Government estimates for next fall’s corn supply held steady Thursday, a factor that should keep food prices high in the new year.  The U.S. Department of Agriculture estimates there will be 846 million bushels of corn on hand at the end of the summer. The forecast was mostly unchanged from last month’s estimate.  The surplus would satisfy demand for less than 25 days. A 30-day supply is considered healthy.  A low supply of corn pushed food prices higher last year because corn is a key ingredient in everything from soda to cereal to animal feed.   Still, corn futures fell 40 cents a bushel in morning trading, the maximum allowed by trading boards. That’s because traders were expecting a decline in the monthly estimate.