The Pilbara region covered in this guide
The Pilbara pasture condition and management guide covers about 192 000 square kilometres of the Pilbara region of Western Australia (Figure 1). We used information from the Rangeland inventory and condition survey of the Pilbara region, Western Australia and the Pasture condition guides for the Pilbara to prepare this guide.
Pilbara pasture types with links to condition and management information
We report on 12 pasture types in the Pilbara, classified into 3 major groups: those dominated by spinifex (hummock grasslands); tussock grasses (tussock grasslands); or shrubs (shrublands).
Spinifex pastures
Tussock grass pastures
- Roebourne Plains grass pastures
- Ribbon grass pastures
- Mitchell grass alluvial plain pastures
- Mitchell grass tableland pastures
- Buffel grass pastures
Shrub pastures
- Bluebush–saltbush pastures
- Snakewood chenopod pastures
- Hardpan mulga shrub pastures
- Acacia mixed shrub pastures
Pilbara pastures and grazing management
Pasture types
Each pasture type represents a broad group of similar vegetation associations or ‘site types’ which need similar management for pastoralism.
A few pasture types in the area are minor in a regional context but are important locally in the management of individual stations. For the sake of simplicity and brevity these have not been given separate descriptions. For example, kangaroo grass pasture of the Hamersley Plateau is only found in two areas. The Mitchell grass alluvial plain pasture type in this guide has the greatest similarities in productivity, sensitivity and management requirements to kangaroo grass pasture.
In the Pilbara, there is a gradual change from dominantly shrub understoreys in southern parts, to tussock grass or hummock grass (spinifex) understoreys in the north. For convenience, snakewood and mulga tall shrublands with understorey dominated by grasses have been described within their relevant grass pasture types, and those with low shrub understoreys have been described in their relevant shrub pasture type. We have assumed that soils with crabholes (gilgai microrelief) were once dominated by tussock grasses.
Carrying capacity and grazing management
We provide suggested levels of pasture use (per annum) in hectares per cattle unit (ha/cu) for three levels of pasture condition: good, fair and poor. This can be considered as a guide to present carrying capacity. See Table 2 for the relationship between pasture value (not condition!) and carrying capacity.
In the rangelands, the term 'carrying capacity' is a complex one, but not very complicated. The idea is to match grazing to the conditions at the time, and to the impact that grazing is likely to have over much longer periods. The terms used by DPIRD are:
- Potential Carrying Capacity (Potential CC): the estimated long-term average carrying capacity for an area (paddock, station or region) while maintaining or improving pasture condition, and assumes that all pasture types are in good condition; the area is fully developed for grazing (particularly water-point distribution); all feed is accessible to livestock; grazing by native and feral animals is controlled; and there is good practice grazing management.
- Present Carrying Capacity (Present CC): the Potential CC discounted for an assessed decline in pasture condition, based on defined ‘discount factors’ for each land system. Each of the pastures has a suggested level of use based on the estimated Present CC for the 3 levels of pasture condition (good, fair, poor).
- Seasonal Carrying Capacity (Seasonal CC): the number of livestock units a paddock or management area can carry in the current season while maintaining or improving pasture condition; this carrying capacity can markedly fluctuate between seasons.
The long-term productivity of rangeland pastures relies on having desirable perennials as a major component (mostly grasses in the north, and mostly shrubs in the south). Perennials have the ability to survive drier than average seasons, and recover in good seasons which gives a more even production than annuals..
However, desirable perennials are palatable to stock, and are called 'decreasers' because they are preferentially grazed and can be killed by overgrazing. Always assess grazing pressure in a paddock by the condition of the desirable perennials – there may be large amounts of 'fodder' still available, but the desirable perennials get grazed out first.
The grazing value and appropriate stocking of a particular pasture at any time varies enormously with seasonal conditions, fire history, perennial pasture condition and degree of recent use. Browse and herbage accruing from periods of growth will, in practice, be taken by stock and many other herbivores including feral donkeys, kangaroos, small mammals, termites and other insects.
Spelling of pastures (destocking) is the most important means of improving pasture condition.
