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Heavy stubble loads demand grower flexibility

Quambatook, Victoria, grower Gregor Knight burned stubble in 2023.
Photo: Supplied by Gregor Knight

Decades of research have established region-specific guidelines for managing retained standing stubble at different times of the year.

In most cases, what growers do largely depends on their seeding system, but it also can be influenced by crop rotations, weeds, diseases, pests, stubble load, harvest machinery and how much ground cover is desirable.

After three consecutive La Nina years contributed to three big winter crops, many growers have found heavy stubble loads increasingly challenging at sowing.

There is still no single answer, but one thing that has not changed since GRDC’s five-year $17.5 million Stubble Initiative concluded in 2018, according to CSIRO senior experimental scientist Tony Swan, is the need for growers to be flexible.

Mr Swan says the key message from the initiative is still relevant: do not let stubble compromise “the big things” of the system – which are weeds, diseases, pests and timeliness of sowing.

“It’s been our take-home message for many years now: make sure you can get next year’s crop in,” he says.

“In other words, if you expect to have too much stubble for your seeder to handle or for the following crop to emerge from, you need to start preparing before harvest to manage this so that you can sow your next crop on time.”

Mr Swan says disc seeders, operated by about 20 per cent of growers, are better able to cope with heavy stubble than tyne seeders, but they are not infallible.

“Many growers who primarily use disc seeders and long-time no-till farmers were forced to burn large areas of stripper harvested cereal crops over the last couple of seasons,” he says.

“That was simply because the stubble load was so large that the surface soil stayed too wet to sow into.”

Circumstances lead to burning

Among those who burned stubble in 2023 was Gregor Knight, who farms with his parents Malcolm and Jan at Quambatook in Victoria’s southern Mallee.

Gregor says they began retaining stubble as a method of conserving moisture over summer, as well as preserving ground cover to reduce the likelihood of sheep compacting the soil and causing erosion.

“Lots of burning went on this year,” he says. “Nearly all the farms around here would be no-till stubble retention, but there seemed to be a lot go up.”

Gregor says they had no issues sowing into cereal stubble with the John Deere 1830 air hoe drill they have used since 2019 on 254-millimetre row spacings.

But it was a different story when it came to legume stubbles that had been flattened by flooding.

“Our bar doesn’t have coulters or anything to cut through it, so it just came up like a mat,” he says.

“We’ve taken on some leased ground that was on a different spacing and we had no chance. They had a heap of stubble there, so we had to burn that 200 hectares and 250ha of our field pea stubble. It would be four or five years since we last had to burn.”

Apart from reducing the stubble load, Gregor says another advantage of burning the new block was cleaning up problem prostrate weeds, such as hogweed, which tended to wrap around tynes.

“We should have that fixed up and also, being on our tyne spacing now, that should minimise our need to burn in future,” he says.

“We’d rather not burn, but if we have to, we just do it.”

Teething issues

Another who burned stubble was Dallas Hobbs, who farms with his father Stephen at Douglas, south-west of Horsham.

They converted to a 12-metre controlled-traffic system with an NDF single disc opener for seeding on 300mm row spacings about five years ago.

Dallas says they wanted to be able to drill straight through stubble without having to spend time working it or burning it, but experienced what he calls “teething issues”, which included hair pinning.

With cereal crops yielding more than five tonnes per hectare, they aim to leave about 30 centimetres of stubble, striking a balance between stubble that is too tall and will shadow the next year’s crop, versus stubble that is too short and results in more residue in the inter row.

“We did some burning last year on areas that were flooded out, washed out and got pretty hairy-looking,” Dallas says.

“They had a lot of grass germinate through it and stuff like that ... that’s the first year we’ve burned probably in over 10 years.”

Apart from weeds, Dallas says benefits from occasional strategic burning included removal of snails and food sources for mice – but burning stubble is not a quick fix.

“It takes a lot of time,” he says. “We’ve probably been able to use that time more effectively with a liming program.”

Sowing into thick stubbles

By contrast, Jarred Schlitz, who farms with wife Natalie and his parents Ron and Deid at Normanville in Victoria’s southern Mallee, did not find it necessary to burn stubble.

They converted from knife points and press wheels to a 12m Horsch Avatar disc seeder and 200mm row spacings in 2019.

