Less herbicide use as weed seed destroyed a possible silver lining for farmers after Wheatbelt fires

Fire-affected Wheatbelt farmers are holding on to hope their herbicide needs could plummet after last year’s weed seed banks and stubble-borne diseases were destroyed in the devastating blazes.

Agronomists believe the fires could have created 80 per cent control of surface ryegrass seeds and 10 per cent of wild radish seeds, depending on how long and hot the blazes burnt across each paddock.

Any cost reduction would be welcomed by growers whose joy of a record harvest last year was dampened by the cost of essential inputs including fertiliser and chemicals in 2022.

Elders Narrogin agronomist Helen Wyatt said depending on how hot the fire burnt in certain areas, seeds of several weed varieties could have been destroyed.

Of the three burn types — cool-moderate, hot or very hot — she said the blazes fell into the latter two.

The Shackleton-Corrigin and Wickepin-Narrogin blazes tore through more than 50,000ha across seven shires after igniting on February 6, reducing paddocks to dust, with farmers likening the landscape to the “Sahara desert”.

Any stubble in the line of the fire was destroyed, with the lack of ground cover creating “horrific” dust storms for farmers as topsoil was blown away in the wind.

However, with the burnt stubble, the fire was also anticipated to have burnt last year’s seed banks of weeds, which could have a positive flow-on effect for the growing season.

Ms Wyatt said a burn at 400C for 10 seconds would kill ryegrass, while other grasses including barley, silver and brome — and also wild radish — needed up to 30 seconds at that temperature.

However, she said seed buried deeper in the soil would not have been burnt and farmers needed to keep an eye out. She also said calthrop came back quite badly after the 2015 Esperance fires and was something to be on top of.

“That was only the previous year’s seed bank,” Ms Wyatt said.

“The cropping system is burying the seed from two to three years before and we’re probably not touching a lot of that.

“So if you’ve got a paddock that is historically bad for ryegrass, don’t think that the fire that burnt a lot of last year’s seed will solve your longer-term ryegrass problem.”

With the soil so bare, she even floated the idea of letting the weeds establish slightly longer before a knock down to provide soil stability.

“There probably is an argument to let those weeds to get that three leaf stage where they’ve got a bit of root system holding the soil together before we do kill them,” Ms Wyatt said.

“Chemicals are not necessarily cheap and we don’t want to blow out the chem budget, but if we can keep that cover even if it’s weed cover, then knock that down and then go back in with a little bit more confidence into a canola, that’s probably something to look at.”

She said insecticides should be treated in a normal seasonal scenario, but a reduction in stubble-borne diseases like canola blackleg and YSpot could lead to less herbicide and fungicide use.

“We haven’t had a big green bridge to carry things through,” Ms Wyatt said. “So hopefully we’re looking at a season with less herbicide.”

Given the amount of erosion, Ms Wyatt said growers needed to consider the increased risk of herbicides moving from the inter-row to the furrow and coming into contact with crops.

To minimise the risks, she said separation of chemical and seed was “key,” encouraging growers to sow deeper and apply herbicide once there was soil moisture to increase binding. She said they would not need “anywhere near as much” of some chemicals as there was no stubble to intercept it and it was 100 per cent on the ground.

“The first ones to start would be the ones that the crops have a natural tolerance to. . . so the safety of them if they do end up in furrow is really high,” she said.

Seeding speed also played a role in minimising the risk, she said, moving fast enough to get chemical out of furrow, but slow enough to prevent soil throw into the next furrow.

“Have a really good look behind your bar when you’re seeding just to see where that soil is ending up,” she said.

She said growers needed to be “a lot more cautious” with more mobile herbicides or decreasing rates, which could see them run into herbicide resistance issues down the track.

If growers were unsure of a certain paddock, she encouraged them to use the motto, “if in doubt, leave it out”.