Part F - Choosing the right sub clover variety

Importance of choosing the right variety

Good clover pastures contain a dense population of one or more   well-adapted varieties. With enough fertiliser and sound management, these   stands contribute to farm profit by:

• producing an abundance of high-quality feed for livestock;
• improving soil fertility;
• reducing soil erosion.

Poor pastures:

• contain more weeds;
• produce less feed from thin or patchy stands;
• lack vigorous growth;
• make little contribution to improving soil fertility;
• have low seed reserves.

The >most important characteristics to consider when choosing which   variety to sow are:

• oestrogen content;
• persistence and productivity;
• time of flowering and seed maturity;
• hardseededness;
• special adaptive features;
• resistance to diseases (see Part B) and insects (see Part C).

Table F1 lists the characteristics of current sub   clover varieties.

Oestrogen content

Clover disease is caused by moderate to high levels of the oestrogenic plant   substance >formononetin, and is an important cause of infertility in   sheep. Each year in New South Wales, clover disease costs the sheep industry   over $2 million. The main clover varieties responsible for this loss are:

• Dwalganup
• Yarloop
• Geraldton
• Dinninup.

Pastures must not be sown to these varieties, and old pastures with a   high proportion of oestrogenic varieties should be resown.

Superior low-oestrogen varieties are readily available (see Table F1). >Only varieties that have a low or very   low formononetin content should be sown.

Persistence and productivity

In the long term, the most productive varieties are those that persist   reliably in pastures. Persistent varieties set enough seed each year to produce   a dense crop of seedlings the following autumn. Winter growth is greatest in   pastures with a high seedling density (more than 500–1000 plants/m2).   Pasture growth in spring is less dependent on density, and is greater from   late-maturing varieties rather than early varieties. But these late varieties   may fail to set seed in dry environments or if rainfall in late spring is   inadequate. It is important to match maturity of the cultivar with the length of   the growing season to achieve good persistence.

Time of flowering and seed maturity

Sub clover varieties flower at various times (see Table F1). Very early strains (Nungarin, Dalkeith)   begin flowering as early as late July; late varieties (Goulburn, Denmark, Leura)   do not start flowering until September or early October. Flowers are produced   over a period of about 6 weeks, sometimes longer in good seasons. A further   4 weeks is necessary to complete seed development.

For high seed yields, good soil moisture is needed for 10 weeks from   the start of flowering. Choose a variety that flowers and completes seed   development before soil moisture runs out, which usually occurs in spring.

The pasture mix should contain the latest maturing variety that persists and   sets seed reliably in the environment in which it is to be grown. Varieties that   set enough seed to persist well along the drier western fringe of the clover   belt in NSW are those that mature earliest. Late-maturing varieties produce the   most feed in spring, but they need a lot of soil moisture late into spring.   Where spring rainfall is erratic, it is advisable to mix an early strain and a   later maturing strain. The short-season variety provides stability of   production, while the longer season variety can take advantage of good seasonal   conditions and grow extra spring feed.

Importance of hardseededness

Hard seeds are impermeable to water and will not germinate until the seed   coat becomes permeable. Most sub clover seeds are hard shortly after maturity,   particularly if moisture conditions at maturity do not restrict development of   the seed coat. During summer, some of these hard seeds ‘break down’ (become soft   or permeable to water) in response to high temperatures and day–night   temperature fluctuations. These eventually cause the seed coat to split. If it   then rains, the softened seeds will germinate.

Hard seeds break down most quickly during hot summers, particularly if held   in burrs on the soil surface. Hard seed breakdown is slower where there is a   large amount of surface residue, and in cooler tableland environments. Varieties   differ in their resistance to hard seed breakdown (see Table F1). Seed of later maturing varieties such as Woogenellup rapidly becomes soft, leaving   very little residual hard seed by autumn.

By contrast, hard seed of the varieties Nungarin and Dalkeith breaks down slowly. Hard seed also   protects the stand against depletion of the seed bank when summer storms and   false autumn breaks cause a succession of early germinations without follow-up rain. In such circumstances, young seedlings will die after a few days of hot   weather. Hard seed also provides a bank of seed capable of germinating after a   year when seedset has failed or has been very poor.

Thus varieties with >high levels of hard seed are necessary in central and   southern NSW, where summer rainfall is common or where there is a strong   probability of seedset failure. Where early autumn and late spring rainfall is   reliable, satisfactory regeneration and good early growth will come from   varieties which have less hard seed.

In >northern NSW, hardseededness is less important, as softer seeded   varieties may germinate on summer rain and survive until more reliable growing   conditions in late autumn.

Burr burial can also influence seed size and the rate of hard seed   breakdown. Buried burr tends to be larger, have a higher viability and become   soft more slowly. The amount of seed buried is influenced by the choice of   variety and by grazing management. Close grazing until the onset of flowering   encourages burr burial, while lenient grazing, which allows the stand to become   very leafy, results in a higher proportion of burr being set on the soil   surface.

Special adaptive features

Waterlogging

Sub clover prefers well-drained soils but will tolerate temporary   waterlogging. Varieties which belong to the yanninicum subspecies that   have cream or light brown seed, such as Trikkala, Gosse, Riverina, Larisa, Napier and Meteora (which are low in dangerous oestrogens)   and Yarloop (highly oestrogenic), are better   adapted to waterlogged or poorly drained conditions than other sub clover   varieties.

Soil pH

Sub clover will grow at a pH(CaCl2) as low as 4.2,  but growth   will be reduced by up to 30% and rhizobia survival will be poor. Most sub   clovers prefer slightly to moderately acid soils, pH(CaCl2)   5.0–6.0.

Clare, Rosedale, Antas and Nuba are varieties of the subspecies brachycalycinum. They grow better on neutral to alkaline soils and grey,   cracking soils than other sub clover varieties. This is due in part to their   greater tolerance of low levels of available zinc, a common feature of such   soils, but also due to their burr burial characteristics.

Sub clover selection guide according to NSW climatic   zones

Figure F1. Map of NSW showing climatic zones and isohyets

Isohyets show   average annual rainfall (mm) for stations with more than 20 years of   records.

Key:

Source: Bureau of Meteorology, Sydney.

Now refer to Table F2 below for the selection of sub clover variety according to the climatic zone.

Table F2. Selection of sub clover variety by NSW climatic zone

Further information

For further information on subterranean clover, see the other sections of this Agfact:

• Part A  Subterranean clover in New South Wales - identification and use
• Part B  Diseases of sub clover
• Part C  Insect pests of sub clover
• Part D  Viruses in sub clover
• Part E  Sub clover varieties and their identification
• Part G  Establishing sub clover