* Chicago wheat erases big gains
* Brazil rain forecasts in focus for soybeans, corn
* soybeans slightly rise, corn declines
CHICAGO, Dec 27 (Reuters) - Chicago wheat declined on Wednesday, erasing most of the gains made the day before when worries that escalating wartime tensions would impact shipping in the Black Sea exporting region sent grains markets higher.
Soybeans ticked higher while corn moved lower as traders assessed a rainy forecast in South America, after drought exacerbated concerns over crop yield in top soy and corn exporter Brazil.
The most-active wheat contract on the Chicago Board of Trade (CBOT) fell 13-1/4 cents, or 2%, to settle at $6.23 a bushel. Without signs of strong demand, the contract wiped out most of the gains of Tuesday, when it closed more than 3% higher after Ukraine struck a large Russian landing warship.
"The demand isn't good enough to move the market itself," said Mark Schultz, chief analyst at Northstar Commodities. "It's not a demand-driven market, on the grains or oil seeds, at this juncture, it's just not."
CBOT's most-active soybean contract settled up 1-3/4 cents at $13.19 a bushel and corn gave up 3-3/4 cents at $4.76-1/2 a bushel.
Weather charts showed uneven showers in the week ahead in dry parts of central and northern Brazil, before widespread heavy rain expected in early January.
For corn, precipitation in the coming weeks will also influence yields in the upcoming first-crop harvest and affect planting prospects for the larger second crop in Brazil.
"Otherwise there haven't been any significant sales being announced to give you some type of encouragement or something else besides just watching the weather day in and day out," Schultz said, referring to a lack of announcements by the U.S. Department of Agriculture of export sales to China and elsewhere this week. (Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; editing by Barbara Lewis and Josie Kao)
