| Abstract | The project BSS25 ?Breeding of clones with high early sugar content? concluded that the potential for increasing CCS through breeding and selection was greatest early in the season. BSS25 commenced a recurrent selection program with short generation interval aimed mainly at population improvement. The aim of BSS93 was to continue a recurrent selection program for early CCS and to assess the realised genetic gain made in the previous project.At the start of BSS93, BSES had changed its selection program from family assessment in clonal 4-sett plots to family assessment as original seedlings. The recurrent selection program for high early CCS reflected this change. Twenty families with high early CCS parents were selected for planting in New South Wales, southern, central, Burdekin, Herbert and northern regions from 1993 to 1996. At each location, the best 600 out of 1 200 seedlings (based on visual appearance) were sampled in May and June in the following year for CCS. The best 10 clones, based on mean CCS, were selected as parents and sent to Meringa for further crossing. Two hundred clones in total were selected as parents. The 10 parent clones and up to 10 additional clones were selected for testing in Clonal Assessment Trials. A total of 377 clones were selected over the duration of this project. Of these 377 clones, 107 clones were derived from families with at least one recurrent parent from the previous project. Good performing clones from this stage were promoted to advanced selection stages. A number of clones from both the current and previous projects have performed well in advanced trials. To date, two varieties have been released, Q185A (central region) and Q205A (southern region).Replicated trials were planted in the southern, central and northern regions to assess the genetic gain realised in the selected clones from the previous project (BSS25). Parents and elite (selected) clones from the families tested were included along with a base population (a group of 29 randomly selected clones from the breeding population) and a core population (a group of 30 clones from core selection programs with known high early CCS). Trials were sampled for CCS in May and June in plant and first-ratoon crops. Mean CCS was calculated and the various populations were compared.At all locations, the parent population had significantly higher CCS than the base population, and the core and elite populations had significantly higher CCS than the parent population. At Bundaberg, the elite population had significantly higher CCS than the core population, but there were no differences in mean CCS between these two populations at Mackay or Meringa.In terms of realised genetic gain, at Bundaberg both southern parent and elite populations showed steady gains from 1987 to 1991, averaging about 0.26 unit of CCS per year. There were no indications of a decrease in variability in these populations and it was concluded that it was likely further genetic gains would be sustained in the future.At Mackay, the central parent populations showed a modest but somewhat inconsistent improvement over the period and this was repeated for these populations tested at Bundaberg and Meringa. The central elite populations showed good improvement for the first 3 years, but this was not sustained over the subsequent 2 years. Extremely difficult selection environments (flooding and extreme moisture stress) impacted on the clonesselected in the final two elite populations and may explain this decline. It was difficult to come to a firm conclusion on continued genetic gain for the central region.At Meringa, the northern parent populations showed a small, but significant improvement over the 4 years of about 0.13 units of CCS per year. However, the northern elite populations showed no improvement over this period. This was not expected, as the parent populations showed a fairly steady improvement. Interestingly, good improvement was shown by the northern elite populations (first 3 years only) when grown at Bundaberg (0.39 unit CCS per year) and Mackay (0.21 unit of CCS per year). It is difficult to explain these results, but it may indicate that the wet tropics pose some unique difficulties in breeding and selection for high early CCS. |
