Spear grass/wallaby grass pastures in the southern rangelands of Western Australia

Pastoral potential – moderately high

The pastoral value of speargrass/wallaby grass pastures is moderately high. In good seasons these pastures are highly productive and during the active growth phase are readily grazed. Speargrass has maximum nutritional value when it has new green shoots, becoming less appealing as it becomes dry and harsh. Wallaby grass is highly palatable and is preferentially sought by herbivores.

Suggested levels of use (per annum)

Table 1 provides a rough guide to the range of pastoral values for good condition pastures, which must be checked against conditions in each region and paddock. Carrying capacities for fair condition pastures might be 75% to 50% of good, and poor condition pastures less than 50% of good.

See Introduction to pastures in the southern rangelands of Western Australia for an explanation of how carrying capacities are estimated.

¹ DSE is based on the feed energy required to maintain a 45 kilogram liveweight Merino wether with zero weight change, no wool growth additional to that included in maintenance, and walking 7 km/day. 1 DSE has an energy requirement of approximately 8.7 MJ ME/day.
² CU in the southern rangelands is based on a 400 kg steer at maintenance and equivalent to 7 DSE.
³ AE is based on the feed energy to maintain a 450 kg Bos taurus steer 2.25 years of age, walking 7 kilometers each day. 1 AE has an energy requirement of approximately 73 MJ ME/day and equivalent to 8.4 DSE.

Managing spear grass/wallaby grass pastures in the southern rangelands

Total grazing pressure must be managed to prevent wallaby grass being grazed out and the pasture becoming dominated by only speargrass and unpalatable species such as toothed saltbush, Ward’s weed, balsam and roly poly, reducing the long-term carrying capacity of the grassland. Once degraded these pastures rarely grow grass in poor seasons and are therefore unreliable as a source of feed during extended dry periods.

Patches of speargrass/wallaby grass pastures are likely to have always existed in a mosaic state and transition pattern between saltbush and bluebush low shrubland. The vegetation has become irreversibly altered as a result of increased fire frequency and through grazing by rabbits in plague proportions.

These pastures are usually stable in good seasons, but wind erosion is common in dry conditions or on recently burnt areas which have yet to re-establish vegetation cover.

Fire management

These pastures are fire-induced vegetation associations. Patches are likely to have always existed in a mosaic state and transition pattern between saltbush and bluebush low shrubland. The vegetation has become irreversibly altered as a result of increased fire frequency and through grazing by rabbits in plague proportions. These pastures now dominate extensive areas, having replaced other habitat types by increasing the fire susceptibility of much of the Nullarbor.

Pasture condition

Good

See Figure 1. There is an even coverage of grasses and the stand will include wallaby grass. Cryptogamic crusts are present.

Fair

See Figure 2. The coverage of grasses may appear patchy or uneven, with the areas in between being either bare or supporting occasional annuals and/or unpalatable perennial plants (e.g. toothed saltbush). Speargrass may come to dominate the stand as palatable wallaby grass is grazed out. Cryptogamic crusts may be breaking up.

Poor

See Figure 3. Larger bare patches occur among the grass plants. Palatable grasses lack vigour or are absent and speargrass dominates the sward if grasses are present. Unpalatable and annual shrubs such as toothed saltbush, Ward’s weed, balsam, roly poly, hairy bindii, limestone bindii, spear-fruit copperburr and twinleafs may dominate the area. Cryptogamic crusts may be absent.

Figure 1 A speargrass/wallaby grass community in good condition. Abundant mixed perennial grasses protect the soil from wind erosion. Cryptogamic crusts are intact but hard to see. Note that with subsequent dry conditions after the good seasonal conditions that have stimulated such growth, this grassland is now highly susceptible to fire. The site is an open depression drainage floor with calcareous shallow loam in the Nightshade land system.

Figure 2 A speargrass/wallaby grass community in fair condition. The density of grasses has been reduced. Much of the desirable wallaby grass has been heavily grazed, resulting in its replacement by annuals and semi-perennials. The site is a clay plain with red/brown non-cracking clay in the Kybo land system.

Figure 3 A speargrass/wallaby grass community in poor condition. Perennial grasses are sparse and have been replaced by seasonally dependent annuals and semi-perennials. In dry seasons such areas have limited carrying capacity. Stock pad lines are common and scalds are developing leading to breaking down of cryptogamic crusts and exposure of soil surfaces to wind erosion. The site is an open depression, clay plain with red/brown non-cracking clay in the Shakehole land system.

Vegetation structure and composition

Speargrass/wallaby grass pastures are dominated by speargrass and wallaby grass. Where speargrass and wallaby grass have been completely eliminated, these grasslands may transition into seasonally dependent bindii or annual herbfields.

Occurrence

Speargrass/wallaby grass pastures dominate large areas of the Nullarbor region on both the extensive limestone plains and in the depressions within the plains on clay plains and claypans, covering about 3.26 million hectares (4% of the southern rangelands). Soils range from calcareous shallow loams and loamy earths to red/brown non-cracking clays in claypans.

Patches of these grassland pastures are likely to have always existed in a mosaic state and transition pattern with Nullarbor chenopod pastures. However, many chenopod-dominated pastures have become irreversibly altered as a result of increased fire frequency and through grazing by rabbits in plague proportions.

These grasslands now dominate extensive areas, having replaced other habitats by increasing the fire susceptibility of much of the Nullarbor region. Speargrass/wallaby grass pastures are considered a fire induced vegetation association. Speargrass/wallaby grass pastures are the major habitat type on Bullseye, Carlisle, Chowilla, Gafa, Kybo, Oasis, Nightshade, Nurina, Shakehole and Skink land systems, and are common to many others.

Figure 4  Map of the estimated distribution of speargrass/wallaby grass pastures

Associated plants

* D = desirable, U = undesirable, I = intermediate, N = no indicator value