Signal grass

Page last updated: Thursday, 16 May 2019 - 2:24pm

Please note: This content may be out of date and is currently under review.

Seasonal growth pattern

Signal grass is one of the first warm season grasses to stop growing as the temperature cools down in late autumn and has negligible growth over winter even in areas with mild temperatures. It is often one of the last sub-tropical grasses to start growing actively in spring and then grows opportunistically in summer if moisture is available. It flowers in response to short day lengths.

Table 1 Soil–climate adaptation of signal grass
Rainfall (estimated minimum) >500mm
Drought tolerance Moderate
Frost tolerance Sensitive
Soil type Adapted to coarse-textured soils including acid soils
Soil fertility requirements Will persist on low fertile soils, but requires fertile conditions (high P and N) for good production
Soil pHCa >4.0 (estimated)
Aluminium tolerance Good
Waterlogging tolerance Low to moderate
Salt tolerance Nil
Ability to spread naturally Spreads very slowly under WA conditions
Nutritive value: dry matter digestibility 60–70% (immature forage), 50-60% (mature forage)
Nutritive value: crude protein 7.5–13.2% (when N fertiliser increased from 0-150kg/ha)

Establishment

Signal grass has a comparatively large seed (220 000/kg), so it usually establishes readily. Typically sown at 2–4kg/ha when sown alone and at a depth of 5–10mm it can achieve full groundcover within three months.

In WA signal grass was often included in the grass mix for non-wetting sands, as its larger seeds and ability to germinate from depth acted as insurance against establishment failure from sowing too deep or following sand infill due to strong winds after seeding.

Signal grass is no longer recommended in WA.

Livestock disorders

Signal grass contains steroidal saponins which can cause secondary photosensitisation in grazing stock.

Results from a survey of commercial paddocks in WA indicate that signal grass has high to very high concentrations of steoidal saponins all year round. All of the signal grass samples had high to very high concentrations of diosgenin equivalents (5.9-24.1mg/g), which is 7-30 times above the critical level than can cause photosensitisation. The signal grass contained high to very high concentrations of protodioscin, and moderate to high concentrations of dioscin.

As a result, there is always a potential health risk to livestock grazing pastures containing signal grass, whether the plants are stressed or not. These results reinforce the conclusion that signal grass is the main cause of secondary photosensitisation in stock grazing sub-tropical grass-based pastures in this state.

Young plants of signal grass showing 'bunch' growth habit
Young plants of signal grass showing bunch growth habit

Management

In Queenslands where it is well suited, signal grass is aggressive and out-competes other species to form a pure sward. It can also tolerate heavy grazing. The feed quality of signal grass is similar to other warm season grasses, but it requires heavy grazing to maintain quality when actively growing, as feed quality declines rapidly with maturity. The palatability is good except when the flowering stems are mature.

For existing perennial grass-based pastures, producers need to be vigilant when grazing livestock on pastures containing significant amounts of both signal and panic grasses. The risk of secondary photosensitisation is heightened when the perennial grasses represent all or most of the palatable green feed-on-offer.

It is important to note that the majority of WA pastures containing a mix of perennial grasses, including signal grass, have not caused any issues in stock that have grazed them. It seems that an appropriate interaction of weather, pasture, grazing and animal factors is required before the potential for photosensitisation occurs. This happens infrequently and only for short periods of time, explaining why these pastures are usually productive.

Cultivars

Basilisk (public variety) often simply called signal grass is the only cultivar available in Australia.

Contact information

Geoff Moore
+61 (0)8 9368 3293
Jeremy Allen
+61 (0)8 9368 3466

Author

Geoff Moore