For biofuels, the future won’t look much like the past. We’re heading, some believe, for a post-ethanol age.

Today, nearly all plant-based liquid fuels are either used to make ethanol, which is blended with gasoline, or biodiesel. But efforts to increase the amount of ethanol in gas are opposed by auto makers and others who say the environment and economy are better served by more efficient engines and the shift to hybrid, electric and natural-gas vehicles. There are critics, too, who say making biofuel from edible plants—most ethanol is based on corn or sugar cane—is a poor use of land and crops needed to feed growing populations.