(Adds market reaction, analyst quote)
By Mark Weinraub
WASHINGTON, June 30 (Reuters) - U.S. farmers seeded a record amount of soybeans this spring, the government said on Friday, in the hope that strong export demand would soak up much of the fall harvest despite bumper crops in Brazil and Argentina that have swelled global stocks.
All-wheat plantings fell to a record low of 45.657 million acres, with ample global stocks and weak export prospects pushing U.S. farmers away from that crop.
Corn plantings rose unexpectedly from the U.S. Agriculture Department's March estimate.
Corn futures, which had been trading sharply higher, briefly dropped into negative territory after the report was released. Many analysts had expected corn acreage to fall from the March outlook due to excessive wetness in the eastern U.S. Midwest.
"The acres on corn were a little larger than the estimate and that's giving a little resistance on the corn market," said Jason Roose, an analyst at U.S. Commodities.
Soft red winter wheat futures rallied to a 1-1/2-year high while soybeans hit their highest since late May.
Spring wheat plantings dropped to a 45-year low, fueling further market gains. Prices for spring wheat have surged to their highest levels since 2014 this week as drought conditions in the northern U.S. Plains threaten crop yields.
Supplies for all three commodities remained robust, with stocks of both corn and soybeans as of June 1 the third-biggest ever for the period. Wheat stocks as of June 1 were the largest in 29 years.
USDA said farmers planted 90.886 million acres of corn, up 890,000 acres from the government's March forecast. Analysts had been expecting corn plantings of 89.903 million acres, according to the average of analysts estimates in a Reuters poll.
Soybean plantings came in at 89.513 million acres. In March, USDA had forecast soy plantings of 89.482 million acres.
USDA pegged spring wheat acreage at 10.899 million acres, down from its March estimate of 11.308 million acres and below the low end of analysts' expectations of 10.990 million to 11.589 million.
On the supply front, corn stocks as of June 1 were 5.225 million bushels, up from 4.711 million a year earlier and the most since 1987.
Soybean stocks as of June 1 were 963 million bushels, up from 872 million a year earlier. The June 1 soy stocks reading was the biggest in 10 years.
Wheat stocks rose to 1.184 billion bushels from 976 million a year earlier.
Reporting by Mark Weinraub; Editing by Paul Simao and Richard Chang