U.S. old-crop corn ending stocks are 32 percent larger than a year ago, surging to a 30-year record of 2.295 billion bushels, according to the U.S. Department of Agriculture’s (USDA) quarterly Grain Stocks report last week.

Even with the huge supplies, corn prices rallied after the number came in below the trade’s pre-report average estimate of 2.35 billion bushels. Stocks were also reduced from the USDA’s estimate in the Sept. 12 World Agricultural Supply and Demand Estimates (WASDE), which pegged ending stocks also at 2.35 billion bushels.

Old-crop soybean ending stocks, at 301 million bushels, also came in well below pre-report estimates of 338 million bushels. They were also down from the Sept. 12 WASDE ending stocks estimate of 345 million...