Highly productive sub-tropical grass–serradella pastures in southern Western Australia

Page last updated: Wednesday, 13 June 2018 - 11:18am

A highly productive sub-tropical grass–serradella pasture is achievable – but there are a number of prerequisites for success. The most important of these appear to be a good perennial grass density to suppress the broadleaf weeds and a suitable soil for successfully growing serradella. The prerequisites for a highly productive perennial grass plus serradella pasture are described along with a guide to establishment and management.

perennial pasture + serradella pasture
Figure 1 Possible to combine a perennial grass pasture (left) with the serradella dominant annual pasture (right)

Most producers would be happy with the perennial grass pasture on the left and similarly with the serradella dominant annual pasture on the right (Figure 1); but it appears possible to combine the two, not as a one-off, but every year. The outcome would be a productive pasture with sub-tropical grasses like panic grass and Rhodes grass growing strongly in spring and autumn and following out-of-season rainfall, and serradella as the dominant pasture species over the winter growing season.

Evidence from medium-term field trials suggest that a perennial grass plus serradella pasture is sustainable. That is, it appears possible to maintain a productive perennial grass pasture together with serradella as a dominant companion legume every year.

Perennial grass field trials with annual pasture control plots show that a perennial grass pasture with good density will partially or completely remove the broadleaf weeds.

Likewise, a long-term perennial grass paddock at Badgingarra Research Station has maintained a dense serradella annual pasture component for seven years (Figure 2). The paddock has not received any special management inputs (i.e. no herbicide or insecticide sprays), apart from being rotationally grazed with lenient grazing in spring when serradella is flowering and setting pods. 

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Figure 2 Excellent regeneration of serradella at Badgingarra from 2011 to 2016 

Prerequisites for a highly productive perennial grass plus serradella pasture

  • right soil type – to support a productive serradella pasture
  • good perennial grass density – to out-compete and replace broadleaf weeds like capeweed
  • low weed burden when establishing serradella – use summer sowing or conventional winter sowing after a knockdown, depending on the size of the ‘weed’ seed bank
  • establish serradella seed bank – need to maximise pod (seed) yield in the initial years
  • rotational grazing – manage for serradella during the winter growing season (lenient grazing during seed set) and manage for the perennial grasses from late spring to autumn (allow time for the grasses to recover after heavy grazing)
  • adequate nutrition - apply phosphorous (P) and potassium (K) based fertilisers regularly.

How to establish serradella and perennial grasses

In general, sub-tropical grasses are established first and then the annual legumes introduced. Both summer sowing and conventional sowing after the break of the season are proven methods for successfully establishing serradella into perennial grass pastures. However, they are not universally applicable.

Consider the questions below and then develop a plan:

1. Is the paddock suitable for growing serradella?

Yellow and French serradella are well adapted to well-drained, sandy soils, but can struggle on leached deep sands with low fertility. This is most likely due to a combination of factors (e.g. low fertility and low water holding capacity).

The critical P and K levels are still to be determined; however, if the paddock is suitable or marginally suitable for growing a cereal crop, then serradella is a viable option.

2. Does the paddock have a good perennial grass density?

A good perennial grass density (8-10 plants/square metre) equates to full rows of perennial grasses distributed uniformly over the paddock. Also ensure there is a low density of broadleaf weeds. Perennial row-spacing trials have shown that broadleaf weeds are suppressed when the rows of perennial grasses are 44 centimetres (cm) apart and can still be effective out to 88cm.

Perennial grasses also act as a nitrogen (N) sink and are very responsive to added nitrogen (especially panic grasses, Figure 3). Nitrogen fixed by serradella is taken up by perennial grasses and increases growth and feed quality.

pasture crop trial
Figure 3 Response of Gatton panic to nitrogen fertilizer at Dandaragan 

For annual-only pasture systems, a legume dominant phase will increase the soil N and as a consequence lead to nitrophilous broadleaf weeds and annual grasses dominating. Broadleaf weeds like capeweed compete strongly with regenerating serradella seedlings, making it difficult to maintain a serradella dominant annual pasture without the use of herbicides.

A paddock with a low perennial grass density will behave more like an annual pasture with a high proportion of broadleaf weeds which can adversely impact on serradella content.

3. Is summer sowing serradella an option?

Summer sowing refers to sowing pod (or unscarified seed of low germination) in late summer/early autumn. Success with summer sowing relies on there being little competition from annual volunteer species, and pods being sown between mid-February and early March.

With perennial grass pastures there are two main opportunities:

  • the first summer after the perennial grasses are sown, as there was full weed control in the establishment year
  • following one to two years of pasture cropping with good in-crop weed control, hence there is a low ‘weed’ seed bank.

With summer sowing, the pod needs to buried beneath the soil surface for seed softening and germination to occur (i.e. pods/seed sitting on the soil surface are less likely to become germinable). Drilling the pod at 5-10 millimetres is the most reliable method of establishment. Broadcasting pod and then using stock to trample the pod into the soil is unreliable – also the pod segments are susceptible to being ‘harvested’ by ants while on the soil surface.

The annual legume options for summer sowing include: hard-seeded Margurita French serradella (pod) plus the long-season Avila yellow serradella (pod). Avila should only be used in high rainfall areas. The yellow serradella experimental line 87GEH72.1a (pod) is also broadly suitable, but is yet to be commercialised.

Sowing rates of serradella pod are 20-30 kilograms/hectare (kg/ha), up to 50kg/ha. With the high sowing rates required for summer sowing consider on-farm seed production.

4. Is conventional sowing of serradella after the break of season an option?

Conventional seeding after the break of the season can be successful if full annual weed control is achieved.

  • Weed control – for panic grass pastures, a glyphosate knockdown can be safely used as the perennial grasses will recover strongly in spring. However, for pastures containing Rhodes grass, knockdown options are limited to Spray.Seed as glyphosate and the grass selective Clethodim can adversely affect Rhodes grass persistence. A knockdown with Spray.Seed can be successful if the annual pasture seedlings are all small; however, any volunteer annual pasture plants which are not killed and subsequently recover, will compete strongly with the serradella seedlings, resulting in a poor or failed legume pasture.
  • Sowing time – serradella needs to be sown before the end of May so seedlings establish before growth slows down with the onset of cold conditions in winter.

If there is a late break to the season consider deferring seeding to the following year.

All of the annual legume species/varieties are suitable for sowing after the break of season using scarified (highly germinable) seed.

5. What are the management requirements?

The over-riding objective in year one is to set up a large legume seed bank; so all management decisions should have this focus. Some grazing in July-August is beneficial, but the paddock needs to be rested in spring when serradella is flowering to maximise pod yield. Spraying for native budworm (Helicoverpa punctigera) in the first year may also be beneficial.

In subsequent years, the pasture should be managed for the sub-tropical grasses from November to April and for the serradella pasture from May to October.

Sub-tropical grasses require some form of rotational grazing, while regenerating serradella should be spelled after the break of the season to allow seedlings to establish. Serradella can be grazed hard over winter and then either lenient grazing or rested in spring when flowering and forming seed. If rested in spring, the dry serradella biomass and pods can be grazed with the green perennial grasses in early summer.

Acknowledgements

This research was co-funded by the Department of Agriculture and Food, Western Australia (DAFWA), Meat and Livestock Australia (MLA) and the Grains Research and Development Corporation (GRDC) EverCrop Project.

Contact information

Geoff Moore
+61 (0)8 9368 3293
David Ferris
+61 (0)8 9690 2117

Authors

Geoff Moore
David Ferris
Christiaan Valentine