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5. Accounting for effects of extraneous matter on commercial cane sugarIn all results presented in the previous section, as with selection trials in sugarcane breeding programs to date, measurements of CCS were made on samples of sound stalks taken by hand. However, CCS of sound stalks is not the only important factor affecting sugar content of cane supplied to mills. Cane is harvested mechanically in many countries, and therefore,commercial cane sugar content of cane delivered to a sugar mill depends on both the sugar content of the sound stalks and the amount of extraneous matter and non-sound stalk component (see Berding et al., 2005 in this issue). The latter fraction can comprise tops,trash, dead stalks, and suckers, which may comprise a significant portion of cane supplied to mills (e.g.20%), especially with mechanical harvesting of green cane (Crook et al., 1999).Genotypes are known to differ in traits that affect the non-sound stalk component of cane supply, such as lodging propensity and suckering propensity. Genetic variation in these traits may be of considerable value but in many cases are not well accounted for in selection systems in breeding programs (Jackson et al.,2000; Singh et al., 2000). McRae et al. (1998) proposed the use of a billet sampling device to be used in association with mechanical harvesting of selection trials to obtain an unbiased sample of CCS. However,McRae et al. (1998) also showed, in the trials they examined, that the relative ranking of clones did not differ greatly for measurements made based on samples of billets and measurements based on hand sampling of sound stalks. This conclusion partly reflected the high level of genetic variation for CCS in sound stalks in the study. However, in considering varieties close to commercial release, differences between such varieties and standard cultivars in sound stalk CCS are likely to be smaller, and differences in the non-stalk component may assume greater relative importance in determining ranking. Therefore, for final stage evaluation of promising experimental clones, an unbiased methodology, such as a billet sampling proposed by McRae et al. (1998), is morelikely to be of value than in earlier phases in the selection process.
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