*

Forecasts of freezing weather in U.S. Midwest support wheat

*

Soy pressured by rainfall in Argentina

CHICAGO, Dec 22 (Reuters) -

Chicago soybean futures dropped on Thursday, pressured by South American forecasts for more rain which could relieve parched soy crops in Argentina.

Corn and wheat traded just below even, holding on to some of the previous day's gains as extreme winter weather threatens winter wheat crops in the U.S. Plains and Midwest.

The Chicago Board of Trade most-active soybean contract dipped 10-1/4 cents to $14.74-1/4 a bushel by 10:53 a.m. (1653 GMT).

Corn last traded 1/4 cents lower at $6.62 a bushel, while wheat eased 3-1/4 cents to $7.64-1/2 a bushel.

"We got some unexpected heavier rains in Argentina overnight. The forecast turned a little wetter going forward," said Karl Setzer, commodity risk analyst at Agrivisor. "Some of the risk premium that was put into the market yesterday, we're pulling back out."

Brazil, the world's biggest soybean exporter, is suffering far less from the dry weather, and is expected to begin harvesting soybeans by late January, capping gains in U.S. futures.

Wheat traders are watching forecasts for temperatures well below freezing across the U.S. Heartland in the coming days, which may threaten winter crops not insulated by snowcover.

"This is pretty extreme. I'm surprised we haven't seen a little more short covering in some of our wheat," said Dan Hussey, senior market strategist at Zaner Group.

Winterkill damage is hard to detect until later in the season, Setzer said, which may be why traders are not reacting more to the threat.

Corn and soybeans

export sales

dipped to the low end of

trade estimates

during the week ended Dec. 15.

U.S. exporters sold 876,000 tonnes of soybeans last week, compared to trade expectations of 800,000 tonnes to 1.4 million tonnes, the U.S. Agriculture Department said.

Corn export sales totaled 636,800 tonnes, near the low end of trade forecasts ranging from 625,000 to 950,000 tonnes.

Exporters sold 334,200 tonnes of wheat, in line with analysts' forecasts of 200,000 to 550,000 tonnes. (Reporting by Christopher Walljasper; Additional reporting by Michael Hogan in Hamburg)