Sub-tropical grass establishment

Page last updated: Thursday, 16 May 2019 - 2:28pm

The establishment of sub-tropical grasses has improved dramatically in the past few years. Leading producers are now regularly achieving uniform establishment across the paddock, in spite of variable seasonal conditions. Major contributing factors have been furrow sowing, better weed control and the development of an establishment package for sub-tropical grasses as described below.

Background

Until the mid-2000s, establishment of perennial grasses in WA at the paddock scale was typically patchy, with areas of good plant density interspersed with many areas of poor density. Seeding failures also were common. However, a few producers were regularly having success, which showed that uniform establishment at the paddock scale was possible. Overall, the establishment success of sub-tropical grasses has improved dramatically since then. Major contributing factors have been furrow sowing, better weed control and the development of an establishment package for sub-tropical grasses as described below.

Summary – key steps for establishment

The following 10 points summarise the key steps or ‘must-dos’ for successful establishment. They need to be followed closely for success, which we define as a density of 8–10 plants per square metre in the autumn after sowing.

For more information refer to Bulletin 4840 Establishment guide for sub-tropical grasses.

Table 1 A summary of the 10 key steps for establishing sub-tropical grasses
1 Plan ahead A year ahead of sowing, reduce weed seedset, especially of difficult-to-control weeds and commence control of rabbits and kangaroos if they are a potential problem. On erosion-prone sites, consider sowing a cereal crop, so that perennial grasses can be sown into standing stubble
2 Species selection Select suitable species and varieties for your soils and climate
3 Seed quality Purchase good-quality seed. Consider buying species with post-harvest dormancy (for example, panic grass) the year before seeding
4 Presowing weed control Fully control weeds. However, do not sow into a bare paddock. The final knockdown herbicide should be just prior to seeding to ensure there is some above-ground plant material and decomposing root matter to hold the soil together. Apply a broad-spectrum insecticide with the final knockdown or immediately after sowing
5 Sowing time Sow into a full profile of soil moisture in late winter to early spring (the sowing window depends on the district). If stored soil moisture is limiting, then consider deferring sowing until the following year
6 Machinery set-up Set up the seeder to sow seed into large furrows, with following press wheels for good seed-to-soil contact and with a row spacing of 50–60cm
7 Sowing rate and depth Sow 2–5kg/ha of seed, depending on seed quality and whether seed is coated or uncoated. Sow at a depth of 5–10mm as most sub-tropical grasses have very small seeds that will not emerge from depth
8 Sowing speed Don’t sow too fast, which can result in excessive soil movement and reduce the accuracy of seed placement
9 Post-seeding checks Control weeds and pests, including insects, kangaroos and rabbits
10 Grazing management Defer the first grazing until the grasses are well established

Video - Key steps for successful establishment of sub-tropical grasses

Video - Sub-tropical grass establishment – Machinery demonstration

Key steps for establishment

1. Plan ahead

Plan a year ahead and reduce weed seed-set by grazing and spray-topping, especially for difficult-to-control weeds.

In exposed sandy paddocks that are highly susceptible to wind erosion, consider sowing a cereal crop the year before establishing sub-tropical grasses. The sub-tropical grasses can then be sown into the standing cereal stubble, which reduces the potential for soil erosion while they are establishing.

Rabbits and kangaroos can severely damage perennial grass seedlings. If they are likely to cause problems on your paddock, start control the year before sowing.

2. Species selection

Select perennial grass species and varieties suited to your district, soil type and to meet an identified need, in terms of the livestock enterprise (for example, wool, meat, maintain weight), a critical feed gap and/or soil conservation requirement.

In general, sub-tropical grasses are limited to the south coast, west coast and northern agricultural region (Figure 1).

When grown in other regions most species will either have poor persistence over winter or low productivity. Select species and varieties suitable to the climate and soils of the paddock to be sown.

‘Sub-tropical grasses - Table 1 outlines suitable species for different soil types on the south coast and in the northern agricultural region. Refer to Table 1 under Documents.

3. Seed quality - purchase good quality seed

Purchase good quality seed as poor seed quality can be a major limitation to successful establishment. The quality of sub-tropical grass seedlots can vary widely in terms of germination, purity and dormancy. The germination of commercial sub-tropical grass seed batches typically varies from 10–60%. Kikuyu (commonly >80%) is an exception.

Send to an accredited seed laboratory if concerned about the expected germination of a seed batch. Several seed companies coat their seeds. While this helps with handling and flow of fluffy seeds in machinery, it reduces the number of viable seeds per kilogram.

Seed dormancy: Panic grass, setaria and signal grass have a high proportion of seeds, referred to as fresh seeds, which are viable, but remain dormant for 6–10 months after harvest. Consider purchasing these species the year before seeding and storing under dry conditions. Rhodes grass and kikuyu do not have such dormancy.

4. Presowing weed control and residual insecticide

A weed-free seed bed is essential, as sub-tropical grass seedlings are weak competitors. For erosion-prone sites, develop a weed control strategy that ensures complete weed control at seeding but leaves some above-ground plant material and decaying root mass to help bind the soil and prevent erosion.

