Research reveals better ways to establish winter legumes

NSW DPI soils research officer Dr Belinda Hackney, has been involved with this research for several years with trials and large-scale farm demonstrations of successful adoption of the technology.

Comparison sowing (late February versus more traditional late May) at Harden this year, for example, highlights the common superiority of earlier sowing.

Early sowing of species especially suited to this technology includes gland clover, bladder clover, pink serradella and arrowleaf clover. Biserrula and some varieties of yellow serradella can also be successfully established this way depending on the district, soil and climatic conditions.

Early sowing is generally successful in varieties with high levels of "hard seed". "Hard seed" refers to the proportion that will not germinate for up to several years after the seed set year.

"Hard seed" avoids most of the seed germinating after so-called "false germinating" rain in summer/early winter, when such germinations mainly fail because temperatures are too high and follow up rain is unlikely to sustain them.

Sub clover is not suited to summer sowing and must be sown after the danger of a false break has passed.

Commonly "hard seed" varieties are processed via scarifying or dehulled Serradella), so that all seed is "soft" and ready to germinate when sown at more normal later autumn. However, when summer sowing "hard" unprocessed seed must be sown as it is protected against early "false" germinating rains.

Also, importantly the hot and cold temperature fluctuation over the remainder of summer and early autumn, experienced post sowing, causes natural seed "softening". It is then ready to germinate on opening autumn rain.

Alosca clay granules, infused with legume species-specific rhizobia bacteria, is part of a strategy to successfully sow early when germination is months away, and when hot conditions would normally mean death of rhizobia applied with the seed via more traditional methods.

Alosca pellets are bentonite clay-based, impregnated with commercial strains of root nodule bacteria and to my knowledge are currently the only marketed product with this feature.

Unlike other application techniques, rhizobia bacteria can survive in the clay granule for a long time and that increases success in many more situations. Granules can be top-dressed or drilled into the pasture with seed or/and fertiliser ahead of the autumn break.

Legumes, either crop or pasture species, without their specific rhizobia, which grows as nodules on their root systems, will not build soil nitrogen. Non nodulated legumes will be dependent on available soil nitrogen for their nitrogen supply (for protein).

Legume species require different rhizobia groups - sub clover requires Group C, biserrula Group BS and serradella Group G/S. When sowing mixes, use the right group for every species.

Where a legume has formed nodules with effective rhizobia it can build soil nitrogen by up to 20-40 kg/ha per 1.0 t/ha of above ground dry matter. This means an effectively nodulated legume pasture that produces 4 t dry matter/ha could supply 80-160 kg nitrogen/ha.

Logic supports why Belinda Hackney research works so successfully. Having seed in the ground, ready to germinate, and with its appropriate viable rhizobia, commonly means successful establishment weeks ahead of more traditionally sown winter legumes. Commonly this means several t/ha extra dry-matter, plus extra nitrogen fixed.

All other aspects of pasture establishment, such as good soil fertility, weed control and choosing species suitable for given soil conditions, especially acidity, are also necessary. Species like biserrula, gland clover, serradella and the like require grazing management that ensures these aerial seeders are allowed to build a soil seed bank.

Next week: Narrabri research worth hundreds of millions to farmers.