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Australian Herbicide Resistance Initiative (AHRI)

Burning wet windrows

People in field

Ten years ago in WA we made a big song and dance about burning narrow windrows to destroy weed seeds and many grain growers jumped on board for the first time.

You’ll never guess what happened! Summer rain, and plenty of it.

When we came out of hiding (expecting to be lynched by farmers with burning torches) we were pleasantly surprised to find that the windrows actually burnt really well after they were given a couple of weeks to dry out. Another up-side was it was easy to keep the fire in the windrow.

In 2011 when AHRI ran workshops in eastern Australia this catalysed a huge uptake of windrow burning and of course the wettest summer on record!

So perhaps the message is “set up narrow windrows for guaranteed summer rain!”

History is repeating itself.

Many growers in SA, Vic and NSW have recently adopted narrow windrow burning and many have now had beautiful soaking rain – happy days! Wet windrows are definitely a challenge, but a good result is still possible. AHRI researcher Dr Michael Walsh measured the effect of rain on subsequent windrow burning temperature in 2007. He found that rain did reduce the burning temperature of the windrows, but they did burn hot enough to destroy weed seeds when given two weeks or more to dry out. This research, along with a few tricks from experienced growers in WA tells us that all is not lost.

Earlier research by AHRI researcher Michael Walsh and (then) student Darren Chitty determined the burning temperature required to destroy weed seeds. 400oC was the magic number, what mattered was the duration of exposure to this temperature. The figure below shows the temperature and duration required to destroy weed seeds. Weed seeds can also be destroyed by lower temperatures with longer exposure time.

burning_temps

Figure 1. Burning temperatures and duration required to destroy 100% of weed seeds.

In 2007, Michael Walsh with help from Vanessa Stewart (DAFWA) set out to determine the effect of rain on subsequent burning temperatures. They wet a section of wheat windrow with the equivalent to a 50mm rainfall event every 7 days over 3 weeks in March and then burnt the windrows on 27 March. This resulted in treatments with different drying times. The treatments were Nil rain compared to 50mm rain either 15, 22 or 29 days prior to burning.

Graph of burning temperatures effect on wheat

Figure 2. Burning temperature at the soil surface of wheat windrows exposed to nil rain or 50mm simulated rainfall event either 15, 22 or 29 days prior to burning on 27 March 2007.

All of the rainfall treatments reduced the maximum burning temperature. However, burning temperatures of 300 to 400oC were maintained for at least 10 minutes. These temperatures are more than adequate to destroy weed seeds.

The message is simple, rain does reduce burning temperature but a good result is still possible if windrows are given two weeks or more to dry prior to burning.

The answer my friend is to burn in light wind

We know that most of the weed seeds in a windrow are at or near the soil surface so we need to achieve these hot temperatures at the soil surface during a windrow burn.

Michael Walsh and Peter Newman undertook research to determine how to reliably achieve hot burning temperatures at the soil surface. They concluded that the answer is to burn in light wind. Light wind at burning of windrows is essential for the windrow to burn all of the way to the soil surface. Common sense and experience tells us that this is more important than ever when the windrows have been exposed to rainfall.

The ideal wind is a light cross wind (8 to 10kph). A big roaring stubble fire may look impressive but can burn quickly over the top of the weed seeds sitting on the soil surface. A slow burn is a good burn. Burning in dead still conditions can create a slow burn but the down-side is that the fire can choke itself out as there is not enough oxygen to fuel the fire at the soil surface. A light cross wind fixes this problem.

Peter Newman simulating strong wind during windrow burning research

Figure 3. Peter Newman simulating strong wind during windrow burning research. Don’t try this at home!!

Burning conditions – download the PocketFire App

To minimise the risk of fire escapes, it is a good idea to use the McArthur grass fire index to pick the right conditions for burning windrows. This is available through downloading an App called NSW PocketFire. A grass fire index is calculated by inputting the ambient temperature, humidity and wind speed (set grass curing to 100%). A grass fire index between 2 and 10 is ideal to minimise the risk of fire escapes. This App is applicable to all regions of Australia.

PocketFire App screenshot

Tips from experienced windrow burner Andrew Messina

Andrew Messina, with his brother Rod, crop 12,500 hectares near Mullewa in Western Australia. Andrew and Rod have used narrow windrow burning over very large areas of lupin, canola and wheat for 12 years on their farm. They have shared a lot of their windrow burning expertise with Australian grain growers through being involved in many AHRI workshops, videos and press releases. We recently contacted Andrew for a few tips on burning wet windrows.

Windrows and farmers

  • If most of the windrow is dry but the bottom 1 to 2cm of windrow are wet and the soil is wet they will still burn hot enough to destroy the weed seeds. There may be a 1cm layer of un-burnt trash on the ground but most times this would have been exposed to enough heat to destroy the seeds even though they are not totally cremated.
  • Canola windrows always burn really hot so these will burn well if left to dry for a week or more.
  • Burning with an 8 to 10 kph cross wind is ideal for all windrow burning (wet or dry). You can burn in slightly windier conditions after rain as the risk of fire escapes is reduced, especially for lupin and canola windrows. You still have to be careful burning cereal windrows but it is easier to keep the fire in the windrow after some rain. It is also possible to burn on a warmer day after there has been some rain.
  • Even if you burn a windrow and it doesn’t get hot enough to destroy the weed seeds a narrow strip can be targeted by other means eg. Spraying a knockdown with a firebreak sprayer

Rain is the enemy of a good campfire and it is the enemy of a successful windrow burn. However, all is not lost. If wet windrows are exposed to two to three weeks of warm, dry weather, a good windrow burning result is still very achievable.

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