One strategy is to use a selective broadleaf herbicide 6 weeks from sowing, followed by a general knockdown herbicide 1-2 weeks from sowing. This allows grass residues to bind the soil and reduce erosion risk.

In paddocks with a low grass density, a single knockdown herbicide two weeks before sowing can be used, but higher rates than autumn-winter applications will be needed to kill difficult-to-control weeds.

Two knockdown sprays (six weeks and two weeks before seeding) result in good weed control, but can leave the paddock prone to wind erosion (unless stubbles have been retained). It also reduces the amount of winter grazing and the ability to change plans if the conditions for seeding deteriorate.

Apply a residual insecticide with the final knockdown herbicide (or at sowing) to control caterpillars, cutworms, aphids and redlegged earth mites.

5. Sowing time

In general, sow as early as possible to maximise the opportunity for follow-up rain. The sowing window varies from late winter to early spring depending on the district – Figure 1.

The best sowing times for the main regions suited to sub-tropical grasses are:

  • Dongara-Kalbarri districts - early to late August
  • Perth-Eneabba districts - mid-August to early September
  • South coast - early September to early October

This coincides with sufficient soil temperatures for germination and the likelihood of sufficient moisture for good root development before summer.

If stored soil moisture is limiting, consider deferring sowing until the following year.

Map of south-western Australia showing the sowing window for sub-tropical grasses
Figure 1 Sowing window for sub-tropical grasses

6. Machinery set-up for best results

Successful establishment of sub-tropical grasses can be achieved with a range of seeding machinery and configurations (tynes, discs) provided that the machine:

  • forms stable furrows that scalp away non-wetting sand, remove weed seeds and harvest rainfall
  • uses press wheels to provide good seed contact with moist soil
  • is set for wide row spacing (typically 50–60cm).
Paddock of established perennial grasses
Good sub-tropical grass establishment - photo taken in March after a dry summer

Formation of furrows - furrows capture rainfall and increase seedling survival, particularly in dry springs. Furrow formation also scalps away non-wetting sand and removes weed seeds. Furrows should be formed to minimise sand in-fill and sides should not be too steep. Furrows of 50mm depth are sufficient if the soil surface is moist. Deeper furrows can be used if the soil surface is dry or highly non-wetting.

Press wheels - press wheels provide good seed contact with soil moisture. They should press soil in the furrow bottoms and minimise sand in-fill from the sides. Rounded or flat-bottomed wheels give the best results.

Optimum row spacing - optimum row spacing is 50–60cm. This reduces soil movement into adjacent furrows and allows annual pastures to grow between rows in the cooler months. Wider row spacing can be used in low rainfall areas to reduce moisture competition.

7. Sowing rate and depth

Use a sowing rate of 2–5kg/ha, depending on seed quality and whether seed is coated. For uncoated seed with >40% germination, 2kg/ha is sufficient. Higher rates should be used for seed of lower germination. Higher rates are also required for coated seeds to sow the same number of seeds per area. With light, fluffy seeds such as Rhodes grass, use a carrier if the seed is not coated to ensure uniform flow through the seeder and to prevent bridging (blockages).

Sub-tropical grasses require shallow seeding to a depth of 5–10mm. Seeds sown too deep will not emerge, because sub-tropical grass seeds are small and very sensitive to seeding depth. A common method is to drop the seeds in the bottom of a furrow and press them into the soil using press wheels to ensure good seed–soil contact. Sowing directly onto the surface with no soil cover is unreliable.

8. Sowing speed - don’t sow too fast

Tractor speed can impact on establishment success. Driving too fast causes excessive soil movement, reducing the accuracy of seed placement. Sand in-fill increases and furrows can collapse, causing deeper seed burial than intended.

The optimum speed (for example, 5–10km/h) will depend on the type of machinery and good operators adjust the sowing speed according to the soil conditions and furrow formation. Check seed placement regularly and make adjustments if needed.

9. Post-seeding checks

Summer growing weeds compete strongly with sub-tropical grass seedlings for soil moisture and their control will maximise establishment. However, if there is a major erosion risk they should be retained.

Monitor the paddock for insect damage, especially over the first 6–8 weeks when the grass seedlings are emerging and vulnerable to insect attack and control if needed. Good control of kangaroos and rabbits is essential to protect young sub-tropical grasses.

To determine whether emerging grasses are those that were sown, the different species can be identified at the seedling stage using the photographs and descriptions in the video and Bulletin 4775, Identifying sub-tropical grass seedlings.

10. Grazing management

Careful grazing management over the first summer is critical to ensure a strong perennial stand in subsequent years. Sub-tropical grass seedlings have a weak primary root system and as a result are susceptible to uprooting and grazing damage over the first summer. Uncontrolled grazing during this time will also deplete carbohydrate reserves, which can result in plant death.

The first grazing of sub-tropical grasses should be deferred until they are well established and actively growing. This will vary with seasonal conditions and may not be until after the break of season. Ensure plants are firmly anchored before introducing animals.

Video - Assessing newly established perennial grasses

Contact information

Geoff Moore
+61 (0)8 9368 3293

Authors

Geoff Moore
Phil Nichols