Agriculture Reference
In-Depth Information
and Swanston and Newton [64] also observed convergence in the expression of certain traits
among contrasting mixture components. The 6-row variety Morex, grown as a component in
a mixture, with two 2-row varieties, produced fatter grain, with a higher thousand grain
weight, than when grown, in monoculture, in the same trial (Figure 5). Recent work [41] has
shown that certain varieties, or combinations of varieties, appear to exert a much stronger
influence on the performance of mixtures than do others. Conversely, some varieties appear
to respond differently within mixtures, compared to monocultures [41] and this may, at least
in part, explain the greater stability of mixtures across sites [68]. Statistical techniques have
been developed, to identify components that operate in the most synergistic way [32], [41]
and a future trialling system, which assessed new varieties for their performance within
mixtures, could be highly beneficial within arable crops.
B
IOMASS AND
S
ECOND
-
GENERATION
B
IOFUELS
Richards [48] noted that the energy balance, associated with the use of wheat as a fuel
source, improved greatly when energy was derived from the whole crop. Straw can be burned
to produce heat and/or electricity and its potential use in this area has been considered for
more than a decade. Culshaw [12] noted that the low sulphur and high calcium contents of
straw, compared to coal, reduced emissions of acid gases on combustion. A number of small,
combined heat and power plants have been constructed in Denmark. These were straw-
burning, with a typical annual consumption of 40,000 tonnes and a capacity of 9 MW of
electricity and 20Mj of heat [8]. This could supply the requirements of two small towns, with
a combined population of 14,000 inhabitants [8]. Culshaw [12] also noted the potential for
electricity generation, citing the building of a 31 MW capacity power station, in eastern
England, with a projected annual requirement of 180,000 tonnes of straw.
Straw has also been considered as a potential feedstock for bioethanol production.
Shepherd [58] suggested that cellulose-based feedstocks were, potentially, the least
expensive, but, technologically, the most complex, in terms of extracting the sugars for
fermentation. Keller et al. [29] indicated that a priority was to reduce the severity of acid
hydrolysis required in the initial stage of cellulose breakdown. A further problem is that
ligocellulosic feedstocks, such as straw, contain hemicellulose in addition to cellulose and
microorganisms capable of converting the full range of 5- in addition to 6-carbon sugars,
occurring as breakdown products, are required [56]. Schubert [54] reviewed many of the
current research and development projects in these areas, which included improved enzymes
for cellulose breakdown and a range of micro-organisms for fermentation, including
thermophylic bacteria as well as pathogenic species, such as
E.Coli
, which have been
genetically manipulated.
If wheat is to be widely utilised for fuel, in addition to potable alcohol, conversion of
cellulose and hemicellulose is likely to be necessary. Shepherd [58] suggested that a 5%
replacement of petrol in the UK would be possible if a quantity of wheat grain, equivalent to
that currently exported as surplus to domestic requirements, was converted into ethanol.
However Sylvester-Bradley and Kindred [71] noted the concerns being expressed about the
diversion of food crops into non-food uses, especially at times when global grain reserves are
low and, as a consequence, food prices are escalating. Since the late 1940s, wheat breeders
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