Comparison of the energy performance of fibre sorghum, sweet sorghum and wheat monocultures in northern Italy

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Four monocultures (fibre sorghum, sweet sorghum and wheat at high and low nitrogen doses) were compared at a field-scale over 3 years (1997–1999) in terms of net energy, net energy ratio and energy use efficiency. Two nitrogen fertilisation levels were assessed for wheat (80 and 120 kg ha −1 of N) to evaluate the energy use efficiency of the applications. For all the crops, fuels, lubricants and farm inputs together formed around 92% of the total input, mainly due to nitrogen fertilisation. A year×crop significant interaction was found for all considered parameters. In fact the monoculture did not lower the biomass yield of both sorghum types, while it involved a drop in wheat yield starting from the second year (third considering that wheat was also cultivated in the same field in 1996). The average (1997–1999) net energy supplied by the monoculture of sweet sorghum was 17, 40 and 50% higher than those of fibre sorghum and wheat at high and low nitrogen doses respectively. The energy use efficiency (EUE, i.e. the energy (MJ) requirement to produce a kg of dry matter) ranged from 0.78 to 0.96 for fibre sorghum, from 0.69 to 0.85 for sweet sorghum and from 1.00 to 1.23 and 0.91 to 1.33 for wheat at low and high nitrogen levels respectively. With ethanol as the end-product of the 3-year monoculture of sweet sorghum, the production process would be a bit less favourable in term of energy balance: the net energy yield would be 90% of that obtained by dry matter gasification (with an efficiency of 50%). If straw was not processed, ethanol from wheat was generally unfavourable. On average of the two nitrogen doses, net energy ratios were 0.99, 1.05 and 0.97 in the first second and third year respectively. If bagasse was not considered also sweet sorghum had a very low net energy ratio, but always higher than wheat (1.14, 1.12 and 1.24 over the 3 years).