Tropical Maize and Lipid Cane As Sustainable New Bioenergy Crops

Two new sustainable bioenergy crops have been recently developed for the US.  One is Tropical maize, a high-biomass, high-sugar corn hybrid that accumulates sucrose in the stalk and produces negligible grain.  Second one is Lipid cane, a sugarcane engineered to produce non-food oil, as drop-in fuels, in place of sugar.  Both these crops present excellent potential to serve as a renewable fuel crops.

Tropical maize can produce large amounts of biomass (9-11 ton/acre, dry weight) and accumulate high levels of sugar (10% sucrose) when grown without supplemental nitrogen (N).  Theoretical ethanol yield from combined plant components is very impressive: 1500 gal/acre when grown with supplemental N (180 lb/acre) and 1175 gal/acre when produced without supplemental N.  Additionally, tropical maize is well-adapted to many regions of the U.S., unlike sugarcane.  Because tropical maize grows vigorously with little or no N fertilizer, it also provides excellent nitrogen use efficiency, making it very positive from an agricultural sustainability perspective. The subequatorial origins of tropical maize’s genetic background provide the photo-period sensitivity trait.  When grown under the short-night environment of our Midwestern latitudes, tropical maize displays delayed flowering and remains in the vegetative state much longer than commercially grown U.S. corn hybrids, resulting in very tall (15 ft.) plants that produce little, if any, grain.  Reduced grain production is offset by accumulation of sucrose in the stalk and also decreases the need for nitrogen fertilizer. 

Lipid cane is a crop suitable for land in the South Eastern US that is marginal, or unsuited, for food crop production.  At the current yields of sugarcane in the SE US, this would produce about 33 barrels of oil per acre, compared to about 1 from soybean.  By increasing the photosynthesis, even higher barrels of oil per acre can be produced.  Sugarcane is far less demanding on soils and fertilizers than food crops in general, and can be grown on land unsuited to food/feed crops.  The US south-east has large areas of land that have dropped out of food and fiber crops agriculture, a decline that continues to this day.  Yet this area receives high rainfall, sufficient to avoid the need for irrigation, and the long growing season maximizes the amount of sunlight these crops can capture over the year.  By modifying the plant’s own triaclyglyceride (oil, TAG) pathway to up-regulate synthesis in the mature stem and down regulate consumption, so causing accumulation.  TAGs, in lipid cane are similar to those of soybean and can be easily converted to biodiesel.  Preliminary estimates indicate that using the 23 billion acres of marginal land in the SE US that is not in food production, more than 50 billion gallons of oil could be produced with these crops.  Fermentation data of tropical maize and techno-economic evaluation of lipid cane as biofuel crops will be presented.