Iowa lawmakers kept up the heat on Trump administration officials last week over the practice of providing so-called hardship waivers that exempt refineries from blending bio­­fuel to comply with the Renewable Fuel Standard (RFS).

In hearings with Scott Pruitt, administrator of the Environmental Protection Ag­­ency (EPA), and U.S. Agriculture Secretary Sonny Perdue, Iowa lawmakers sharply questioned the waivers, which have reportedly slashed the EPA’s 15-billion-gallon volume requirement by 1.5 billion gallons. Those waivers, renewable fuel advocates say, effectively gut the...