Apparently from a recent article in Science magazine.
I saw similar statements in Scientific American.
• We use about one litre’s worth of fossil fuels to grow, harvest, process and transport a litre of corn-base ethanol.
• Substituting just 10 % of fossil fuels to biofuels for all vehicles would require about 40% of the entire cropland in Europe and North America. That is simple not sustainable.
So ethanol from corn is likely a red herring.