Stubble management still a burning question

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Do not rule out burning as an effective management tool, particularly with windrow burning rather than whole-paddock burning

Photo of windrow burning

Occasional windrow burning is a low-risk option for lowering weed seed and snail pressure.

PHOTO: Kellie Penfold

We are all aware of the benefits that have resulted from retaining stubble. Our soils have more organic matter on the surface, are more friable and better retain water. This organic matter provides the soil with protection against wind and water erosion. The surface cover helps to get crops up and going in marginal moisture conditions. This has contributed to huge improvements in crop establishment, vigour and yield over the past 30 years.

The problem is that we have become so paranoid about burning, even when presented with good reasons to do so, that many will refuse to burn stubble at any cost.

Like anything in agriculture, it is a matter of finding a balance. There are ways to burn stubble that can still provide a good amount of surface cover to achieve all the benefits of retaining stubble, but that also give us all the benefits we know are available through the use of fire.

We also know that we can minimise the risks of burning whole paddocks by using effective techniques to burn only windrows and chaff dumps.

Windrow burns

Windrow burning to manage weed seeds is more effective than whole-paddock burning. Windrow burning for ryegrass and radish, for example, is where you concentrate the chaff out of the back of the header and then burn the windrow.

The temperature in the burning windrow is higher than the temperature in a whole-paddock burn because there is more material. This means that the same length of burn is not needed. In 10 seconds, temperatures of about 500ºC can be achieved in a windrow. A standing stubble burn might only reach temperatures of about 400ºC.

That temperature difference and burn duration can be important in terms of controlling ryegrass and broadleaf weeds such as radish. The trick is containing the fire to the windrows and leaving most of the stubble cover on the paddock. When windrows are burned effectively up to 90 per cent of the soil cover can be retained, along with its associated benefits, as well as the benefits of weed seed and snail control.

Cereal windrows

Getting an effective windrow burn in cereal stubbles is more difficult, but experienced growers who are able to take advantage of ideal conditions in late autumn have achieved some excellent results.

Humidity, wind speed and wind direction are all factors that will determine the effectiveness of the burn and whether the fire will cross the windrows and damage the intervening stubble.

The job can be made easier by managing stubbles from harvest. For example, if growers are able to chop the paddock very low after harvest and even put the stubble through the header a second time it will help break down the straw into a chaff fraction. During reasonable summer and autumn rain, high levels of stubble can break down to the point where there is probably not enough material in the inter-row to carry a fire.

Initial stubble loads are important here. If dealing with a two-tonne-per-hectare crop that has not produced a lot of stubble and the management of that stubble is by slashing or putting it through the header chopper again, there will not be a lot of material left in the windrows. The likelihood of fire crossing those windrows is not very high.

If dealing with a 5t/ha crop with an 8t/ha stubble load the grower has a much harder job ahead. But it is still worth trying to achieve stubble breakdown between the windrows.

Even if growers end up with a full paddock burn just prior to planting in late April / early May before going into a cereal, that can still be acceptable. A very good level of weed and pest control can still be achieved while reducing that exposure gap between burning and planting a crop.

Weed seeds and snails

Burning to control snails is very effective – potentially 100 per cent – as snails are quite likely to be concentrated on the highest points in a windrow.

In the case of weeds, growers can expect about 85 per cent control of ryegrass seed in a windrow burn and perhaps a little less (75 per cent) in a standing stubble burn. Radish control varies from 70 to 80 per cent depending on the technique used and the duration and intensity of the heat in the fire created.

Chaff dumps

Burning chaff dumps can also be effective, with weed control levels often between 90 and 100 per cent. Chaff dumps give an intense burn and the fire can be more easily managed than in windrows. One of the problems with chaff dumps is that under most of the existing systems the chaff fraction is quite fine. Dumps take longer to burn and often have to be let burn right out. Some regional councils have restrictions requiring fires to be out before 9pm.

A judicious, occasional burn done carefully keeps a lot of weeds and pests under control in the long term. We know that some growers who burn every three to four years do not seem to have the weed and snail problems that others who do not burn are experiencing. The message is ‘don’t throw the baby out with the bathwater’. Burning has been around since agriculture began and it is an effective tool to manage weeds and pests. Used carefully, we do not need to expose ourselves to the damage that careless, blanket burning can cause.

More information:

Bill Long, 0417 803 034, bill@agconsulting.com.au

Watch this video from Doug Smith of Pingrup, WA, on windrow burning: www.youtube.com/watch?v=Hp_3tAI-VZY or press play on the video below.


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