Energy efficiency of potato production practices for bioethanol feedstock in northern Japan

In 2007–2009, field experiments were conducted to identify agronomic practices affording the lowest energy inputs (i.e. total energy inputs from fuels and other agricultural material inputs required to produce 1L of ethanol) under potato-based bioethanol feedstock production in northern Japan. On a hectare basis, for a standard 4.4m−2 planting density, conventional practices [two inter-row cultivations (weeding and preparation for ridging) and final ridging] yielded an estimate of 4.85kLha−1, representing an energy input of 5.86MJL−1. The energy input savings arising from the lesser fuel consumption associated with fewer tractor operations under no- and low-ridge cropping practices were outweighed by a reduction in ethanol yields, resulting in slightly greater energy inputs (6.09±0.65 and 5.89±0.30MJL−1, respectively). Similarly, poorer ethanol yields outweighed the reduction in energy inputs arising from lessened seed potato production-associated energy inputs under lowered planting densities of 3.8 and 3.3m−2, resulting in ethanol yield-based energy inputs of 5.98±0.33 and 6.01±0.41MJL−1, respectively. Omitting fungicide applications significantly lowered biocide-related energy inputs, but yielded 20 and 63% lower ethanol yields for Phytophthora-resistant and -susceptible genotypes, respectively, substantially worsening energy efficiencies (6.24±0.42 and 12.2±6.3MJL−1). In northern Japan, use of high starch-yielding genotypes served as the only way to increase ethanol yields and improve energy efficiency for potatoes used in bioethanol feedstock production. A 29% greater ethanol yield (6.26±0.46kLha−1) and 21% better energy efficiency (4.63±0.23MJL−1) were achieved by replacing the standard potato cultivar with a high starch-yielding variety. The yield-based energy inputs with a high starch-yielding potato variety were significantly lower than those with conventional sugar beet in northern Japan (5.82MJL−1).