map
EnviroNorth > VRD-Sturt > Grazing > Fire and grazing research

Fire and grazing research

by Rodd Dyer, Department of Business, Industry and Development (Primary Industries)
From Managing for Healthy Country in the VRD, Tropical Savannas CRC 2000.

Pastoral management trials

On pastoral lands, controlled fire can be used for a variety of reasons. It can be used to manage native tree and shrub populations, maintain pasture condition and quality, reduce fuel to prevent wildfires, manage grazing distribution, control exotic weeds, and establish and manage improved pastures. The effectiveness of fires in each of these roles, however, depends on a range of factors that can be managed to achieve the most desirable result. These include the frequency and intensity of fire, the season in which it burns, the amount of fuel available and the nature of grazing management both before and after the burn.

Methods

A project established at Kidman Springs in 1994 investigated what sort of fire regime is most effective for management of woody vegetation and pasture condition on both black (cracking clays) and red soils. Early and late dry season burns were trailled on arid short grass on red soils and ribbon grass on black soils in two paddocks with a range of fire frequencies over the five year period between 1994 and 1998. Frequencies ranged from no burning to burning up to three times in five years. Specific effects of burning on pasture response and woody plants were examined.

Effects on trees and shrubs

Tree and shrub species in both vegetation communities are well adapted to tolerate fire. Survival of plants after burning is very high. Following burning, plants re-sprout from undamaged buds, either from branches or from the stem base if aerial branches are killed (top-killed). Plant mortality following burning for all species remained below five percent and was not affected by frequency, intensity or season of burn. Regular fire acts on woody plants by suppressing growth and reducing plant height. The impact on trees and shrubs is largely determined by fire intensity and plant height. A range of factors such as fuel load, continuity and weather conditions influence fire intensity.

The role of fire in grazing lands is to manipulate plant height structure, rather than reduce plant density. This can be achieved by using periodic fire to top-kill taller plants of emerging populations, thereby reducing canopy height and suppressing woody plant growth.

When regular fire is removed, the growth of woody plants continues unhindered. Woody plants one metre in height will take between seven to 12 years to grow two to three metres. Once plants exceed two meters in height, rates of top-kill decrease significantly and only hot to extremely hot fires are able to achieve sufficient top-kill of aerial branches. Under normal conditions fuel loads of at least 2000 kg DM(dry matter)/ha are required for effective fires. Late dry season burns are generally more intense and effective at controlling woody plants compared to early dry season fires.

Fire frequency

The most appropriate fire frequency depends on annual rainfall, as this will determine the rate of fuel accumulation and woody plant regrowth. Burning frequency should be every:

  • two to four years in high rainfall areas;
  • three to six years in medium rainfall areas; and
  • five to ten years in low rainfall areas.

The most desirable fire frequency will be determined so that plants do not exceed the height where they become difficult to control with fire during the interval between burns.

Pasture quality and condition

The use of fire can have significant impact on pasture quality and condition. Without burning and under low grazing pressures dead grass material accumulates, reducing pasture vigor and making pastures unattractive to livestock. Burning pasture in good condition during the dry season generally has little long-term impact on pasture production and composition and increases the availability of nutritious green regrowth to cattle.

Ribbon grass pastures

Ribbon grass pastures on cracking clays are resilient and well adapted to periodic burning. Under all but the most frequent fire regimes burning generally results in only small short term reductions in total yield and cover and has negligible impact on species composition. Frequent burning (biennial fires) resulted in significant longer term reductions in yield and increases in annuals grasses such as Flinders grass. The minimum interval between burns therefore should be no less than three years when implementing prescribed burning regimes.

Arid short grass pastures

In contrast, arid short grass communities are sensitive to grazing and burning. Burning in arid short grass communities (regardless of fire frequency) may promote the dominance of annual grasses at the expense of perennials. An increase in the proportion of bare ground may also occur even during good rainfall years. When these pastures are in poor condition, the detrimental effects of burning are exacerbated. In the absence of burning, rapid recovery of pasture condition is possible when grazing pressure is low or absent.

Recovery is accelerated during periods of above average rainfall. Despite increases of trees and shrubs in this vegetation community, fire should therefore be used with extreme care due to the potential adverse impacts of burning on species composition and ground cover. Fire may have a role in pasture management if pastures are in good condition and if grazing pressure is low.

The sites established for this project have also been used to examine the impacts of fire on vertebrate and invertebrate animals, discussed under the 'Grazing, vertebrates and soil type' link above.