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In the second study, a more recent group of 12 sugarcane cultivars was examined in the same(Herbert) region in 1995–1998. Each cultivar was evaluated at between four to six sites across plant and first ratoon crops. The trial design at each site was a randomised block design with two replicates and plots four rows 10 m. Cane yield and CCS weredetermined using procedures described by Jackson and Hogarth (1992). The broadly adapted cultivar,Q124, which was the dominant commercial cultivar at the time of the study, was included in all trials. In the Herbert region, large variation due to genotype X environment (GE) interaction is common, and some varieties are recognised as being adapted to specific sites but not to others. To compare varieties only at sites to which they are suited, only data from the three sites where each cultivar performed the best were considered. Data were expressed as a percentage of Q124 (Fig. 2, top). In this experiment, no significant differences in trends between the plant crop and ratoon crops were observed, and the results shown in Fig. 2 are averages across all crops. The results provide evidence for ongoing improvement in sugar yield in newer varieties. However, there was no difference in CCS between older and newer varieties, so that as with the first study above, improvements arose through gains in cane yield.A third study was reported by Wallis (2001), of 15 cultivars of varying ages established in a trial near Atherton, Qld. Results in only the plant crop were reported. These results again demonstrate a significant trend for improved cane yield arising in the newer varieties (Fig. 2, bottom). However, for clones bred after 1960, there was no evidence for any improvement in CCS.The above three studies present a consistent picture of higher contributions to sugar yields by breeding in the last three to four decades from improved cane yields than CCS. In all three studies, it appears that there is a plateau in terms of CCS improvement.Possible reasons for this are explored later in this paper. The first study presented also clearly shows evidence for dramatic improvements in ratooning performance between old and new varieties. This effect was not clearly apparent in the second experiment in the Herbert, which featured varieties between the 1960s and 1990s.
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