Getting the best from your legume pastures
Legumes have a symbiotic relationship with a type of bacteria called rhizobia which attaches to the roots of a legume plant forming nodules. These nodules can extract atmospheric nitrogen from the air spaces in the soil and also provide some nitrogen to the legume plant. During the growing season some of the nitrogen is used by the rhizobia bacteria and any excess nitrogen is taken up by plant roots.
Once the annual legume plant such as sub clover or medic dies off in early summer, or the perennial clovers and lucerne dry off and drop leaves and replace roots, nitrogen is released into the soil for use by other pasture plants. This assists the cycling of nitrogen through the pasture sward. In addition, as the legume plants are eaten by livestock, nitrogen in various forms is redistributed back onto the pasture in urine and dung. Nitrogen has a major role in the production of protein in both plants and animal.
Legumes are very palatable and are favoured by grazing livestock, particularly sheep, which will actively seek out and select legumes from the pasture sward. As legumes are high in energy and protein, this increases livestock growth. The fresh green legumes are easily digested and pass through the digestive system quickly allowing for more pasture to be eaten each day thus helping to lift livestock performance. Legumes are also known to contain slightly more of the minor trace elements in the leaf and stems compared to some other plants.
However, there are a few negatives to a very legume dominant pasture:
- the total dry matter production from legume pastures is less than a well-managed grass-based pasture
- bloat in cattle grazing rapidly growing immature clover dominant pastures has caused many deaths in past years
- some cultivars of subterranean clover such as Yarloop sub clover have been found to have high oestrogens contained within them leading to infertility in ewes through embryo loss in the early stages of pregnancy.
The important thing is that a well-balanced grass and clover pasture will outperform a monoculture pasture, is more adaptable to seasonal conditions and provides a safe well-balanced diet to both sheep and cattle.
To get the best from your legume and grass-based pasture you need to have suitable soil conditions such as a well-drained soil, in the 5.3 to 5.9 pH range, with adequate fertility. The soil fertility will vary with the soil type, location, and local climatic conditions. As a general rule a good pasture paddock would have an Olsen phosphorus range of 15-25 Olsen P, a Colwell potassium range of 150-230 depending on soil type and a sulphur KCL – 40 tests in the 8-12 range. It is recommended to take a soil test of your pasture paddocks every 3-4 years in the spring months when pastures are actively growing and base your lime and phosphorus, potassium and sulphur (PKS) fertiliser application on the soil test results in consultation with your local agronomist to fine tune your local soil requirements.
Grazing is the next management task that needed to the best from your legume pastures. Rotational grazing or on/off grazing gives the legume plants a chance to recover and build up stored energy between grazings. Rotational grazing prevents livestock from over grazing the very palatable legume plants.
To establish a new legume pasture, the important steps are:
- reducing the weed burden in the previous season
- the preparation of a fine seedbed
- sowing lime coated seed inoculated with specific species of rhizobia into a shallow firm seedbed and
- rolling the surface post sowing to ensure good seed soil contact.
Autumn sowing of pastures gives best results. An application of lime presowing if the soil pH is low, using insecticide coating on the seed will give protection against red legged earth mite and lucerne flea for the first few week’s post germination.
Information on selection of legume species to sow for your locality and soil type is best gained from a local agronomist or seed retailer but a general rule of thumb is medics are suited to below 450 mm rainfall, subterranean clover and Balansa clover from 450 mm to 600 mm and above 600 to 1100 mm rainfall white clover, Strawberry clover Red clover and lucerne are best suited.
Most clovers and particularly lucerne don’t perform well in waterlogged soils with the exception of Strawberry clover which will grow in damper soils and is a good summer grower if moisture is available on river flats or with irrigation. Lucerne is not suited to low pH soils and can be grown in lower rainfall areas as a dryland pasture at low densities and is able to make use of subsoil moisture or summer thunderstorms. Both Balansa and Subterranean clover have some salinity tolerance, while Arrowleaf clover is an annual long season cultivar producing an abundance of feed in the late spring.
In summary to get the best from your clover, medic and lucerne pastures, ensure adequate PKS fertilisers, apply lime to achieve a soil pH above 5.4 pH, avoid waterlogging and use rotational grazing to allow a rest period between grazings. Select a species and cultivar to suit your local soil and climate conditions.