Grazing triggers for southern Australia

Summary

Cool-season burning has been suggested as a low-cost management option for steep hill country to increase the nutritive value of pasture and improve animal production. The focus of this study was on a 250 ha area of hill country within a case study property in north-eastern Victoria. The area is currently used to run 30 Angus breeding cows, calving in mid-August with calves weaned at 5 months of age. This analysis of cool-season burning, conducted using the SGS Pasture model, has highlighted the tradeoffs in production and natural resource management risks.

Cool-season burning was shown to provide some advantages to the production system, in particular by increasing both pasture nutritive value (digestibility and metabolisable energy (ME) content) and annual pasture growth. The increase in nutritive value was due to higher legume content and an increase in the ratio of green: dead plant material. There were also disadvantages to the production system associated with cool-season burning, because of low pasture mass following burning limited pasture intake. This was particularly the case with burning carried out later in the season (June-August) compared to burning in Autumn, and when a large percentage (>50%) of the farm area was burnt on an annual basis.

The tradeoffs in cool-season burning between the advantages of increased pasture growth and nutritive value, and disadvantages of lower pasture mass, were assessed in terms of animal production (cow liveweight and calf weaning weight).

From the simulation analyses conducted in this study, two conclusions were drawn:

1. Under conditions where 50% or more of the area underwent cool season burning late in the season (June-August), lower cow and/or calf weaning weight was simulated.
2. Under conditions where 25% of the area underwent cool season burning early in the season (March-May), cow and/or calf weaning weights were similar to the baseline scenario. When limitations of the model were considered, small increases in animal production may be possible in these scenarios.

The major change in the risk of degradation to the natural resource base with cool-season burning was increased frequency of breaching the minimum pasture mass threshold (used as an indicator of ground cover). Under current management, the threshold was breached in 21% of years, but burning breached this threshold each time it was carried out. Runoff was also predicted to increase in burnt areas. Burning increased the risk of soil erosion losses.

Knowledge of soil water content (SWC) in autumn can assist in making a decision on whether or not to burn because it alters the risk associated with low pasture mass. For the cool season burning option of 25% of the area on 15 March, if SWC was in the highest one-third of historical values on the date of burning then pasture mass increased more quickly than if SWC was in the mid or lowest thirds of historical values. Burning when SWC is high minimised the time that pasture mass was low, as such knowledge of SWC at time of burning can be used to minimise the risk of soil erosion.