* Corn at record high, closing in on $8
* Tight U.S. corn supplies lifting market
* May/December spread remains wide
* Soy hit by S. America harvest, China demand
* Coming up: US corn planting progress Monday afternoon (Recasts, updates prices to close of U.S. trading session, adds details of corn spreads, fresh analyst quotes)
By Sam Nelson
CHICAGO, April 11 (Reuters) - U.S. corn futures notched another record high on Monday as strong demand across the livestock, ethanol, export and investment sectors faced the tightest stocks since the 1930s.
But soybeans saw their biggest decline in a month, tumbling nearly 2 percent as South America harvests a bumper crop and concerns mounted that China may slow its soy buying.
New-crop corn futures came under pressure as U.S. farmers appear to be off to a fast start seeding this year's crop. They are expected to plant the second largest area to corn this spring since World War Two.
The dwindling U.S. corn supply lifted futures to a record high of $7.83-3/4 in overnight Globex trade.
"Funds keep building a longer position in corn. If there are any issues in the upcoming planting season it will be pretty explosive; with stocks the way they are we can't have a hiccup," said Dan Dempsey, an analyst for Iowa Grain.
A government report later in the day was expected to show the fastest start ever in U.S. corn seedings.
Corn rose for the second straight trading session, nearing $8 a bushel and on track to post gains for five consecutive weeks after government reports showed demand from all sectors despite record high prices.
"The fundamentals are still taking the lead on corn, the interest is still there to own corn," said Dax Wedemeyer, an analyst for U.S. Commodities in Des Moines, Iowa.
CBOT May corn closed up 8 cents per bushel at $7.76. May soy was down 23-3/4 at $13.68-1/2 and May wheat was up 3/4 at $7.98-1/4.
Wheat was firm, following corn. However, gains in Chicago Board of Trade wheat were slowed by forecasts for showers in drought-plagued areas of the U.S. Plains that could enhance development of the struggling hard red winter wheat crop grown in that region. That weighed on the HRW wheat contract traded at the Kansas City Board of trade.
Traders said unwinding of KCBT/CBOT and MGEX/CBOT spreads lifted Chicago wheat at the expense of the higher protein wheat contracts traded on the KCBT and MGEX.
INVESTORS TO KEEP FOCUS ON CORN DEMAND
Traders said the corn market should remain on investors' radar until there are signs demand is cooling off.
The United States, the world's largest grain exporter, runs the risk of depleting its corn supplies before the autumn harvest unless demand wanes.
The 2011 U.S. corn crop must be seeded in a timely manner to ensure an adequate supply for next year.
Wet weather in the Midwest is slowing spring fieldwork but plenty of time remains to plant the 2011 crop. The bulk of U.S. corn plantings usually occur in late April and early May.
The May/December corn spread remained at a lofty $1.20-1/4 per bushel, premium May. In early March, that spread widened to a life-of-contract high $1.30-3/4 per bushel, premium May.
The big premium for May reflects shrinking old-crop stocks and outlooks for farmers to plant the second largest U.S. corn area since 1944.
"The spread will remain wide unless we see some planting problems and December could then begin recovering against May, which would narrow the spread," Wedemeyer said.
POOR MARGINS IN CHINA
Soybeans fell more than 1 percent as the harvest of South America's bumper soy crop advanced, some buyers took profits, and traders worried about demand from China, the world's leading soybean importer.
"As for soy, news that China might cancel cargoes is the impetus behind lower prices," said Matt Pierce of GrainAnalyst.com.
An official with China's state-owned trading house COFCO Co Ltd attributed a possible slowdown in soy imports to poor crushing margins. In addition, he said soy supplies are expected to be ample in the first half of the year. (Reporting by Sam Nelson; additional reporting by Suzanne Cosgrove in Chicago; editing by Jim Marshall)