New analysis by researchers from the International Council on Clean Transportation in the journal Biomass and Bioenergy suggests that estimates for potential energy crop yields from some widely cited research studies may overstate those yields by as much as 100%. The findings could have significant implications for long-term renewable energy targets.

The ICCT study examined reported yields of five important energy crops (Miscanthus, switchgrass, poplar, willow, and Eucalyptus). The authors found that the highest predicted yields, and associated expectations of how much biomass could be grown for energy, could not be supported by an overview of studies in this field.

A handful of studies on biomass production report very high yields, but the ICCT found that these were all extrapolated from very small experimental plots, intensively irrigated and weeded, and carefully hand-harvested. These are conditions that would not be replicable at commercial scale. Studies that grew energy crops over larger areas and used conventional harvesting techniques have shown much more modest results, but these more realistic experiments are not always the ones highlighted by bioenergy researchers.

Not only are commercial-scale energy crop yields lower than often thought, but they are not likely to improve rapidly over time. The ICCT note that agricultural practices and achievements in plant breeding or genetic modifications that have increased yields of food crops over the past several decades, like intensive fertilization and increasing the ratio of grain to straw, generally do not work on energy crops. Miscanthus, for example, often considered a top candidate for large-scale biomass production, is a hybrid between two types of plants and cannot reproduce by seed, which significantly slows down research.

The authors stressed that their findings should not be interpreted to mean that policies supporting cellulosic biofuel are misplaced.

We need more, not less, support for cellulosic biofuels right now to get the industry going,. There is an important place for cellulosic biofuel in reducing greenhouse gas emissions from transportation, but that makes it all the more important to be realistic about how much biomass can be produced.