* Chicago wheat strengthens on drop in global output
* Soybeans fall as U.S.-China trade war dents demand
* Corn firms after three-day decline, market eyes USDA data (Updates throughout, adds comment, detail)
By Ana Ionova
LONDON, Nov 1 (Reuters) - Wheat futures rose for a second session running on Thursday, supported by an expected boost to demand for U.S. supplies after a drop in output in key exporting countries.
Soybeans slid after a two-session winning streak, hit by curbs on U.S. shipments to China, the world's biggest buyer, as the Sino-U.S. trade war rumbles on.
The most-active wheat contract on the Chicago Board of Trade
was up 0.5 percent at $5.03 a bushel by 1114 GMT.
"The EU, Russia and Australia have much less wheat to offer this year," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia. "Price is never irrelevant but the available wheat right now is in the U.S. and we expect it to flow."
U.S. wheat exports have also struggled in the face of competition from Russia, the world's biggest supplier. But lower output in some of the top exporters is likely to boost demand for U.S. wheat early next year.
December wheat futures on Paris-based Euronext were up 0.25 percent at 199 euros a tonne.
CBOT soybean prices slipped, with the most active contract down 0.3 percent at $8.49-1/4 a bushel. Corn prices rose 0.34 percent to $3.64-1/2 a bushel.
The market focus was on monthly supply-and-demand forecasts due from the U.S. Department of Agriculture (USDA) next week, with some predicting that the agency will trim its corn yield estimate.
Uncertainty over demand is adding pressure on the soybean market, with the trade row between Washington and Beijing virtually halting U.S. soybean exports to China.
On Thursday a vessel carrying soybeans from the United States to China changed its destination to South Korea, shipping data showed.
Data on Thursday also showed that China increased its purchases of Brazilian soybeans by 28 percent year on year in September as it looks to other sources of the oilseed.
"The trade conflict with the U.S. is thus having a visible impact," Commerzbank said in a note. "This is disastrous news for U.S. farmers, who are harvesting a record soybean crop and are likely to have trouble finding alternative buyers."
Soren Schroder, chief executive of global grain merchant Bunge, said he expects a small cut in China's year-on-year soy imports because of the trade dispute.
U.S. soybean crushing in September is likely to have totalled 5.131 million short tonnes, or 171 million bushels, according to the average forecast of nine analysts polled by Reuters ahead of a monthly USDA report. (Reporting by Ana Ionova and Naveen Thukral Editing by Sunil Nair and David Goodman)