(Adds start of US trading)

By Tom Polansek

CHICAGO, Nov 21 (Reuters) - U.S. soybean futures edged

lower on Friday at the end of a see-saw week during which

Chinese purchases of U.S. supplies pushed prices to a 17-month

high before doubts about whether China would sustain such buying

punctured the rally.

Corn and wheat futures were nearly unchanged.

The U.S. Department of Agriculture this week confirmed that

China bought more than 1.5 million metric tons of American

soybeans. Traders said they had largely expected deals of that

size before the USDA reported them, and they had factored

China's buying into the market already.

"They buy, and it's already behind us," said Don Roose,

president of U.S. Commodities in Iowa.

The most-active soybean contract on the Chicago Board of Trade

(CBOT) was 2-1/2 cents lower at $11.20 a bushel by 10:05

a.m. CST (1605 GMT). This was down from Tuesday's peak of

$11.69-1/2, the highest since June 2024.

China's soybean purchases remain far off the 12 million tons

that U.S. officials said it promised to buy by year-end. The

U.S. also faces tough competition for global export sales from

cheaper soybeans in Brazil, analysts said.

Some traders said China could bear the cost of making larger

purchases of U.S. soybeans in order to preserve the trade truce

agreed with Washington in late October but expected the market

to remain cautious until volumes are confirmed.

"We wouldn't be surprised if Chinese purchases from the U.S.

are closer to a trickle than a flood," said Rod Baker, an

analyst at Bendigo Agribusiness Insights.

The USDA reported on Thursday that China also bought 132,000

tons of U.S. white wheat. The deal failed to spark a strong

rally in CBOT wheat futures as global supplies were plentiful,

traders said.

CBOT wheat was up 3/4 cent to $5.41-1/2 a bushel, while

corn futures dipped 3/4 cent to $4.37 a bushel.