The U.S. Department of Agri­culture (USDA) raised production estimates for corn and soybeans in its September crop production report last week, surprising farmers and grain traders for the second month in a row.

Based on conditions as of Sept. 1, U.S. corn yields are expected to average 169.9 bushels per acre, up 0.4 bushel from the August forecast. Soybean yields are expected to average 49.9 bushels per acre, up 0.5 bushel...