GROWING WHEAT FOR HIGH ALCOHOL YIELD –HOMOGENEOUS AND HETEROGENEOUS APPROACHES - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 170

G ROWING W HEAT FOR D ISTILLING AND B IOETHANOL P RODUCTION
The response of wheat, in terms of grain yield, to increased application of nitrogen (N)
fertiliser has been studied fairly extensively, to enable optimal applications to be calculated.
Factors such as the timing of fertiliser application [13], the source of N used [14] and the
effect of the previous crop [74] have also been assessed, though essentially for their effect on
improving breadmaking quality. For distilling wheat, the key objective is to maximise alcohol
yield per hectare [61]. While grain yield is thus highly important, an increase of 1% in grain
protein content will reduce alcohol yield by about 6 litres/tonne [31], so a balance has to be
achieved. As an example, studies on the variety Istabraq [71] suggested that 184kg of N
fertiliser would have been required to maximise alcohol yield compared to 236kg/ha to
maximise grain yield, in the site and season in which the study was carried out. This
illustrates the potential for optimising resource use and profitability and the importance of
developing both the crop genetics and agronomy with alcohol production as a specific
objective.
The economic optimum for N fertiliser application may, however, be lower at times of
relatively low grain prices and high fertiliser costs [61] and the environmental impact of N
fertiliser application is also coming increasingly under scrutiny. Richards [48] assessed the
energy required to grow and process wheat for bioethanol production, in comparison to the
energy released on combustion of the fuel, i.e. the energy balance. In growing the crop, the
largest single contribution to energy consumption was the production of inorganic N fertiliser
(Figure 4), so reduction of fertiliser application would be a key element in enhancing the
energy balance for bioethanol production from wheat [65]. Additionally, Sylvester-Bradley
and Kindred [71] noted that most (around 75%) of the greenhouse gas emissions associated
with growing a wheat crop result from the use of N fertiliser - half from the use of fossil fuels
in its manufacture and the other half from emissions of nitrous oxide in the field. Limited
reductions in N application could be achieved without large adverse effects on grain yield
[28], while Vaidyanathan et al. [74] noted that the optimal N application for wheat was
reduced considerably when wheat followed rape or legumes, in a rotation, rather than another
cereal. More precise monitoring of the growing crop, in addition to the soil, combined with
variable rate fertiliser application [47] would improve the efficiency of N utilisation and
could also reduce fertiliser application. Swanston and Newton [65] calculated that an overall
25% reduction in N fertiliser would equate to an energy saving approaching 2000Mj/ha.
The other major contributor to energy consumption, in growing a crop, is vehicle fuel
use. Swanston and Newton [65] pointed out that fertiliser and fuel, together, accounted for
75% of the energy requirement. One field operation requiring a significant amount of fuel, i.e.
24% of total consumption [48], is ploughing, so reduced or minimum tillage (min-till)
appears to be a possible means of significantly reducing fuel use. A further advantage of
minimum tillage, particularly in some environments, can be reduced soil erosion [56], but, in
the absence of ploughing, requirements for weed control and, therefore, herbicide use could
increase [65]. Herbicides, along with other protectant chemicals, do not make a large
contribution to energy consumption [48] or greenhouse gas emissions [71] in growing a crop,
but may raise concerns about toxicity [75]. Comprehensive, prophylactic spraying regimes
are not likely to affect the safety of an end-product, especially when a crop is being cultivated
for non-food use, but may not be perceived as appropriate, if environmentally-benign claims