent more than the average for the prior 10 years, according to a Bloomberg News survey of 33 analysts.

When combined with reserves left over from the record crop of 2014, total U.S. supply this year will be the biggest ever. Soybean futures are down 14 percent in 2015, and hedge funds are betting that prices will fall further.

U.S. exports for delivery before Aug. 31 have dropped 38 percent from a year ago, according to USDA data. Purchases of American soybeans by China plunged by more than half in the period.

Overall, China is buying more soybeans. It imported 7.78 million metric tons last month, up 29 percent from a year earlier, with most of that coming from Brazil. July imports were at a record, customs data show.

Brazilian shipments are also up. The country's ports, once beset by bottlenecks and outmoded infrastructure, are finally raising their capacity thanks to public and private investment.

The planting season is just beginning in the Southern Hemisphere. Farmers in Brazil, the largest exporter, are set to boost output to a record for a fourth straight year, said Michael Cordonnier, owner of Soybean & Corn Advisor Inc. in Hindsdale, Ill.

As a result, the world is awash with soybeans. Global reserves for the start of the Northern Hemisphere harvest may rise 7.7 percent to a record 86.66 million tons, compared with 80.48 million tons forecast this year, according to a Bloomberg survey.