Corn, soy futures sag on improved South America weather forecasts

* Soy, corn ease as Argentina rain outlook tempers crop fears

* CBOT corn touches lowest price since Jan. 3

* USDA increased U.S. wheat planting, stocks forecasts

CHICAGO, Jan 13 (Reuters) - U.S. corn and soybean futures tumbled on Thursday on expectations that rains forecast for dry crop-growing areas of South America may limit harvest losses, traders said.

Weather forecasts show parched areas of Argentina, a major corn and soy exporter, may receive significant rainfall from late this week, traders said.

Concerns about hot, dry conditions in Argentina and Brazil in December lifted corn futures to six-month highs.

A return of adverse South American weather is needed to stabilize prices, said Rich Feltes, head of market insights for broker RJ O'Brien.

"Incoming rains will stabilize crops for now," Feltes said.

The most actively traded corn futures on the Chicago Board of Trade were down 9 cents at $5.90 a bushel by 11:15 a.m. CST (1715 GMT) and reached their lowest price since Jan. 3. Most-active CBOT soybeans were down 23-1/4 cents at $13.76 a bushel.

"The market's focus now returns to South America's weather as the trade tries to determine the impact of the current La Nina on Argentina and Brazil's corn & soybean output," said Jerry Gidel, analyst for Midland Research.

The major Rosario grains exchange on Wednesday slashed its forecast for 2021/22 corn production in Argentina to 48 million tonnes, down a huge 8 million tonnes from its previous outlook, scuppering what had been expected to be a record harvest.

The U.S. Department of Agriculture, in a separate crop report on Wednesday, reduced its forecasts of soybean and corn production in Brazil and Argentina. Some private analysts have cut their outlooks more steeply.

The USDA also on Wednesday pegged U.S. winter wheat acreage, U.S. end-of-season stocks and world wheat stocks above average analyst estimates.

The most-active wheat contract on the CBOT was down 14-1/2 cents at $7.43-1/4 a bushel. (Reporting by Tom Polansek in Chicago, Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Sherry Jacob-Phillips, Chizu Nomiyama and Hugh Lawson)