Chaff carts paying off for harvest weed control

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Chaff carts help avoid the cost of the nutrient loss of windrow burning.

PHOTO: Evan Collis

When considering machinery investments, a chaff cart may prove an economic option for taking pre-emptive action against problem weeds and herbicide-resistance issues.

Research and grower experience in WA in recent years indicate a chaff cart system will cost about $15 per hectare, based on an initial outlay of $75,000 to $80,000 for a new machine, combined with $7 to $8 per hectare in operating costs and $2 to $2.50/ha in likely nutrient replacement (based on producing a 3 tonne/ha wheat crop).

This may seem more expensive than simply windrow burning, but the nutrient removal cost from windrow burning is significantly higher and burning whole paddocks is not ideal in many areas – especially in medium and high-rainfall zones. That is the message from Australian Herbicide Resistance Initiative (AHRI) leader of communications and long-term weeds specialist Peter Newman.

“Chaff carts can be towed over every hectare of the farm every year and chaff dumps can be grazed before residue is burned, which can also provide extra feed value for livestock in mixed farming operations,” he says. “Sourcing a second-hand machine will obviously lower the costs of this system.”

Mr Newman says a chaff cart is one of several harvest weed-seed control (HWSC) tactics that is proving highly effective in driving down large viable seedbanks of WA’s most damaging cropping weeds. These include annual ryegrass (Lolium rigidum), wild radish (Raphanus raphanistrum), wild oats (Avena fatua) and brome grass (Bromus diandrus and B. rigidus).

“If freely allowed to produce seed that enters the seedbank, the consequences of not controlling these weeds at harvest is intense competition with crops next year that reduces establishment and potential grain yields,” he says.

GRDC-funded research has found all HWSC measures commonly used in WA tend to be equally effective. The measures typically remove about 60 to 70 per cent of standing annual ryegrass and 70 to 80 per cent of wild radish seed, leading to about 55 to 60 per cent less weed germination the following year. “If you do employ HWSC for enough years in a row, the seedbank is dramatically reduced and growers can get on top of weeds and herbicide resistance evolution relatively fast,” Mr Newman says.

A recent GRDC-funded national survey indicated 43 per cent of Australian grain growers already use some form of HWSC and 83 per cent plan to implement HWSC in the next five years. Mr Newman says the key to a successful chaff cart system is optimising the set-up of the harvester. To maximise the amount of weed seed captured and destroyed, his experience has found value in:

  • threshing chaff hard in the rotor of the harvester;
  • opening rotor grates to get more weed seeds on to the sieve; and
  • using a baffle above the sieve on some harvesters to get weed seeds into the cart/system being used.

About 10 to 20 per cent of stubble residue is removed in a chaff cart system, compared with about 50 per cent where windrow burning is used. This means there are significantly fewer nutrients taken out of paddocks where chaff carts are used – equating to about $2 to $2.50/ha in nutrient costs (mainly in replacing nitrogen and potassium), Mr Newman says.

 “This may be less where sheep are grazing chaff dumps and respreading residue across paddocks in droppings. “It is worth looking at all the HWS controls available because windrow burning can be expensive when you consider the nutrient implications and potential problems with getting windrows to burn consistently across the whole farm.” Mr Newman outlines his top tips for setting up chaff carts, managing residue and considerations for nutrients in a new GRDC ‘Know More’ video. This short guide also outlines the effectiveness and economics of an HWSC system and is available on the GRDC’s YouTube channel.

More information:

Peter Newman, AHRI,
0427 984 010,

Useful resources:

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