Crop weeds: reduce weed seed numbers in the soil

Page last updated: Monday, 11 March 2019 - 11:27am

Burning residues

Burning crop residues can reduce the surface seedbanks of many weeds. All crop residues (canola, wheat, lupin and others) can produce a sufficiently heated burn to kill weed seeds. A narrow windrow will burn at a higher temperature and improve weed seed kill. More information can be found at GRDC IWM hub: Managing the weed seedbank.

Burning stubble residues to destroy weed seeds

 Burning stubble in a paddock

Burning in summer is illegal in Australia, but autumn burns effectively destroy weed seeds. All crop residues can produce a sufficiently heated burn provided that adequate tonnage of residue is present. Hence, higher temperature burns will be obtained by concentrating residue into a narrow windrow. Burning a narrow windrow also reduces the percentage of the paddock that is burnt, thereby reducing the area prone to wind erosion. Seeds close to the soil surface are more likely to be killed than seeds that have been buried.

Benefits

  • Burning destroys weed seeds. In WA, burning windrows of wheat, canola or lupin trash has been found to destroy 75% of wild radish seed and 98% of annual ryegrass seeds.
  • Late autumn burning of crop residues may also kill emerged weed seedlings (including self-sown volunteers such as wheat).
  • Burning can stimulate weed germination of some weed species. Fire can be very effective at stimulating germination of hard or dormant seeds, which allows for subsequent control with another tactic.
  • Burning removes residues and thereby allows more effective incorporation of pre-emergent herbicides.
  • Burning residues makes it easier to sow the subsequent crop, improves disease and pest management and eliminates short-term nitrogen tie-up.

Issues to consider

  • Best success is achieved by a high temperature burn, accounting for seasonal risks.
  • The area to burn should be prepared to ensure best placement of weed seeds. Seeds should be on or just above the soil surface. Grazing should be avoided or reduced to ensure that sufficient residue remains for the burn and that weed seeds are not buried by trampling.
  • Time burning to suit residue conditions and legislative requirements. Burning early in summer gives best weed seed control, but is illegal and also increases the chance of erosion while reducing the efficiency of water conservation.
  • The impact of burning depends on residue placement and quantity. Placing the residue in windrows will result in a slower, hotter burn.
  • Not all weed seed banks can be decreased by effective burning. Some weeds such as wireweed are not affected by burning and others benefit from burning.

Potential disadvantages of burning

  • Environmental concerns about carbon dioxide emissions and pollution from burning crop residues, as well as respiratory health issues like asthma.
  • The risk of soil erosion following burning, especially a total residue burn.
  • Adverse effects on soil fertility, organic matter and soil structure, especially if burning is used frequently.
  • Reduced soil water infiltration and increased evaporation and run-off due to crop residue removal.
  • Reduced numbers of macro and micro-organisms, especially earthworms, and therefore reduced biopores.
  • A shortened sowing window after rain.

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