Narrow windrow burning is a simple, low cost way of destroying weed seeds and can significantly reduce weed seedbanks.
Concentrating weed seed bearing chaff and straw fractions into narrow windrows during harvesting increases the potential for weed seed destruction when the residue is burned in autumn.
South Kukerin grower Barry Gray has set up a chaff direction system on his harvester for narrow windrow burning as part of an integrated weed management strategy aimed at cutting herbicide use and lowering the weed seedbank. Photo: Cox Inall Communications.
With more fuel concentrated in the narrow windrows, the residue burns hotter and longer than standing stubble and conventional windrows.
Effectiveness of narrow windrow burning
Australian Herbicide Resistance Initiative (AHRI) research, supported by the Grains Research and Development Corporation (GRDC), shows 99 per cent of annual ryegrass and wild radish seeds that enter the harvester are destroyed in narrow windrow burning systems.
But often only 70-80 per cent of weed seeds get into the front of the harvester – so there is 70-80 per cent efficacy of destruction of total weed seed set.
AHRI research associate professor Dr Michael Walsh says narrow windrow burning is just as effective for other annual weed species, including brome grass and wild oats, and works in cereal, canola and lupin stubble windrows.
He says when conditions are right and weed infestations at harvest are moderate - at about five plants per square meter - it is likely weed numbers will drop significantly in the first year.
If weed numbers are high at harvest - about 10-50 plants/m2 - it may take several years to see a reduction in populations.
Tips for harvesting
The narrow windrow burning system requires only simple modifications to the harvesting process and is cheap and easy to use.
To optimise the number of weed seeds entering the front of the harvester, it is recommended crops are cut as low as possible – e.g. beer can height or lower.
Using a mounted chute at the back of the harvester, straw and chaff fractions are funnelled into narrow windrows of about 500mm to 600mm wide.
Many growers in the northern WA wheatbelt are using a chute that is the width of the shaker tray and has a minimum gap at the base of about 500mm for trash to flow out, which is especially important in canola crops.
Some growers in southern areas have set up a system of reverse spinners on the harvester that direct chaff to adjacent heavy duty PVC tubes and deposit it on top of the straw windrow.
Regardless of the modifications used to make narrow windrows, they will all potentially block when the harvester stops.
It is recommended that when there is a stop, the machine is shut down and then reversed-up before starting again.
This is because there is still material coming out of the harvester and it is best to avoid it piling up in the chute and then blocking up the header.
Where to place the chaff in the windrow
Some growers claim that placing the chaff and weed seed fraction on top of the straw windrow destroys more weed seeds because these are in the hottest part of the fire when stubble rows are burned in autumn.
But AHRI research has shown that regardless of where chaff is placed on the windrow, the weed seeds have generally settled on the soil surface by the time the burning season starts the following autumn.
Tips for achieving a burn that kills weed seeds
The longer the duration of high temperatures at the soil surface, the more weed seeds are destroyed.
AHRI has found the ideal soil surface temperature for annual ryegrass seed kill is 400C or hotter for at least 10 seconds.
This same temperature maintained for 30 seconds will ensure the destruction of wild radish seeds, or the same result can be achieved for wild radish from a hotter fire of at least 500C for 10 seconds.
Pulse and canola stubbles burn well - and safely – and AHRI researchers say these are the best to trial first when starting a narrow windrow burning system.
Burning narrow windrows in high yielding (more than three tonnes per hectare) wheat and barley crops is risky due to high residue levels - and should be avoided if possible.
The efficacy and safety of windrow burning is dependent on environmental conditions. High temperatures, poor wind conditions and rainfall can restrict burning efficacy.
AHRI recommends using a fire weather index that takes into account temperature, humidity and wind speed to rate burning conditions.
Higher temperatures increase the risks of fire escapes, especially when burning larger cereal crop windrows.
Rainfall reduces burning temperatures and if the windrow is wet, it will not burn right to the soil surface. In this situation it is best to wait for about two weeks - or least until all but the bottom 2-3cm of the windrow is dry.
The most important factor is wind. A light breeze – of about 5-10km/hour – is best to fan the fire.
A light cross wind at right angles to windrows supplies oxygen for the fire and ensures that fire burns slowly right through the windrow.
Dr Michael Walsh, AHRI
08 6488 787
GRDC Integrated Weed Management information hub, including ‘How to’ videos: www.grdc.com.au/Resources/IWMhub
WeedSmart information hub – including harvester chute designs for narrow windrow burning: www.weedsmart.org.au
Australian Herbicide Resistance Initiative (AHRI): www.ahri.uwa.edu.au
AHRI Fact Sheet Financials of windrow burning: www.ahri.uwa.edu.au/files/files/1170_HWSC_financials_narrow_windrow_burning.pdf
WeedSmart ‘How to’ videos: http://www.weedsmart.org.au/videos/how-tos/
GRDC Integrated Weed Management Manual: www.grdc.com.au/IWMM
GRDC RCSN Case Study Booklet The Effectiveness of on-farm methods of weed seed collection at harvest time: case studies of growers in the Albany port zone: www.grdc.com.au/CaseStudy-WeedSeedHarvest-Albany
GRDC Project Code
CSP00146; DAW00213; UWA00124; UWA00146; UA00124; DAW00196; DAW00218; DAW535; UQ00062