Choosing the right cultivar
Tables 1 and 2 show agronomic data for ssp. subterraneum and ssp. yanninicum cultivars suitable for use in WA, while Table 3 shows data for ssp. brachycalycinum cultivars. Outdated cultivars and brands not registered with the Australian Seeds Authority are excluded. The following factors should be considered when choosing the most appropriate cultivar for sowing.
Select the best adapted subspecies for the paddock to be sown. A mixture of subterraneum and brachycalycinum subspecies may be appropriate for paddocks with variable drainage.
Flowering time is the most important character that determines cultivar persistence in an environment, as it determines the amount of seed that can be set. The most appropriate flowering time is a balance between producing high quantities of feed, and setting high levels of seed. Later flowering cultivars produce more spring feed, whereas earlier flowering ones can set more seed before the summer drought. Low rainfall, short growing-season environments require early flowering cultivars for persistence, whereas high rainfall, long growing-season environments should be sown to later flowering cultivars.
Mixing two or more cultivars with up to two to three weeks flowering time difference is one way to account for seasonal differences. Earlier flowering cultivars will set seed in seasons that finish early, while later flowering cultivars will be able to take advantage of favourable seasons.
Some older cultivars contain high levels of the compound, formononetin, which can cause infertility and difficult birthing in ewes. All newer cultivars have been selected with formononetin levels ≤ 0.2% of dry matter, giving them a low potential for oestrogenic problems. Two other less important oestrogenic compounds, genistein and biochanin A, are also present in subterranean clover, but are of less concern to graziers.
Cultivars vary in the proportion of seeds that remain hard (dormant) in the winter following seed set. Cultivars for lower rainfall areas and those used in crop rotations need to be hardseeded, to enable them to regenerate reliably after a year of little or no seed set. For example, the greater hardseededness of the new cultivars Tammin and Forbes allow them to persist better through crop rotations than less hardseeded cultivars. On the other hand, higher rainfall environments need less hardseeded cultivars, as their seasons tend to be more reliable. In these areas seedling germination needs to be high to enable subterranean clover to compete with other pasture components.
It should be noted that most subterranean clover cultivars are among the least hardseeded of the annual pasture legumes. This means they are poorly adapted to intensive crop rotations, such as 1 pasture:1 crop. The new, more hardseeded cultivars, Tammin and Forbes, are better adapted, but other more hardseeded species, such as biserrula (Biserrula pelecinus), bladder clover (Trifolium spumosum), yellow serradella (Ornithopus compressus) and annual medics (Medicago species) are well suited to such rotations.
Several diseases can affect subterranean clover, particularly in high rainfall areas. Clover scorch, caused by the fungal pathogen Kabatiella caulivora, is the most important leaf disease. Two races are known, with Race 1 being the most widespread and Race 2 being confined to the Esperance sand plain. Most of the newer cultivars aimed at medium and high rainfall areas have some resistance to both races. A range of other leaf diseases can affect subterranean clover, including leaf rust (Uromyces trifolii-repentis), powdery mildew (Erysiphe polygonii) and leaf spots caused by Cercospora zebrina and Pseudopeziza trifolii. Screening of advanced breeding lines is conducted to ensure cultivars released onto the market are not highly susceptible to these diseases. Several fungi cause root rots of subterranean clover. Of these Phytophthora clandestina is the most important and screening has been conducted to select cultivars with some resistance to the most important races. A range of viruses can also affect subterranean clover.
Redlegged earth mite resistance
Redlegged earth mites (RLEM), Halotydeus destructor, are the major pest of subterranean clover, particularly at the seedling stage. All older cultivars are susceptible to RLEM as seedlings. However, the new cultivars, Rosabrook, Narrikup, Bindoon, Tammin and Forbes, have been released with increased cotyledon resistance to RLEM. Field studies have shown these cultivars have higher seedling densities in regenerating pastures than similar older cultivars, resulting in increased autumn-winter production. However, the resistance is not absolute and pesticides should be used at sowing to ensure good establishment and in regenerating pastures when RLEM densities are high.