"...Corn has long been king in Iowa, the nation's top corn-producing state, implanting in Iowa voters a sentiment that every candidate must cheer Washington's backing for ethanol. Since 2011, though, that universal backing has been eroding. Congress decided at the end of 2011 not to renew a tax credit that cost the government $6 billion a year. Critics of the government’s ethanol policy then set their targets on the ethanol mandate, which requires refineries to blend an increasing amount of biofuels into the U.S. gasoline supply each year. At the ethanol summit Tuesday, Mr. Trump also read a prepared statement...