Persistence needed to bring down weed seeds

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Using a chaff cart at harvest has become an integral tool for tackling herbicide resistance for Tarin Rock growers Mark and Lisa Pearce.

The couple have been facing widespread ryegrass and wild radish resistance to Group A, Group B and triazine herbicides since buying their cropping property in 2004.

Brome grass is also becoming increasingly hard to control, even though testing shows it has not developed resistance to selective herbicides.

A chaff cart that collects the seeds of these problem weeds for destroying through grazing and burning has been used by the Pearces since the 2005 harvest. They support this practice by mixing herbicide modes of action, changing crop rotations and varieties, and maintaining label-recommended herbicide rates.

These combined management tactics have significantly reduced weed burdens and herbicide expenses.

“It is hard to gauge the cost of weeds to our business, but with poor control we stand to lose up to 0.5 tonnes per hectare of cereal crop yields, coupled with higher herbicide costs,” Mark says.

In the first year Mark tried narrow windrow burning after harvest in canola, barley and wheat stubbles, but the heat was not intense enough and a high proportion of weed seeds continued germinating in the windrow over the next four years.


Owners: Mark and Lisa Pearce
Location: Tarin Rock
Farm size: 2000 hectares and 1100ha leased
Enterprises: cropping and first-cross South African
Meat Merino–Merino sheep
Average annual rainfall: 320 millimetres
Soil types: sandy gravel to medium loam
Ideal crop rotation: pasture, canola, two cereals –
depending on soil type
Crop program 2013: wheat (600ha), canola (930ha),
barley (450ha), oats (220ha) and serradella (200 to 400ha) 

This first-year result prompted the shift to a chaff cart, although initial efforts with a blower system were problematic. The addition of a conveyor belt for the 2012 harvest made the chaff cart easier to use and piled more straw to achieve a hotter burn and destroy more seed.

This pile also burned faster, reducing the risk of fire escaping across paddocks.

The Pearces also grazed some of the chaff dumps until April, providing valuable feed and further reducing weed seed numbers before the residue of the chaff dumps was later burnt.

Mark says the conveyor belt on the chaff cart reduced stress on the harvester by eliminating blockages and improving the flow of chaff fraction from the header to the cart.

“Since using a chaff cart, we have seen a significant reduction in wild radish and ryegrass numbers,” he says.

“We are no longer having to grade ryegrass seeds out of wheat at harvest, which has also reduced costs, time and stress.”

To address this, the Pearces are changing crop rotations and increasing plantings of imidazolinone-tolerant barley and wheat so they can target the worst of the brome grass with herbicides.

Southern DiRT’s Jane Kowald (left) and DAFWA
weeds project manager Alexandra Douglas are
preparing a book about harvest weed seed
management in southern regions.

Mark says herbicide rates have not changed with the adoption of a harvest weed-seed-destruction system, and the chemicals reduce competition from weeds in germinating crops.

He says this allows him to delay post-emergent spraying to achieve a good knockdown when the bulk of weeds have germinated, without affecting crop emergence and density.

Total herbicide use on the farm has fallen with the reduction of pasture phases and removal of a high proportion of wild radish. This now means a spray as required, rather than blanket applications twice a year.

Mark says the downside of chaff collection and burning and/or grazing at harvest is removal of nutrients for subsequent crops, especially potash, which he now adds to his fertiliser regime.

Mark and Lisa’s harvest weed seed management system is featured in a new booklet containing grower case studies from the GRDC’s Albany port zone.

This is an initiative of the Albany zone’s Regional Cropping Solutions Networks group and is available on the GRDC website.

Department of Agriculture and Food, Western Australia, weeds project manager Alexandra Douglas co-prepared the booklet with Jane Kowald, of Southern DiRT, and says it provides practical tips about low cost, innovative methods of harvest weed seed collection and destruction in the south – from strategic burning to chaff carts and baling.

More information:

Mark Pearce
0428 959 106,

Alexandra Douglas
08 9821 3246,;

Julianne Hill, RCSN,
08 9726 1307,;


See May–June 2013 Ground Cover Supplement ‘Herbicide Resistance’ for more information:

Image of Ground Cover Supplement (May/June): Herbicide resistance

Ground Cover supplement –
Herbicide resistance

A fact sheet on Herbicide Resistance is available at:
A fact sheet on In-Crop Herbicide Use is available at:


GRDC Project Code SDI00012

Region West