AHRI's Ken Flower talks about stubble and yield loss

With farms operating a range of different strategies to combat stubble, from burning to running more sheep, it is clear high levels of residue will remain a problem for the upcoming season unless planned for correctly.

Crop residue is important for reducing erosion and evaporation and increasing soil moisture in Australia's dry environment - so it has an overall positive effect on yield.

However, Australian Herbicide Resistance Initiative (AHRI) director Ken Flower said high levels of cereal residue, of much more than two to three tonnes per hectare, could result in yield loss in the case of a frost event, especially with wheat following multiple cereals.

"Obviously, heavy frost will damage wheat regardless but the frost effect is worse with high levels of cereal residue," professor Flower said.

Interestingly, he found that high amounts of lupin or canola residue didn't make the risk of frost damage to wheat worse.

Cereal residue is worse for frost events as it creates "quite a thick cover" on the soil, which stops solar radiation from heating the soil.

This means at night-time, heat that would normally be radiated back out to keep the temperature up, is not there - making the crop more prone to frost.

In comparison, lupin and canola residue have thicker stems and don't fully cover the soil surface, enabling solar radiation to reach the soil.

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Another contributing factor, professor Flower said, could be that lupin residue was a darker colour - which means it does not reflect as much heat, but this 'albedo effect' still has to be tested.

As heavy cereal residues break down, the soil microbes can also lock up mineral nitrogen - resulting in a deficiency of nitrogen, therefore more fertiliser nitrogen may be required to maintain yield and protein in these situations.

After extremely high yields, professor Flower said the amount of residue in the field could be a potential issue for growers.

"Spreading residue uniformly really helps with seeding next year because you don't get so many blockages behind the header, so it's really important to spread that residue," he said.

Also, crops like canola may do better following a legume rather than seeding into heavy cereal residues, which can reduce emergence.

Professor Flower said that windrow burning was a better option than burning whole paddocks, because it reduces the stubble load by 40-50 per cent and, importantly, still maintains some cover.

This method also kills more weed seeds than burning whole paddocks.

With more stubble than usual to burn, and fire bans typically not ending until the end of March, it's going to be a tight window for growers.

With harvesters getting substantially bigger, it is getting harder to spread residue right up to the full extent of the cutting front.

"So what you tend to find over a number of years is a build-up of nutrients and organic carbon closer to your wheel tracks compared with the outer rows that might not receive as much stubble, because more residue is accumulating directly behind the harvester over time," professor Flower said.

"That can have an impact on the uniformity of your crop, with better growth in this area and the effect is even greater if the residue is concentrated in the windrow."