Inoculating pasture legumes

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Pasture legumes form a symbiotic (mutually beneficial) association with specific soil bacteria (rhizobia) to meet their nitrogen requirements. Nodules develop on the plant roots and house millions of rhizobia that convert nitrogen from the air into a form the plant can use in a process known as nitrogen fixation.

The association between the host plant and its rhizobia is very specific and pasture legumes must be inoculated with the correct rhizobia strain (or Group) for maximum nitrogen fixation. Techniques to successfully inoculate pasture legume seeds are described here.

Types of inoculants

Inoculants come in four different carriers:

  • peat
  • freeze dried powders
  • granular
  • a pre-coated seed form, with inoculum as part of the pellet.

All forms of inoculant carry live cells of rhizobia and must be stored correctly to preserve high numbers. The shelf life of these products varies from several weeks in the case of some pre-coated seeds to three years for the freeze dried powder.

The cost of inoculation can vary from $5-25/ha depending on the product. Peat-slurry is the cheapest form of inoculant to purchase but there are additional costs in time and labour to consider. The more expensive options can be easier to use and offer greater flexibility for sowing operations.

Peat-slurry inoculants

For traditional peat-based carriers, the inoculum is applied as a slurry to the seed coat so that rhizobia are in direct contact with the seed. Packet size of inoculant is generally 250g and this amount is recommended to treat between 10-50kg of pasture seed (Table 1). When seeding mixtures of different pasture legume seeds, each cultivar should be inoculated separately and then mixed together.

Table 1 Seed size groupings of some common pasture species, the required inoculant group and the maximum amount of seed to be treated by 250g inoculant packet.
Inoculant group Common name Scientific name Seed size Maximum weight of seed to be treated
AL Lucerne Medicago sativa Small 25kg#
AL Strand medic M. littoralis Small 25kg
AM Burr medic M. polymorpha Medium 50kg
AM Barrel medic M. truncatula Medium 50kg
Bis Biserrula Biserrula pelecinus Small 10kg#
B White clover Trifolium repens Small 25kg
B Strawberry clover T. fragiferum Small 25kg
C Balansa clover T. michelianum Small 25kg
C Persian clover T. resupinatum Small 25kg
C Gland clover T. glanduliferum Small 25kg
C Arrowleaf clover T. vesiculosum Small 25kg
C Subterranean clover T. subterraneum Medium 50kg
C Rose clover T. hirtum Medium 50kg
C Crimson clover T. incarnatum Medium 50kg
C Bladder clover T. spumosum Medium 50kg
S French serradella Ornithopus sativus Medium 50kg
S Yellow serradella O. compressus Medium 50kg
Sulla Sulla Hedysarium coronarium Medium 10kg#

# Pasture legume requires higher inoculation rates because of either small seed size, poor survival of rhizobia during inoculation or sensitivity of the symbiosis to low pH soils.

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Ronald Yates
+61 (0)8 9368 3665
Email Ronald Yates
Clinton Revell
+61 (0)8 9368 3596
Email Clinton Revell

Inoculating pasture legumes

  1. Types of inoculants
    1. Peat-slurry inoculants
    2. Peat-slurry inoculants (continued)
    3. Freeze dried soluble legume inoculant
    4. Granular inoculants
    5. Pre-coated seed technology
  2. Inoculant quality control
  3. Interaction between nodulation and nitrogen fertiliser
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Authors

Ronald Yates
Clinton Revell