(Adds quote, details on flooding; closing prices)
CHICAGO, March 29 (Reuters) - Massive supplies of both U.S. corn and soybeans remained in storage bins around the country ahead of spring planting, U.S. Agriculture Department (USDA) data released on Friday showed.
USDA also said that farmers planned to boost their corn plantings by 4.1 percent but severe flooding in the U.S. Midwest may curtail final acreage.
"Nearly all of the data were collected before the flooding began," Lance Honig of USDA's National Agricultural Statistics Service said on Twitter. "It will take time to determine what impact it has on final plantings."
In its quarterly stocks report, USDA pegged domestic corn supplies as of March 1 at 8.605 billion bushels, down from 8.892 billion a year ago but still the third-biggest on record.
Analysts had expected corn stocks of 8.335 billion bushels, according to the average of estimates in a Reuters survey.
Corn prices plunged after the report was released, with the most-active Chicago Board of Trade corn futures contract dropping 4.6 percent to a four-month low. It was the biggest daily price drop for corn in percentage terms since July 2016.
In its prospective plantings report, the government said farmers planned to seed 92.792 million acres of corn and 84.617 million acres of soybeans this spring.
The plantings outlook was based on surveys of farmers conducted during the first two weeks of March.
At least 1 million acres (405,000 hectares) of U.S. farmland were flooded after a "bomb cyclone" storm left wide swaths of nine major grain-producing states under water this month, satellite data analyzed by Gro Intelligence for Reuters showed.
The glut of supplies allowed traders to shrug off concerns about a reduction in corn acres.
"We are lugging around 8.6 billion bushels of corn, which gives us a bigger buffer than we thought," said Don Roose, president of U.S. Commodities.
Soybean stocks of 2.716 billion bushels were the largest on record for the time period and also were above market forecasts for 2.683 billion. Wheat stocks stood at 1.591 billion bushels, the second-biggest in 31 years.
A bitter trade fight with China, the world's largest buyer of soybeans, hindered the U.S. export program during the winter months, typically a period of high global demand for U.S. supplies.
Soybeans dropped to a three-month low while wheat futures hit their lowest in two weeks. (Additional reporting by Karl Plume; Editing by Dan Grebler and James Dalgleish)