- Stubble retention has many benefits for soil health, can improve soil moisture and help maintain soil organic carbon levels
- Different methods of sowing and equipment result in different thresholds for change in terms of maximum stubble load that can be retained
- If full stubble retention is not feasible due to machinery, weeds or disease constraints, there are other options such as shallow incorporation, slashing straw or cutting short at harvest which can reduce the frequency of burning
Benefits of Stubble Retention
Retaining stubble can reduce the impacts of soil erosion. At least 70 per cent ground cover minimises water erosion risk and 50 per cent ground cover minimises wind erosion risk. Stubble height should be at least one-third of the width of crop rows. In general, the shelter provided by a barrier is approximately three times its height; 10 cm tall stubble will protect the adjacent 30 cm of topsoil. (Source: Managing stubble | General agronomy | Crop production | Grains, pulses and cereals | Crops and horticulture | Agriculture Victoria )
Retaining stubble leads to higher soil moisture content through reduced run-off and increased infiltration. Increased infiltration over summer can boost nitrogen mineralisation and availability for the subsequent crop.
Retaining stubble increases the input of carbon into the soil as stubble is approximately 45% carbon by weight so represents a significant source of organic carbon.
Retaining stubbles returns nutrients to the soil, the amounts depend on the quality and quantity of stubble. Wheaten stubble from a high yielding crop may return up to 25 kg of available nitrogen per hectare to the soil. (Source: Benefits of Retaining Stubble - NSW | Fact Sheets | soilquality.org.au) The carbon to nitrogen ratio (C:N) of crop residues governs how quickly residues decompose. Pulse residues, have a lower C:N ratio so are more decomposable than wheat residues. Faster decomposition may improve nutrient availability for the following crop.
In general, retaining stubble is good for soil structure, soil organic matter and microbial activity.
Drawbacks of Stubble Retention
Blockages of sowing equipment is a major barrier to adoption of full stubble retained systems in Southern NSW and Victoria where stubble loads are high. Traditional machinery is limited to sowing through 2-3 t/ha of cereal stubble. Slashing, harrowing can enable sowing through a 4-5 t/ha stubble.
As microbes break down the stubble, they immobolise nitrogen known as nitrogen ‘tie-up’ therefore retaining stubble may require more nitrogen in early crop growth compared to not retained systems.
Disease carry-over in retained stubble can include:
- crown rot, take-all, yellow leaf spot, eyespot and Septoria tritici blotch in wheat
- net blotch in barley
- blackleg in canola
However, burning will not get rid of these diseases completely as inoculum remains below ground and in the case of crown rot burning can make things worse later in the year as the soil is drier.
Another drawback of retaining stubble is higher pest populations – mice, slugs, snails, slatters, earwigs etc. Growers will need to monitor for signs of pests in spring so they can control populations early before they grow. Finding effective and efficient ways to control these pest populations is vital to ensure productivity levels are maintained.
In parts of the country, such as Western Australia and Queensland, stubble retention is common practice to avoid soil erosion through wind and heavy rain. In Southern NSW and Victoria, stubble is more commonly burnt before sowing. Although this has some benefits of the stubble being retained over the summer months, there is a period between burning and when the crop has established sufficient ground coverage, when the soil surface is exposed to moisture loss and erosion.
Management of stubble should start at harvest: cutting stubble short and ensuring an even spread of chaff. If it is not possible to cut short at harvest due to time constraints, stubble can be mulched or slashed to accelerate decomposition. Another possibility is baling the straw, especially in years when demand and prices are high. Long stubble shades the emerging crop, resulting in a delay in flowering and maturity.
The inclusion of legumes and canola in the rotation can help as these crops produce less stubble.
Inter-row sowing allows the stubble to be retained. This technique requires rows are more than 22cm wide.
Stubble management approaches should be considered strategic and flexible. You will need to assess the stubble load and seasonal conditions. It is more difficult to retain stubble following wet years or irrigated crops.
Stubble Retention Project
Riverine Plains carried out a 5-year project on stubble retention measuring the effects of stubble on productivity. The key finding from this project were:
- Stubble management is not a key driver of yield – management should be strategic and flexible
- Long stubble shades the emerging crop, resulting in delayed flowering
- Long stubble did not significantly increase the likelihood of frost damage
To find out more about the Stubble Retention project please visit the Riverine Plains website: https://riverineplains.org.au/wp-content/uploads/2019/11/RP-Stubble-retention-report-19_LR.pdf