Cooler summer temperatures and an extended growing season boosted the size of the country’s corn and soybean harvests, according to a new forecast issued last week by the U.S. Department of Agriculture (USDA). But strong demand, both export and domestic, will keep supplies relatively tight, experts said.

In its first World Agricultural Supply and Demand Estimates (WASDE) report since September due to the government shutdown, the USDA last week forecast a record 13.989 billion bushels of corn and 3.258 billion bushels of soybeans to be produced in the United States this year.

Chad Hart, grain market specialist at Iowa State University, said the forecast met ...