Over the last 25 years, conservation cropping, which includes reduced soil cultivation and retention of crop stubble, has continued to evolve, driven by the need to maximise water use and protect soil in an era of increasing climate variability.
In Victoria, the level of adoption of conservation cropping practices has increased from around 25 per cent to around 75 per cent, as more and more farmers recognise the benefits in terms of productivity and sustainability. Soil properties in our crop growing regions have improved as a result of this change.
Notwithstanding this, strategic burning of stubbles, based on sound agronomic principles, may occasionally be a valid option, for example in some paddocks with very high cereal leaf disease burdens. Farmers should have flexibility to use this option when it is most appropriate.
The key points of stubble burning are:
- Stubble management is one of many complex issues that farmers must contend with.
- There is no single, 'one-size-fits-all' solution for managing heavy stubbles.
- Stubble burning is not the preferred option for the majority of farmers.
- In particular circumstances, such as dealing with herbicide resistant weeds, stubble burning may be a reasonable option.
- Most farmers will only burn stubbles when absolutely necessary, having considered all available options and the potential implications of burning.
- To minimise negative impacts, farmers should rake and burn windrows or cool-burn just before the break of season.
- Stubble retention has many benefits, but requires a systems approach to manage disease, pest and weed pressure.
- A number of techniques, other than burning, can be employed to manage heavy stubble loads.
- Effective stubble management begins at harvest with even spread of residue and appropriate stubble cutting height.
- Decisions about stubble management may need to be reviewed annually.
The advantages of burning are:
- quick and easy
- can assist weed, insect and disease control
- reduced nitrogen tie-up.
The disadvantages of burning are:
- loss of nutrients
- loss of carbon
- impact on soil microbes and fauna
- reduction in soil structure (soil aggregate stability)
- increase in erosion (wind and water)
- can increase acidity over time.
Carbon losses from burning
There are general community perceptions that the carbon (C) component in stubbles is lost by burning and that the process of burning stubbles even occasionally, seriously affects the organic carbon levels of the soil.
Research clearly shows that around 80% of the C in standing stubble will return to the atmosphere as CO2 in the short to medium term. Losses of carbon as CO2 to the atmosphere through burning are often only slightly greater than through natural decomposition, but they are of course immediate.
Actual nutrient losses caused by burning are also known.
After harvest, a 3.45 t/ha wheat yielding crop left a residue of 5.4 t/ha of above ground dry matter. A cool burning of this stubble yielded 437 kg/ha of ash, the balance being lost as smoke to the atmosphere.
The nutrients lost and proportion of straw were:
- nitrogen 16kg (straw 80%)
- phosphorous 0.5kg (straw 40%)
- potassium 17kg (straw 60%)
- sulphur 1kg (straw 50%)
This nutrient loss is well understood by land managers and strongly influences decisions not to burn, except under extreme circumstances.
Wimmera and Mallee transect surveys indicate a downward trend in the amount of stubble being burnt.
While there can be exceptional years when above-average biomass production creates extra challenges for land managers, soil health messages delivered throughout Victoria by Agriculture Victoria (and others) are generating real practice change with a strong trend toward adoption of minimum or no-till, stubble retention and precision agriculture practices.