Des Moines Register: Biodiesel production has fallen to 2006 levels as producers struggle to cope with low oil prices and the imposition of stiff European duties that have cut off a key market for the fuel additive. Production fell to 30 million gallons in March, down more than 40 percent from April 2008. The Energy Department has predicted production will fall 20 percent this year, but that figure could be more. At current levels, production could be half what it was last year, according to the …