The Schlitzes grow lentils, vetch, oaten hay for export, wheat, barley and canola, and have i rrigated pastures for their sheep.

Average yields for last year’s cereal crops were 4t/ha, compared to the long-term average of 2.5 to 3t/ha.

“We changed to a disc seeder to handle more stubble, maintain ground cover and subsequently conserve soil moisture,” Jarred says.

“It’s been good so far. We really like the system. It can handle wet conditions and we’ve had very few issues sowing into thick stubbles where seed to soil contact is pivotal to crop emergence.”

Jarred says they use a Shelbourne stripper front, which leaves taller standing stubble and increases harvest efficiency because the header does not have to process a large quantity of straw.

According to an Agriculture Victoria fact sheet on managing stubble, surveys found stubble retention rates in the Wimmera increased consistently during the Millennium Drought, but levels of burning or cultivation rose in 2012, 2017 and 2021, after wet seasons produced heavy stubble loads.

The surveys also found whole paddock burning in the northern Wimmera peaked at about 18 per cent in 2004, while windrow burning has increasingly replaced some whole paddock burning in the past eight years.

Decision timing

The best time to make decisions about managing heavy stubble loads is before harvest, according to Tony Swan, when growers should consider how they will manage over summer fallow in preparation for seeding the following crop, and set up the header accordingly.

“If it’s a nice dry harvest, where things can go a bit slower, you can harvest at the right height, spread the straw properly and you’ve got time to do it,” he says.

“If it’s one of those extremely wet harvests, we’re recommending harvesting higher and faster or using stripper fronts, getting it off when you can, and then coming back – if you’re using a tyne seeder – and undertaking summer fallow management such as mulching and adding nutrients, hay removal, grazing or simply cutting the stubble to the right height and spreading evenly.”

After three decile eight to 10 years, Mr Swan says some growers who harvested two cereal crops in a row would be facing stubble loads of 12 to 15t/ha by the end of 2023.

“A farmer has to say, ‘What am I going to do? What’s my next crop? And how am I going to manage this stubble?’” he says.

“If they do want to burn, the best option is a late, cool burn at the end of April or early May. By then you’re not losing as much in nutrients, compared to an early, hot burn, where you lose a lot more nutrients from the system.”

But stubble burning is not the only option.

At the Cowra GRDC Grains Research Update in July, grower Tom Johnstone spoke about the impacts of recent heavy stubble loads on the family’s mixed-farming operation and how they have embraced mulching as a strategy to maintain their “no-burn philosophy”.

Along with including legumes, such as faba beans, in the canola/wheat rotation, the Johnstones baled all cereal stubble from the header windrow in 2017–19, and bought a Kuhn Excelerator® vertical tiller/mulcher in 2020 to smash up stubble after harvest.

Tom says they had tested a mulcher in 2016 but opted to cut the straw short and bale the header tailings in 2017–19 when demand was high due to drought.

Grower Tom Johnstone

Tom Johnstone spoke at the Cowra Grains Research Update in July about the impacts of recent heavy stubble loads. Photo: Supplied by Tom Johnstone

By 2021 it had turned wet, and they found two passes of the mulcher offset to cereal stubble at seven to 10 degrees gave an excellent result, providing a level seedbed suitable for the Gyral Sure Strike parallelogram airseeder with coulters on 250mm row spacings. Grazing paddocks, stubble grazed in summer and brown manure paddocks often needed only a single pass with the mulcher.

“We generally mulch stubbles in March on to moist soils, except where we aim to sow an early dual-purpose crop,” Tom says.

“Here we mulch earlier, sometimes directly after harvest to ensure we can sow these crops when the early rainfall occurs.”

Tom says the mulcher had six main advantages: its capacity to level out bog marks and wheel ruts, retain stubble, chop up weed residue, ensure timely sowing and even germination, allowing the deep tynes to open up heavy red soils after grazing by cattle, and eliminating burning that causes smoke pollution, costs nutrients and requires permits.

The two main disadvantages were the time it took to operate and maintain the machine, and the cost, which they estimated at $33.77/ha for each pass.

The next step was to apply a tailored fertiliser blend before using the mulcher in cereal stubble in the wheat/canola rotation, in a bid to increase stubble conversion to humus and boost soil fertility.

