Pasture legumes are showing they can fix large quantities of nitrogen and reduce the requirement for inorganic nitrogen fertilisers for both crops and pastures.
While subterranean clover and annual medics have been the traditional choices for self-sustaining crop-pasture rotations and for forage or fodder conservation crops, growers have reported declining success, particularly in dry conditions.
These species have shallow root systems and a limited capacity to tolerate difficult soil conditions, such as acid soils and soils prone to waterlogging.
Consequently, there has been a substantial investment in new annual legume species over the past 20 years and many are now demonstrating important agronomic advantages over traditional species.
These advantages include more herbage production, increased water use efficiency, suitability for difficult soils, improved pest and disease tolerance and/or superior ability to cope with difficult or unpredictable seasonal conditions.
The new legumes have also demonstrated comparable or superior capacity to fix large quantities of atmospheric nitrogen compared with traditional pasture legumes and, unlike subterranean clover and annual medics, can be harvested with a conventional harvester.
Farmers and advisers have been keen to assess the potential of new annual legumes, but a lack of technical information about regional suitability, productivity, persistence and economic benefit has discouraged a broad uptake. However, a New South Wales project, ‘Agronomy and management of new annual legumes’, supported indirectly by the GRDC, has identified several high-potential annual pasture legumes while also addressing the knowledge gap.
More than 300 farmers and 33 advisers across central and southern NSW have taken part in a series of workshops to examine their level of knowledge, perceptions and current use – or for advisers, recommendation – of legumes.
What emerged was that farmers and advisers had limited knowledge of the characteristics or potential use of new pasture legumes.
The project also identified a range of new annual legumes with great potential for crop-pasture rotations across a range of agro-ecological zones. These are bladder clover (Trifolium spumosum), gland clover (T. glanduliferum), biserrula (Biserrula pelecinus) and French serradella (Ornithopus sativus).
The project covered areas with Mediterranean and temperate climates. Its findings are directly applicable to central and southern NSW areas with long-term average annual rainfall above 325 millimetres and similar climatic zones of Victoria.
The project examined three different methods of establishing crop-pasture rotations:
- stand-alone sowing of annual legumes, followed by cropping in the second year;
- the use of cover cropping or undersowing, stand-alone sowing and undersowing involve scarified or softened seed where the pod has been removed; and
- twin sowing – the use of in-pod or unscarified seed sown under a normal rate of cereal. The seed softens in the crop year and establishes in the following year.
Key to the success of any pasture establishment involving annual legumes is ensuring adequate seed-set in the establishment year.
Results from this project showed that use of cover cropping resulted in seed-set in all species except bladder clover, being well below the critical 100 to 150 kilograms of seed per hectare required for successful regeneration in following years. Further, use of cover cropping resulted in substantial reduction in individual seed size compared with stand-alone sowing (Figure 1). Seed size has a direct effect on seedling vigour and therefore success of the pasture in the long term. The combination of reduced seed-set and reduced seed size is likely to compromise pasture performance in the long term.
Figure 2 shows the herbage produced from cover crop treatments in early spring in the year following sowing, compared with that achieved from regeneration following twin sowing. In 2010, regeneration occurred early in the season – well ahead of the normal pasture sowing time. The herbage produced from these two treatments is compared with that from a normal time (mid-May) pasture sowing in 2010. The results show that twin-sowing produced more than three tonnes of DM per hectare.
Both French serradella and biserrula achieved significantly higher herbage production when regenerating from twin sowing than from cover cropping. The advantage of having a viable seedbank and capitalising on early-season rainfall is clearly shown when comparing the herbage produced from regenerating stands compared with those sown at the ‘normal’ time. The advantage of the new deep-rooted annual legumes and their ability to survive through what might normally be considered ‘false break’ conditions can potentially be of significant benefit to farms by enabling greater pasture production and subsequent utilisation by livestock through the winter period.
The ability of legumes to fix nitrogen depends on effective inoculation of the seed with the correct strain of rhizobium at sowing. Methods of rhizobium delivery include peat, pre-coated seed, freeze-dried and long-life clay-based inoculants.
Research has shown mixed performance of some rhizobia delivery systems, including long-life inoculants. With all seed treatment methods, rhizobium numbers decline after inoculation. The best way to achieve successful nodulation and provide the highest level of nitrogen fixation possible is by freshly inoculating uncoated seed and sowing it within 24 hours. Pre-coated and inoculated seed should be sown as close as possible to the date of coating.
New annual legumes were superior to traditional annual legumes both in the well-below-average rainfall seasons experienced in 2008 and 2009, and in 2010, a significantly wetter-than-average year. They produced both higher quantities of herbage and seed, enabling livestock production to be maintained at higher levels in dry seasons, and more reliable year-in, year-out regeneration. (Figure 2).
The superior performance of new annual legumes across a range of extreme seasonal conditions experienced during the project was a good test of their adaptability.
Flexibility and productivity
Use of new annual legumes in crop-pasture rotations can reduce reliance on inorganic nitrogen fertilisers. Farmers found the flexibility of the self-sustaining crop-pasture rotation provided by these new legumes highly attractive. It meant they did not have to resow pastures at very high cost following each crop phase, as is the case with traditional pasture legumes.
Other benefits include increased feed production and pasture quality.
An increased ability to set seed (a higher proportion of which is hard) was also observed, resulting in greater reliability of pasture production and contribution of nitrogen to the crop phase of the crop-pasture rotation.
Overall, new pasture legumes have the potential to stabilise crop-pasture rotation systems, as well as having potential as stand-alone fodder or forage crops.
The end result of this project was to develop management packages that provide growers and advisers with the necessary technical resources to successfully grow and manage the most widely adaptable new annual pasture legumes.
Dr Belinda Hackney,
02 6330 1200, firstname.lastname@example.org;
NSW Department of Primary Industries, www.dpi.nsw.gov.au/agriculture/pastures
GRDC Project Code