Until now different biofuel crops have usually been compared by looking at their fuel yield for each unit area. For example it is widely reported that the productivity for each hectare of maize-ethanol is roughly half that of sugarcane. It is a simple way of conveying the complex relationships between the starch, sugar and oil content of different crops, and how they translate to potential fuel yields. However, the way that these yields have been presented is misleading.

“The yield tables have been communicated in an optimistic way, using the top producers in the world and the best plots of land. These yields are not necessarily possible elsewhere,” Matt Johnston, from the University of Wisconsin, Madison, US, told environmentalresearchweb.

Climate, soil conditions, agricultural technology, as well as social, political and economic conditions can all influence crop yields. In particular farmers in developing countries, who don’t have access to large farm machinery or expensive fertilizers, are unlikely to reproduce the high yields currently seen for biofuel crops in Western Europe and North America. To remedy this failing, Johnston and his colleagues have used the best available global-agricultural census data to develop a new biofuel yield analysis.

The M3 cropland database (created by the Universities of Madison, Minnesota and McGill), used 22,000 different census reports from around the world to create the most detailed global map of crop area and yield available. Satellite data was used to fill any gaps in the census data. Crop area and yield statistics are given at the scale of 10 km grid squares, for 175 crops in 238 different countries. Also, 10 ethanol and 10 biodiesel crops were included.

Johnston and his colleagues used this data to calculate yield-distribution plots for biofuel crops in all of the different countries. Unlike previous yield tables, which have estimated the maximum yield, this method revealed the median yield and its upper and lower quartile bounds.

Writing in the journal Environmental Research Letters the scientists say that, “for most crops, previous reports have overestimated yields by 100% or more. Barley, cassava, castor, maize, rapeseed and sunflower all show that previous global biofuel yields were overestimated by at least 100%, with wheat–ethanol and groundnut–biodiesel estimates having been overestimated by 150% or more.”

By providing more detailed information about the potential yields of biofuels in different countries Johnston and his colleagues hope that countries will be better placed to decide where they should prioritize their efforts. “It will help them to decide which crops to grow and how to encourage production for those crops holding the most realistic potential,” says Johnston.