Two complementary strategies are required to replace outclassed cultivars. The first is to deplete the existing seed bank so competition against newer cultivars is reduced.
Success in depleting the sub-clover seed bank is determined by the level of hardseededness, along with the ability to break seed dormancy and prevent reseeding. If this can be achieved in three consecutive years, the seed bank of most outclassed cultivars will be greatly reduced or eliminated.
The level of hardseededness in outclassed and oestrogenic cultivars varies (Table 1). Cultivars with low hardseededness can be reduced in just two years, but this can take twice as long for harder-seeded cultivars.
While sub-clover has a natural level of hardseededness, this rate of breakdown and therefore germination can be increased. Deliberately increasing the rate of natural germination depletes the seed bank quicker if subsequent seed set can be prevented.
Hardseededness is broken down by fluctuations in soil temperature over summer and into late autumn. Removal of dry material along with cultivation, which cracks the seed coating allowing moisture to be absorbed, will increase germination.
Preventing flowering is a crucial step and can be achieved by removing the sub-clover in winter with herbicides, creating a grass-dominant pasture, grazing hard in spring to remove flowers and burrs, or applying herbicides in spring to disrupt seed set.5 Silage or hay is ineffective as burrs are usually below cutting height and oestrogenic compounds will remain in the conserved fodder.
A cropping or grass fodder phase, with herbicides to prevent seed set, are alternative approaches to achieve seed bank depletion.