* U.S. corn gains 0.6 percent, soybeans up on weak dollar
* U.S. Fed gives no signs monetary policy tightening
* Tight-supplies, plantings delays to support corn
* Fundamentals bearish for wheat on weather, supplies
(Adds details, quotes; updates prices)
By Martin Roberts and Naveen Thukral
MADRID/SINGAPORE, April 28 (Reuters) - U.S. corn and soybean futures firmed on Thursday as part of a broad-based commodities rally driven by a sharply weaker dollar, while wheat extended previous falls on good weather and supply prospects.
The outlook for U.S. grain exports improved as the dollar tumbled to a three-year low after Federal Reserve Chief Ben Bernanke gave no sign on Wednesday that the central bank was about to tighten monetary policy.
Chicago Board of Trade actively traded July corn gained 0.6 percent to $7.63½ a bushel by 1222 GMT and soybeans for July rose 0.6 percent to 13.93¾ a bushel.
July wheat fell 0.4 percent to $8.08¾ a bushel, adding to a 4.2-percent drop on Wednesday prompted by forecasts of rain needed to help the developing hard red winter wheat crop in Kansas.
There was additional support for the corn market stemming from tight global supplies and planting delays for the U.S. new-crop, analysts said.
"We are getting planting delays and the balance sheet even for the next season looks pretty tight," said Abah Ofon, an analyst with Standard Chartered Bank in Singapore.
"Yields could come down because of delays. There is upside for corn against the backdrop of tight stocks as any adverse incident is going to push prices higher."
There are worries that wet weather in the U.S. Midwest would delay corn planting at a time when U.S. supplies are the tightest since the 1930s.
The U.S. Agriculture Department said on Monday that just 9 percent of intended corn acreage had been seeded as of April 24. A year ago, corn planting was 46 percent complete and the five-year average for late April is 23 percent.
Commodity funds were net sellers of an estimated 15,000 contracts of CBOT corn on Wednesday, trade sources said. They were net sellers of an estimated 6,000 each of wheat and soybean contracts.
WHEAT
Forecasts of improved weather and higher supplies continued to hammer the wheat market, taking losses sustained over three days to more than 6 percent, the biggest decline in about two weeks.
While fundamentals are bullish for corn, wheat is coming under pressure with prospects of higher supplies from the Black Sea region and improved weather forecasts in the United States.
Kazakhstan wheat output is seen rebounding to 14.5 million tonnes in 2011 from the drought affected 9.7 million last year and wheat exports are forecast at 7 million tonnes, an increase of 2 million from last year.
"If you look at the big picture it is one of increasing supplies," Ofon said. "As we head into the rest of the season, the market should decline much more sharply than what we have seen in the last few sessions."
Wheat prices in Europe's biggest exporter, France, fell in line with Chicago and a strong euro made exports from the common currency zone less competitive on world markets.
New crop November milling wheat was down 2.50 euros on the Euronext exchange in Paris, or 1.14 percent, at 217.50 euros ($322.7) a tonne by 1224 GMT, adding to a 4-percent drop on Wednesday.
"In terms of weather market it looks like the situation is improving. It is raining in the U.S., in China and in Germany," a trader said in France.