A long stretch of hot, dry weather has left the Mississippi River so low that barge companies are reducing their loads as shipping costs soar. That comes as Midwest farmers are preparing to harvest crops and send tons of corn and soybeans downriver to New Orleans. The transport restrictions are a headache for barge companies, but even more worrisome for thousands of farmers who have watched drought scorch their fields for much of the summer. Now they will face higher prices to transport what remains of their crops. About 60% of U.S. grain exports are taken by barge down the Mississippi to the Gulf of Mexico.