Agriculture Reference
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eastern Scotland, with the extent and evenness of water uptake and cell-wall breakdown
(modification) determined by tests on individual grain [68]. It was demonstrated that samples
of different varieties, grown at the same site, could be more similar in patterns of
modification than samples of the same variety, grown at different sites. The mixtures also
gave greater consistency between sites in malting performance than all but one of the
individual varieties. This resulted from a reduced effect of site and absence of the change in
ranking order that led to a significant genotype x site interaction, for malting quality
characters, in the individual varieties [68]. Wheat varieties are also subject to genotype x site
interaction for alcohol yield, as has already been shown (Fig 2).
Where barley mixture components differed significantly in quality, problems with
heterogeneity did occur [42]. A similar situation may be observed after processing in wheat
e.g. in breadmaking, where Sammons and Baenzinger [50] suggested that the quality of a
mixture was very similar to the mean of its components, so inclusion of a component with
very low quality would adversely affect the performance of the mixture. However, where
differences between components were less extreme, mixtures comparable to the higher
quality component could be identified [26]. Similarly, Manthey and Fehrmann [33] noted no
problems from the use of mixtures. Osman [44], working with organically-grown wheat,
detected a small advantage, in loaf volume, for a mixture compared to the mean of its
components. Most of these data were obtained from experimental plots, but mixtures and
monocultures have also been compared on a field scale [36]. Inclusion of one component with
slightly lower quality, within a four-component mixture, did not cause the mixture to have
significantly lower baking quality that the best component.
Wheat mixtures have also been accepted for grain distilling, on the conditions that they
are grown from accredited seed and achieve acceptable specific weights [69], and Swanston
et al. [66] noted that a mixture of three components was included in a commercial batch of
grain that passed through the entire distilling process without problems. A major difference
between wheat and barley for distilling is the absence of a malting stage and this appears to
accommodate some variation in quality amongst the components. The variety Deben gave the
highest grain yields of four varieties included in a field trial [66], but its alcohol yield was
significantly lower than that of the other three cultivars, Riband, Claire and Consort. A
mixture, comprised of Claire, Consort and Deben gave both grain yields and alcohol yields
comparable with the best individual variety and alcohol yields significantly higher than those
of Deben [66].
Mixtures provide the opportunity to extend the commercial lifespan of older, high-quality
varieties or to exploit newer varieties with good processing attributes, but agronomic
weaknesses that have precluded their recommendation [69]. The average value for alcohol
yield per hectare, obtained from such mixtures, is likely to exceed that achieved by high
yielding, but poorer distilling quality varieties. In addition, varietal mixtures tend to give
more stable grain yields between locations and seasons [17] and, as a consequence, should
show less fluctuation in nitrogen dilution effects and, thus, grain N levels. Wheat mixtures
should, therefore, be potentially attractive to the grain distiller, while the emergence of
several more varieties with good or moderate distilling quality, in the last few years, also
provides the opportunity to create more complex mixtures.
Newton et al. [40] demonstrated that the efficacy of varietal mixtures, in reducing the
spread of fungal disease and increasing yield, improves with complexity, i.e. a greater number
of mixture components. This was observed, particularly, in winter barley, where mixtures
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