Tom says the benefits of retaining stubble are clear, but many growers had struggled to successfully manage it while growing profitable, agronomically sound crops.

“To achieve this, the system needs to be fluid and interchangeable, and we need a way of capturing the value of carbon in the ground,” he says.

“The ability to capture carbon and maintain our soil is only going to become more important and valuable in the future, particularly in terms of sustainability.”

Research led by CSIRO Agriculture chief research scientist John Kirkegaard has found adding a ratio of nitrogen, phosphorus and sulfur – and then incorporating or mulching it – accelerates the breakdown of cereal stubble into humus, a stable form of organic carbon that feeds beneficial microbes, binds to the soil and does not wash away.

The concept floated by former CSIRO soil scientist Clive Kirkby has been proven in trials at Harden, New South Wales, where nutrients were applied to achieve ratios favoured by microbes of about five kilograms of nitrogen, 1.5kg of phosphorus and 1kg of sulfur per tonne of wheat stubble. Over eight years, soil carbon increased by about 10kg/ha.

Mr Swan says the best time for these post-harvest nutrient applications depends on stubble load, topography, rainfall and temperature.

Earlier, such as in January or February, is better for flat paddocks – if there is rain coming – while later is better for sloping paddocks. It takes a couple of months with the right conditions and sufficient nutrients for a sizeable percentage of the stubble to be converted to stable humus.

Either way, there will be no breakdown without soil moisture, he says.

When burning makes sense

Birchip Cropping Group research project manager Alison Frischke says there are occasions when burning is the most economical solution for trash, weed and disease management.

But burning should be avoided, if possible, due to the loss of ground cover, increasing the risk of erosion of topsoil nutrients and losing soil moisture.

Another reason not to burn is public attitudes towards air pollution, she says.

Scientists at the Centre for Safe Air (CSA) released a report to mark the United Nations’ International Day of Clean Air on 7 September, calling on the Australian Government to adopt policies and implement regulations to tackle the problem of air pollution.

The report highlighted the role of air pollution as the world’s single-greatest environmental cause of preventable disease and premature death – linked to more than 3200 deaths a year in Australia at an estimated cost of $6.2 billion.

It is not just pollution from producing and burning fossil fuels that is responsible. According to a 2020 paper in the journal Environmental Advances, stubble burning is the third-most-significant source of air pollution, and huge volumes are burnt in populous countries such as India and China.

The CSA report called for a unified approach that considers and addresses the diverse sources of air pollution, including agriculture and forestry.

Advantages of burning stubble

  • It controls weeds that can use soil water and nitrogen, and destroys weed seeds.
  • Standing and surface stubble can tie up nitrogen, causing yield penalties of 0.3 to 0.5t/ha, especially in wheat-on-wheat crops.
  • Seeding equipment is less likely to become blocked.
  • Seed-to-soil contact and emergence are maximised, especially in small-seeded crops such as canola.
  • Pre-emergent herbicides may be more effective, depending on stubble load, percentage ground cover and herbicide type.
  • It eliminates habitat for pests such as mice, snails, slugs and insects.
  • It can reduce soil and stubble-borne diseases, such as nematodes, rhizoctonia, blackleg, yellow leaf spot and crown rot.

Disadvantages of burning stubble

  • Maintaining ground cover of at least 70 per cent (equal to two to 3t/ha of cereal stubble) helps avoid soil loss from wind or summer storms.
  • Stubble cover helps protect soil structure and increases infiltration and water storage.
  • About five kilograms of nitrogen/ha is lost for each tonne of stubble burnt.
  • There is potential for fire to spread to crops, bushland and property.
  • It is another task to fit into a short window in the operational plan before sowing once fire restrictions are lifted.
  • Air pollution can drift across nearby properties, towns and cities.

Alternatives to burning stubble

  • Baling straw for stock feed and bedding.
  • Short-term grazing by sheep or cattle to consume residual grain and green pick and aid nutrient cycling.
  • Mulching by using a mulcher, slasher, prickle or disc chaining, or rolling.
  • Incorporating with cultivation using a speed tiller or heavier cultivation if adding lime using a disc plough, Horsch Tiger or similar equipment.
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