CHICAGO, Oct 2 (Reuters) - U.S. corn and soybean futures fell on Friday on expectations of a busy harvest weekend in the heart of the Corn Belt, and on investor caution following news of U.S. President Donald Trump's positive test for COVID-19, analysts said.

Wheat futures firmed on concern about dry weather in Russia, the world's top wheat exporter.

As of 1:12 p.m. CDT (1812 GMT), Chicago Board of Trade December corn was down 2-1/2 cents at $3.80-1/4 per bushel, and November soybeans were down 2-3/4 cents at $10.20-3/4 a bushel. CBOT December wheat was up 2-1/2 cents at $5.72-3/4 a bushel.

All three markets were on track to post higher weekly closes, fueled by tighter-than-expected quarterly U.S. stocks figures reported Wednesday by the U.S. Department of Agriculture.

But attention was shifting to harvest results from farmers ahead of the USDA's Oct. 9 monthly supply/demand report, and the question of whether the government would adjust its U.S. 2020 corn yield forecast from its September estimate of 178.5 bushels per acre (bpa).

"It's harvest time, and ears are going to be (tuned) to what is happening with yields over the weekend," said Don Roose, president of Iowa-based U.S. Commodities.

Private analytics firm IHS Markit on Friday cut its forecast of the average U.S. 2020 corn yield to 177.8 bpa, from 178.1 a month ago. On Thursday, brokerage StoneX lowered its U.S. corn yield estimate to 179.0 bpa, down from 179.6 a month earlier but still above the USDA's figure.

A firmer dollar also hung over the market, making U.S. grains less competitive globally. The dollar index rose while equities and oil prices slipped after Trump said he and his wife had tested positive for COVID-19 a month before U.S. elections, unsettling investors.

(Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Marguerita Choy)