CHICAGO—Soybean prices fell nearly 2% as government forecasters estimated this year’s crop would be by far the largest in history.

Wheat also declined, while corn rose modestly after the U.S. Agriculture Department projected lower-than-expected yields.

Soybeans snapped a four-session winning streak after the USDA estimated this autumn’s U.S. harvest would reach a record 3.82 billion bushels, roughly in line with analysts’ estimates. The government said soybean yields would hit a record 45.4 bushels an acre.

The USDA’s forecast for a bumper soybean crop reflects near-perfect weather in the U.S. Midwest this season and an 11% increase in harvested acres of the oilseeds. As much as three times the normal amount of rain has fallen in the past three months in the Farm Belt, improving soil moisture and leading to the best conditions for the soybean crop in a decade, according to federal data.

U.S. soybean stockpiles in the 2014-15 season will more than triple from a year earlier, the USDA estimated, alleviating concerns about low inventories that have plagued food producers and other soybean buyers for several years.

Traders and investors had eagerly anticipated Tuesday’s monthly crop report because it was the first of the season in which the USDA estimated yields based on surveys of farmers and inspections of fields.

“We got confirmation that there will be plenty of soybeans, and we know there will be plenty of corn,” said Joe Hofmeyer, an analyst with commodity brokerage CHS Hedging in Minnesota.

August soybean futures, the thinly traded front-month contract, dropped 22.75 cents, or 1.7%, to $12.92 a bushel on the Chicago Board of Trade. The November contract, the most-active by volume, declined 12.25 cents, or 1.1%, to $10.61 a bushel.

Corn futures gained after the USDA’s production and yield forecasts—while also at record levels—failed to meet analyst expectations.

The USDA estimated the corn harvest at 14.03 billion bushels, which would break last year’s record of 13.93 billion. But the agency surprised analysts by forecasting average corn yields at 167.4 bushels per acre, breaking the record 164.7 in 2009 but below expectations of about 170 bushels an acre. The USDA didn’t elaborate on the factors that led to its yield forecast.

Ideal weather in prime growing areas such as Iowa, above, led the USDA to predict this year’s soybean crop will be the largest ever. Futures prices fell nearly 2% on the news. Associated Press

“The corn number being sub-170 [bushels an acre] is the big surprise,” said Sterling Smith, a futures specialist at Citigroup C +0.10% in Chicago. “That was way under expectations.”

Corn futures for September delivery, the front-month contract, rose 1.75 cents, or 0.5%, to $3.585 a bushel on the CBOT. December futures, the most-active contract, gained 0.75 cent, or 0.2%, to $3.69 a bushel.

Wheat futures fell after the USDA raised its outlook for global stockpiles at the end of the 2014-15 season that ends next May 31.

The USDA said global inventories of wheat at the end of the 2014-15 season will total 192.96 million metric tons, which compares with analyst estimates of 189.89 million tons.

Wheat futures for September delivery fell 8.5 cents, or 1.6%, to $5.38 a bushel in Chicago trading.