The persistence of lucerne plants or stands (their ability to survive over time) is particularly important in Queensland where the life of lucerne stands is among the shortest in the world. The longer a stand can remain highly productive, the more economic it will be.
This profitability depends largely on the plant population of the stand, and its intended use. When the plant population of a stand sown only for hay production declines, the stand becomes grassy and weedy. With present cultivars, grass and weed invasion becomes critical in haymaking stands when populations fall below about 40 plants/al’. In contrast, grazed, dryland stands with a population as low as about 10 plants/al’ can still make a valuable contribution to animal performance and the profitability of the enterprise. The long life of a stand is most important in extensive grazing systems, with replanting costs comprising a large portion of the total production cost. It is not so important in short-term grazing stands sown in crop/pasture rotation systems or in haymaking enterprises.
An integrated management approach to lucerne persistence
An integrated management approach can maximise both persistence and production. Such a management approach essentially takes into account the natural factors (diseases, pests, root reserves, waterlogging injury, drought injury, competition and crop nutrition) and the management factors (cultivar selection, seeding rates, mowing, grazing and irrigation) that influence persistence and production. The factors affecting lucerne persistence also influence production, either through the effect of a declining lucerne plant population on yield, or by decreasing plant vigour before death occurs.
For example, competition from weeds and sown species may be manipulated by cultivar selection, herbicide application, mowing and grazing patterns, and the sowing rates of both lucerne and any sown companion species. Another example is the control of damaging insects by the selection of resistant cultivars and the use of appropriate insecticides.
Improving persistence and production: the future
The greatest advances in lucerne persistence and production have come from the breeding of cultivars that are both disease and pest resistant. In Queensland, using cultivars with combined resistances to colletotrichum crown rot, phytophthora root rot, spotted alfalfa aphid and bluegreen aphid is essential.
Trifecta and Sequel (both bred jointly by DPI and the CSIRO), Sequel HR and Hallmark (bred jointly by CSIRO and the University of Queensland) and UQL-1 lucernes (bred jointly by the University of Queensland and DPI), all offer this combined resistance and have demonstrated superior production and persistence in irrigated trials in Queensland. Most cultivars now bred in Australia contain moderate to high levels of resistance to these pests and diseases and are also performing well in Queensland. Some cultivars have high levels of resistance to a specific pest or disease and are more suited to conditions where the incidence of that pest or disease is high (e.g. Quadrella for stemphylium and leptosphaerulina leaf spots and Aquarius for phytophthora root rot).
Research and plant breeding in Australia and overseas is aiming to improve the following attributes of lucerne:
- resistance to leaf diseases;
- resistance to Rhizoctonia;
- further improvement in resistance to colletotrichum crown rot and phytophthora root rot;
- nitrogen fixation efficiency;
- waterlogging tolerance;
- salt tolerance;
- grazing tolerance;
- tolerance of acid soils;
- resistance to nematodes.
Bloat tolerance is a difficult goal that is being pursued in some lucerne breeding programs.
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