Sustainable grazing in Mitchell Grasslands

Resilience to grazing

Under conservative grazing practices, Mitchell grass can withstand extended periods of grazing and drought. It is one of the most nutritious pasture species to be found in the tropical savannas and cannot be improved upon, in terms of productivity or hardiness, by other grasses. One of its most valuable attributes is that it  "hays off" (standing matter dries but does not decay or lose its nutritive value) during the dry season and continues to provide nutritious feed. During the wet season stock tend to eat other short-lived herbage and forbs, and rely solely on Mitchell grass over the dry season. Mitchell grass tussocks recover well after the dry-season grazing with the arrival of the next rains.

These plots from the VRD show how consistent Mitchell grass is over time

Grazing pressure

Mitchell grass country can become degraded if the stocking rates are heavy, particularly if there is a series of dry years. Areas around watering points are also vulnerable to overgrazing. Under grazing can also be a problem, as tillering is stimulated by moderate grazing pressure. Ungrazed plants can become moribund as old leaf and roots tie up the supply of available nitrogen in the soil.

The grass tussocks should not be chewed below 10–15 cm in height. Grazing pressure greater than this will damage the plant's productivity in the future by limiting the vigour of root growth.

Dry-season carrying capacity

The carrying capacity of an area over the dry can be estimated on the basis of the amount of feed left over at the end of the growing season. Stock should be in good saleable condition at this point and so adjustments can be made to the stocking rate. Occasional and opportunistic spelling is also recommended for maintaining healthy Mitchell grass pasture (Partridge 1996).

The southern reaches of the Mitchell grass region are some of the most susceptible to drought over the dry. There are several measures which producers can take to better prepare for this possibility. Where viable, some run both sheep and cattle, which can improve market flexibility and facilitate pasture management. Supplementary feeding is also carried out, and in some areas producers cut surplus Mitchell and Flinders grass for hay to be kept in case of dry spells.

Burning Mitchell grass

Fire is rarely used as part of management strategy in this region of the savannas, although it would once have been a part of the natural ecosystem. Burning is not carried out largely because the grass is considered too valuable. In addition, seasons are far less reliable in this region and so burning is considered too risky. Fire is used to control gidgee regrowth after clearing on the margins of this region. It also can remove undesirable pasture species such as feathertop and improve the quality of standing pastures. The major application is firebreaks which are established to protect the country against uncontrolled fires in the dry.

After burning, the Mitchell grass tussocks need to recover to a height of around 15 cm before grazing should resume over the area. Managing kangaroo numbers is important at this juncture, particularly in the south of the region where their numbers are far greater.

Watering points

Managing the distribution of water is critical in a semi-arid area such as the Mitchell grass region. During the dry, cattle graze within a 5 km radius of water. This limitation can have enormous impacts on grazing pressure and paddock utilisation. As this region sits on the great artesian basin, most producers have in the past relied on bore drains. However at present there is a push, partly subsidised, for producers to cap these bores in order to reduce water loss to evaporation. Works completed from the first year of the project replaced 200 km of bore drains and conserved around 3000 ML of ground water.

There are many other benefits associated with the shift from bore drains to piping. Some of these include:

• facilities such as spear traps can be introduced to aid in mustering
• piping can take water to areas beyond the scope of bore drains
• stock weakened by drought will not risk getting bogged in the bore drains
• forms of land degradation associated with these bores, including weed and feral animal invasion, will be more easily controlled

Optimum spacing of waters depends on fencing, preferred pasture types and prevailing wind conditions as cattle find water by smell (Partridge 1996). Again, kangaroos can significantly contribute to overall grazing pressure and must be accounted for in property improvement planning, especially in more southerly reaches of the region.

Weeds

A major management concern in the Mitchell grass region is the invasion of prickly acacia (Acacia nilotica), and to a lesser degree other woody weeds including mesquite (Prosposis sp.) and Parkinsonia. Prickly acacia has currently taken more than 500,000 hectares out of production, and has invaded less intensively around another 6.5 million Ha. Much of this country is to be found in Queensland in the northern half of the region.

Prickly acacia was originally introduced as a fodder and shade tree from the Middle East in the 1890s. The shift in much of the area from sheep to cattle grazing which occurred in the 1970s saw a dramatic increase in the distribution of the trees. Cattle do not digest the seeds of the weed as well as sheep and so act as vectors for the spread of prickly acacia.

As these trees grow close together they shade out all of the grasses underneath, limiting food available to cattle and exposing the soil to erosion. The trees also interfere with mustering efficiency and with stock access to water as they tend to cluster around bores and drains. An individual tree may produce more than 175,000 seeds per year which can remain fertile for a decade or more. Control of prickly acacia can be expensive and time consuming. Methods of control include bulldozing especially around water courses and controlling cattle access to infested areas to slow the spread. Some producers are experimenting with camels which graze the plants much more successfully than cattle, both in terms of the height that they reach and seed digestion.

While mesquite is not yet a problem comparable to prickly acacia, it is widely recognized as having the potential to get out of control. Strategic burning, which may need to be incorporated with spelling to allow fuel to build up, seems to be effective in limiting the distribution of this weed.