Fire affected pastures
Fire changes pastures in different ways according to a number of different factors: the intensity of the fire; the pasture species; the fertility of the soil; the time of the autumn break and follow up rains.
Fire intensity
Three categories of burns can be defined by considering what was burnt and destroyed during a fire and what was left:
- Cool-moderate burn: Most dead plant material burnt; some seed and perennial grasses and clovers survive unhurt. There will usually be a small residue (or stubble) of unburnt pasture remaining.
- Hot burn: All dead plant material, many seeds, young and weaker perennial grasses destroyed. The topsoil usually appears charred and bare.
- Very hot burn: The soil is virtually sterilised. All plant material and seed is destroyed as the fire burns into the top organic matter layer of the soil.
Generally cool-moderate burns occur where there is little dry grass cover before the fire. Hot burns occur where there is heavy plant cover such as lightly grazed pasture or crop stubble. Very hot burns occur under hay bales; windrows; on sheep camps; on soils with a thick root mat or where an intense fire emerges from bush areas onto pasture land.
Plants that bury their seed or have growing points below the surface should be best able to survive the effects of a fire.
Effects on annual species
Most annual grasses produce very little dormant seed. Usually 80 to 90 per cent of the seed in one season will germinate in the following autumn. This means any factor, such as fire which destroys annual grass seed, will cause a drastic reduction in the annual grass component in the pasture. In addition, the annual grass seed that survives the fire is very vulnerable to removal by wind. Subterranean clover has the ability to bury its seed. This substantially reduces the damage to the seed caused by fire.
Effects on perennial species
Grasses
Recently re-sown perennial grass pastures can be seriously damaged by fire. The young perennial plants without well established root systems and reserves are more vulnerable to fire damage, especially if the pasture was sown with a cover crop. Almost all well established perennial grasses survive a cool-moderate burn. The ability to survive a hot burn varies between species. Grasses with growing points below the soil surface survive best.
Legumes
Observations of burnt white clover-based pasture following the 1983 Cudgee fires indicated the survival of white clover is very similar to the survival of perennial ryegrass.
What can be done?
There are several possible courses of action after a fire. The appropriate action will depend on the intensity of the burn, the condition of the pasture prior to the fire and the finance and time available:
- Cool-moderate burn: The pasture should recover to its original density during the following year given adequate moisture and the absence of soil nutrient deficiencies.
- Hot burn: In most cases it is probably best to wait a season and see how the pasture recovers before considering re-sowing. However, in some cases it may be a good opportunity to reliably direct drill new pasture species or top up the old pasture.
- Very hot burn: Almost all plant material will be dead so the area should be cropped or re-sown to pasture following the fire.
Several management practices may improve the recovery of pasture after fire.
Adding new pasture seed: New pasture seed can be added to the pasture in a variety of ways, such as by direct drilling or over-sowing. It is usually best to wait until there is a germination of annuals after the autumn break. If weeds are dense, chemical weed control may be necessary before sowing the seed.
Heavy harrowing: Heavy harrowing can unearth buried seed and improve the germination, especially of subterranean clover. This harrowing may cause further damage to surviving perennial species.
Autumn saving: Leaving stock off the burnt pastures for six or more weeks after the autumn break improves the vigour and growth of surviving plants.
Fertiliser: Fertiliser application will speed the growth and recovery of pasture where there is an adequate density of regenerating plants or pasture seed has been sown.
Broadleaf weed control: Where broadleaf weeds start to dominate the recovering pasture, control measures such as the use of herbicides or spray-grazing are recommended.
Seedset: The pasture plants should be encouraged to set seed in the spring following the fire. This can be assisted by avoiding heavy grazing pressure in the mid-late spring period and not cutting the pasture for hay.
Information Note AG0203:
Pasture recovery after fire
