Evolution of subterranean clover in South Australia. I. The strains and their distribution

Abstract

The distributions of the strains of subterranean clover were determined from the presence of burrs in wool collected at the Adelaide Wool Sales. Of 7600 samples examined, nearly 1300 contained viable seeds. This seed was grown, the strains were identified, and their distributions in South Australia were mapped.

Subterranean clover was found in wool from most areas with more than 400 mm of rain. However, it was absent from the climatically favourable Yorke Peninsula, and it seems clear that distribution of subterranean clover in South Australia is limited by the presence of alkaline soils.

Divergent strains are widespread in South Australia. In all, 435 divergent strains were discovered from 325 farms. The presence of these strains leads us to believe that genetic change in subterranean clover, previously thought to be genetically stable, has been brought about by infrequent outcrossings, mainly in mixtures of cultivars. We discuss evidence for this proposal, and the consequences that it has for seed certification schemes.

The distribution of flowering time amongst the divergent strains differed from the cultivars. Many of them flowered about half way between the mid-season cultivar Mount Barker and the early-flowering cultivar Dwalganup. There was no strain later than Tallarook, the latest-flowering cultivar, and very few earlier than Dwalganup. Flowering time of the divergent strains was weakly related to the length of growing season of the farms which produced the wool. Strains were also discovered which were morphologically identical with a cultivar but which flowered either earlier or later.

The first-released cultivars, Mount Barker and Dwalganup, were the most widely distributed of all strains. Indeed success, in terms of the number of seeds of a cultivar found in the wool, could be broadly related to the number of years since release. The data illustrate the significance of the early-flowering cultivars on the distribution of subterranean clover, but indicate that, apart from T. brachycalycinum, there is little need in South Australia for yet earlier cultivars.