Pastoral and Veterinary Institute, Private Bag 105, Mt Napier Road. Hamilton. Vic. 3300
Subterranean clover is the most important pasture legume in southern Australia and is grown on an area of around 20 million ha. This species, with a wide range of maturities and three subspecies adapted to different soil types, can be used successfully in permanent pastures and in rotations with crops. Its success relies on producing a seed bank, a prostrate growth habit and seed burial. Incremental improvements with subterranean clover will continue to be made as more rigorous selection procedures are applied with traditional breeding and with biotechnology. However, much larger improvements in production and persistence can be achieved with alternative legumes in areas where subclover, or other traditional pasture species, perform poorly. The genus Trifolium, with 237 species, of which only some 15 are used commercially around the world, is just one source where new varieties can be found. If the work is not restricted to a particular species or genus, the potential for improvement is unlimited in the near future.
How will future varieties be developed?
Improvements can be made by using traditional breeding methods, genetic engineering and by developing germplasm collected in appropriate ecological niches in Australia and overseas.
Traditional breeding methods
Cultivars with broad adaptation have been developed in most commonly used pasture species, both grasses and legumes. Improvements will continue to be made as varieties with special adaptation to specific conditions, and pests and diseases are developed. Better winter growth and tolerance to different management systems could be achieved. These improvements are generally made from existing, well adapted species.
This is another tool which can be used to incorporate desirable characters into already successful cultivars. Resistance to herbicides like “basta” and “bromoxynil” has been incorporated into some subclover varieties. This is important to improve pasture quality, to improve nitrogen fixation and reduce disease and weed incidence for future crops. Seed producers, in particular, can benefit greatly from genetic engineering. Herbicide resistance may also be a problem when a pasture cultivar is acting as a weed.
Resistance to viral diseases and root rots may be incorporated. Work is currently in progress at La Trobe University to develop drought resistance in white clover and subclover by the incorporation of fructans into some cultivars.
High sulphur subterranean clover has been developed to increase wool growth, and work continues to obtain high sulphur protein in the leaves with a high level of rumen resistance.
Alternative pasture species
In some areas alternative species can provide a close match, not only to ecological niches but also to different farming systems. For instance, in a 400 mm annual rainfall zone of Western Australia on a waterlogged grey clay, Persian and balansa clovers significantly outproduced medics and subterranean clovers which were recommended for the area (Evans and Snowball, 1993). As these species have small seeds which survive the digestive tract of herbivores, and must be severely grazed during summer, they could complement annual medics by being grazed continuously in summer while the medics, with their easily digested pods, maintain their seed bank through lenient grazing. This management would provide optimum regeneration and winter production both in the medics and the small seeded annuals.
By having a wider choice of species we will also have a choice of management systems. For example, perennials can be used in a pasture phase lasting around 3 years before returning to a crop phase of similar duration.
Alternative legumes can help fill feed gaps. For example, arrowleaf clover (Trifolium vesiculosum), a late maturing annual with a deep tap root, has continued to grow as late as February in 450 mm annual rainfall zones in Western Australia on deep, well drained soils with a perched water table. In Tasmania, this same species when sown in October provides excellent summer, autumn and winter growth before flowering and setting seed early in the second summer.
Species that tolerate salinity and waterlogging, such as Persian and balansa clovers or Melilotus spp., provide an opportunity for high production in areas which are normally considered wasteland. This can increase the stocking rate on these soils while releasing “better” land for cropping.
Diversity of legume species is low in Australia. In an area of Western Australia, Marchant et al. (1987) list 55 exotic legumes, mostly of Mediterranean origin. This contrasts with Davies (1970) who lists 900 legumes from Turkey alone. There are sound ecological reasons for increasing the diversity of Australian grasslands (Marshall, 1977). Specific and genetic mixtures are tolerant of epidemic diseases, can colonise soil mosaics and may survive environmental disasters. They also maintain production and botanical composition at stable levels, and are more likely to provide a balanced diet for grazing animals (P.S. Cocks, pers.comm.).
Harvesting of subterranean clover and medics can be costly and damaging to soils. By using alternative species which are aerial seeders, harvesting should be easier and require less specialised equipment.
How is a new variety developed?
Several steps must be followed to develop a new pasture variety from collections of germplasm:
collection of material
seed multiplication and characterisation
match species with rhizobia
evaluation in appropriate niches
determine ecological and system niche
Once material is collected and enough seed produced, maturity, growth habit and other characteristics are determined in each line before they are tested in the field. Failure to perform in many cases is due to the absence of appropriate rhizobial strains so it is important that, together with characterisation, testing of rhizobial strains is carried out at this stage.
Evaluation will not only determine where the material will grow, but also the systems under which it will perform best and how this will affect other species.
Species used currently will continue to be improved with traditional breeding methods and genetic engineering. In areas where these species perform poorly, rather than attempting to force them into unsuitable ecological niches, the breakthroughs are likely to come from alternative pasture species. This will result in good quality species occupying niches that currently have less desirable ones, such as barley grass. Grazing could assist, instead of reduce, persistence and seed dispersal. Greater persistence and stability should result in higher long term productivity. The availability of several species for similar areas will provide a choice of management systems to suit individual preferences.
1. Evans, P.M. and Snowball, R. (1993). Balansa and Persian clover lines outproduce registered cultivars, and subterranean clover and medics, in a 400 mm annual rainfall zone in Western Australia. Proceedings of the 7th Australian Agronomy Conference, Adelaide, 53-56.
2. Davies, P.H. (ed) (1970). Flora of Turkey and the East Aegean Islands. Edinburgh University Press.
3. Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and McFarlane, T.D. (1987). Flora of the Perth Region, Part I. Perth: Western Australian Herbarium, Department of Agriculture.
4. Marshall, D.R. (1977). The advantages or hazards of genetic homogeneity. Annals of the New York Academy of Science, 287: 1-20.