>>>Does ethanol have to be produced from corn?
No.
Many other things can be used to make ethanol - even the left-overs from growing corn...Iowa State is working on equipment that catches & bundles what's left after the combine harvests the corn kernels to be used in producing ethanol.
Yeah 5500 lbs of the leftover produces all of 180 gallons of ethanol. The tradeoff is more erosion of topsoil and reduced future yields.