An investigation of breeding methods applicable to Tasmanian-grown pyrethrum

Pyrethrum is a perennial daisy that is predominantly out-crossed. It is grown commercially in Tasmania for pyrethrins, which are extracted from the flowers and are used in the preparation of insecticides. A pyrethrum breeding program was conducted by the University of Tasmania from 1978 to 1998, with the aim of developing varieties suitable for Tasmanian growing and production systems. The primary selection character was pyrethrins yield, and the program consisted of population improvement through recurrent selection for two indirect selection characters; visually-estimated flower yield and the UV-assay of pyrethrins at a specific maturity stage (open but not over-blown). Varieties, consisting of either clones or single crosses between two clones, were selected from each successive recurrent selection generation. This study was undertaken to evaluate the selection methods used for the University's breeding program from the period of 1985 to 1998, and to assess the impact on breeding methods of the change in the mid-1990's from the establishment of the crop by clonal splits planted at a contant spacing (0.5 m) to establishment by direct-drilling at a somewhat higher average plant density. Data records for the two main selection characters were analysed in order to identify genetic changes in the breeding population. This indicated that there had been a genetic gain in UV-assays but not in flower yield. The efficiency of direct selection for first year yields in the Univeristy's base population was compared with indices of the indirect selection characters and yield components. A trial was established to obtain estimates of genetic parameters to construct selection indices and predict selection responses. Estimated heritability was moderate for pyrethrins yeld (0.26-0.39) and for the yield components of pyrethrins content (0.24-0.34) and flower yield (0.17-0.30), and low for percent dry matter content (0.00-0.15). Estimates were also obtained for two product quality characters, both derived from the ratio of the six different esters that form the active product in the pyrethrum extract. Heritability for these character was moderate to high. The effect of planting density on selection for pyrethrins yield was also investigated. The index of the two indirect selection characters was the most efficient method for single-plant selection in the planting densities used commercially from 1980 to 1995. However, an index of component characters was more efficient for the densities currently used in commercial areas. There was evidence for a genotype-density interaction for flower and pyrethrins yield but not for percentage pyrethrins or the ratios of pyrethrins esters. The potential to reduce the length of the period of obligate vegetative growth through breeding was also assessed. There was evidence for significant level of additive genetic control for this character and it was concluded that the vegetative period could be reduced by recurrent selection. The performance of new synthetic varieties were evaluated and the relative merits of varieties derived from a single cross of two clones and polycross of several clones were discussed.