CHICAGO, March 9 (Reuters) - The U.S. Agriculture Department (USDA) left its outlook for domestic corn and soybean supplies unchanged at seven-year lows on Tuesday.

While the stocks forecast was slightly bigger than what analysts were expecting, it showed that supplies will remain tight until U.S. farmers begin harvesting crops in the fall when a big-crop will be needed to satisfy robust domestic and export demand.

Chicago Board of Trade soybean and corn futures sank to session lows immediately following the report's release. But all three commodities quickly returned to their pre-report levels, with the trade focused on forecasts for planting weather in the United States.

"In general this was a plain vanilla report that is eliciting a plain vanilla response," said Charlie Sernatinger, global head of grain futures at ED&F Man Capital.

USDA forecast in its monthly World Agricultural Supply and Demand Estimates report that the U.S. corn stocks will stand at 1.502 billion bushels by Aug. 31 and soybean stocks at 120 million bushels.

Analysts had predicted the report would show corn ending stocks of 1.471 billion bushels and soybean ending stocks of 117 million bushels, based on the average of estimates given in a Reuters poll.

USDA also kept its outlook for U.S. wheat stocks unchanged at 836 million bushels, which would be the lowest since the 2014/15 marketing year. Analysts had expected a slight increase.

On the global front, USDA raised its outlook for the Brazilian soybean crop by 1 million to 134 million tonnes and left its outlook for the country's corn harvest unchanged at 109 million tonnes.

Heavy rains in that key export country have slowed the soybean harvest, raising concerns about crop quality as well as delaying the planting of fields intended for corn.

USDA lowered its forecast for Argentina's soybean production to 47.50 million tonnes from 48.00 million. (Additional reporting by Julie Ingwersen; Editing by Caroline Stauffer and Aurora Ellis)