On the Chicago Mercantile Exchange, soybeans futures for March delivery traded at USD14.3525 a bushel, down 1.2% on the day.
The March contract declined by as much as 1.6% earlier in the day to hit a session low of USD14.2938 a bushel, the weakest level since January 24.
Soy prices came under heavy selling pressure for the second consecutive session after the USDA forecast larger-than-expected inventories of the oilseed.
Soybean inventories on October 1 will total 60.12 million metric tons, more than the 59.46 million estimated in January and 8.8% higher than a year earlier
Expectations for bumper crops in major South American soy growers also added to the selling pressure.
The USDA said Brazil will harvest a record 83.5 million tonnes of soybeans this spring, on pace to pass the U.S. as the top producer for the first time.
Combined soybean production in Brazil and Argentina will rise 28% to a record 136.5 million
World production will total a record 269.5 million tons in the current marketing year, more than the 269.41 million forecast in January. In the previous season, global output totaled 238.73 million tons.
Meanwhile, corn futures for March delivery traded at USD7.0612 a bushel, down 0.3% on the day. The March contract fell by as much as 0.45% earlier to hit a session low of USD7.0562 a bushel, the weakest level since January 11.
Corn prices extended losses from the previous session after the USDA said U.S. stockpiles before the next harvest will be 5% larger than forecast a month ago.
U.S. inventories will total 632 million bushels on August 31, up from 602 million forecast in January.
The upward revision reflected slowing demand for U.S. supplies. U.S. exporters will sell 900 million bushels of corn in the current marketing season, down from 950 million forecast last month and the lowest since 1972.
Last year, U.S. exporters sold 1.543 billion bushels of corn.
The data also showed that global inventories of the grain on October 1 will be 118.04 million tons, up from 115.99 million predicted a month earlier.
Elsewhere, wheat for March delivery traded at USD7.5162 a bushel, down 0.6% on the day. The March contract fell by as much as 0.7% earlier in the session to hit a daily low of USD7.5062 a bushel.
Global wheat inventories at the end of the year will be 176.73 million metric tons, up from the month-earlier estimate of 176.64 million
Losses were limited as the USDA cut its forecasts for U.S. wheat reserves at the end of the crop's current marketing year to a four-year low of 691 million bushels, 3.5% lower than the agency’s January estimate of 716 million bushels.
Wheat traders also continued to monitor weather and crop conditions in the U.S. Great Plains-region, where prolonged dryness threatens now-dormant winter wheat crops.
Corn is the biggest U.S. crop, followed by soybeans, government figures show. Wheat was fourth, behind hay.