Projected foliage cover (PFC)
In the pasture type descriptions, projected foliage cover is used to describe the extent of shrub and tree vegetation, and hummock grass (spinifex) cover, as a percentage of ground cover (Table 1). This measure assumes that the tree or bush canopy is quite stable over a year and between years.
| Term | Projected foliar cover (PFC) |
|---|---|
| Isolated | <2.5 |
| Very scattered | 2.5‑10 |
| Scattered | 10‑20 |
| Moderately close | 20‑30 |
| Close | 30‑50 |
| Closed | >50 |
Pasture value and carrying capacity
For the purpose of this report, the following six categories of pasture value or potential are based on estimated present carrying capacity (Table 2).
| Pasture value | Carrying capacity |
|---|---|
| Very high | 5–15 |
| High | 16–30 |
| Moderately high | 31–50 |
| Moderate | 51–80 |
| Low | 81–120 |
| Very low | >120 |
Indicator plants and pasture condition
Each pasture type has characteristic plants known as indicator species that can be used to assess the condition of the pasture for grazing. Plants in the species list for each pasture type have been divided into four categories of indicator value (see Table 3) - decreasers (desirables), increasers (undesirables), intermediates and no indicator value. Some species are more sensitive to grazing than others in the same category. For example, ruby saltbush (Enchylaena tomentosa) is much more easily removed from bluebush–saltbush pastures than tall saltbush (Rhagodia eremaea), but both are classified as decreasers.
More detailed information on many of the common species found in the Pilbara, including their indicator values, is contained in ‘Arid shrubland plants of Western Australia’ (Mitchell and Wilcox 1994) and ‘Plants of the Kimberley region of Western Australia’ (Petheram and Kok 1983). By being able to distinguish plant species, land managers can determine the impact of their management practices.
| Broad group | Characteristics |
|---|---|
| Decreasers (desirables) | Species which decrease in numbers as grazing pressure increases (e.g. barley Mitchell grass - Astrebla pectinata, silver saltbush - Atriplex bunburyana). These are highly palatable preferred species and are also known as ‘desirables’. |
| Increasers (undesirables) | Species that increase in number with grazing pressure (e.g. crinkled cassia - Senna artemisioides subsp. helmsii, feathertop three awn - Aristida latifolia). They can also include palatable species that are poisonous to livestock (e.g. black soil poison - Stemodia kingii). |
| Intermediates | Species which may initially increase under grazing, but being moderately or slightly palatable, later decrease under continued increasing grazing pressure (e.g. three winged bluebush – Maireana triptera). |
| No indicator value (stability desirables) | Species which are largely unaffected by grazing and which usually only decrease in number after natural disturbance such as hail damage or fire (e.g. mulga - Acacia aneura, hard spinifex - Triodia spp., eucalypt trees). These species are not palatable or only slightly palatable (or out of reach of browsing animals) and are known as ‘stability desirables’. They confer stability on the landscape and contribute to important landscape functioning processes such as water retention and nutrient cycling. |
Other resources
- Blood, D, Mitchell, AA, Bradley, J & Addison, J 2015, Field Guide to Common Grasses of the Southern Rangelands, Rangelands NRM.
- Clunies-Ross, M, & Mitchell, AA 2014, Pasture Identification: A field Guide for the Pilbara, Greening Australia
- Maslin, BR (coordinator) 2018, WATTLE: Interactive Identification of Australian Acacias, Version 3, Australian Biological Resources Study, Canberra; Department of Biodiversity, Conservation and Attractions, Perth; Identic Pty. Ltd., Brisbane, viewed 27 August 2019, https://apps.lucidcentral.org/wattle/text/intro/index.html
- Mitchell, AA, & Wilcox, DG 1994, Arid Shrublands Plants, University of WA Press
- Moore, P, 2005, A Guide to Plants of Inland Australia, Reed New Holland
- Pringle, HJ, & Cranfield, R 1995, A key to the species of bluebushes (Maireana species) of the arid southern shrublands of Western Australia. Department of Agriculture and Food, Western Australia, Perth. Resource management technical report 147, viewed 27 August 2019, https://researchlibrary.agric.wa.gov.au/rmtr/132/
- Smith, N, Clark, M 2014, Pilbara native plants: for gardens and landscapes, Greening Australia (WA) Shenton Park, Western Australia, viewed 27 August 2019, https://www.greeningaustralia.org.au/pilbara-native-plants-for-gardens-and-landscapes/
- van Vreeswyk, AM, Leighton, KA, Payne, AL, & Hennig, P 2004, An inventory and condition survey of the Pilbara region, Western Australia, Department of Agriculture and Food, Western Australia, Perth. Technical Bulletin 92.