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Options outlined for resistant weeds in the north's summer season

Paul McIntosh, the northern extension agronomist for the Australian Herbicide Resistance Initiative.
Photo: Rebecca Thyer

Growers are urged to use glyphosate as part of an integrated strategy, or risk losing it.

When Paul McIntosh first encountered glyphosate it was the summer of 1978-79 and his father was using the herbicide to 'attack' Johnson grass growing on his family's farm in the South Burnett region.

He recalls the chemical doing a fantastic job of controlling the weed in the paddock before they planted a successful crop of irrigated lucerne.

That was back in the early days of glyphosate use in Australia, when the high price of the chemical meant it was generally used strategically and exclusively prior to planting.

By the early 2000s, the price of the herbicide had dropped and many growers began to use it as their primary weed control tactic.

In 2019, with rising herbicide resistance issues, the grains industry has again shifted to more strategic use of glyphosate.

Mr McIntosh, the northern extension agronomist for the Australian Herbicide Resistance Initiative (AHRI), is again working with glyphosate, with his focus firmly fixed on helping preserve the herbicide's future use in the face of resistant weeds.

At this year's Australian Summer Grains Conference, he spoke about how resistance has led to a summer weed control challenge in the north, reminding growers that although glyphosate is still economically priced, the cost of not using the product strategically could be losing it forever.

"In the northern region, we have glyphosate resistance in fleabane, sowthistle, barnyard grass and feathertop Rhodes grass. And the number of glyphosate resistant weeds is spreading over a wider area."

Integrated Weed Management

Mr McIntosh says non-herbicide weed management alternatives need to be used alongside chemical controls on-farm to combat herbicide resistance.

"Although none of these tactics provide 100 per cent control, we need to use them in combination to drive down the number of weed survivors."

He highlights the work being done by Darling Downs growers Peter and Kylie Bach, who use an EMAR chaff deck to capture the chaff fraction from the header and channel it on to the tram tracks.

Mr McIntosh says harvest weed-seed control (HWSC) techniques, such as the one used by the Bachs, allow growers to focus on the weed-seed-bearing chaff fraction, driving down weed-seed banks.

HWSC techniques include:

  • Chaff tramlining, where the chaff fraction is diverted on to one or both permanent tramlines in controlled-traffic systems, typically using a chaff deck.
  • Chaff lining, where a chute is placed on the rear of the harvester that concentrates the chaff into a single line in the centre of the header.

In chaff lining and tramlining, the straw is chopped and spread as usual, with the chaff fraction (which contains a large percentage of the weed seeds) left in a concentrated row to rot or mulch. The idea is to drive the header on the same tracks each harvest or convert to tram tracking, repeatedly placing chaff (and weed seeds) in the same place.

Since the Australian Summer Grains Conference, Mr McIntosh has also been in Emerald helping to launch Weed Smart's 'Summer Big 6' - specific practices to control summer weeds.

He says there are challenges involved with integrated weed management, including maximising the efficacy of tactics; timing (weed growth stage); application (nozzles, rates, additives); the cost of applying multiple tactics; and understanding plant life cycles. However, he is bullish that the adoption of alternative tactics is gaining traction.


Mr McIntosh says glyphosate has been instrumental in making no-till or zero-till farming an option.

"When I started this journey, we could lose more than 50 tonnes per hectare of valuable topsoil down the creek in a single year. It was commonplace to end up with a huge crater and deep gutters in the paddock landscape from water erosion. Adding wind erosion was just an extra kick in the pants," he says.

"Herbicides like glyphosate did us big favours in the early days of no-till farming as it allowed weed control while retaining stubble, which is so important for minimising soil erosion. But now we have to face the impact of glyphosate resistance in weeds.

"In the occasionally very wet 1980s, glyphosate was used more strategically. Due to the high price per litre, glyphosate was used almost exclusively prior to planting to stop transplanting weeds at planting time. By the early 2000s it cost around $5 per litre and was often relied on as the only weed control tactic, used multiple times per season and often without a tank mix partner.

"But now it is back to being used more strategically, because of resistance levels and because we understand how important it is that we maintain glyphosate as a viable herbicide option for us."

The 'Summer Big 6'

In response to growing herbicide resistance issues, the grains industry is being encouraged to adopt a more integrated approach to weed management, incorporating both chemical and non-chemical tactics.

The 'Summer Big 6' were developed by AHRI as a guide for growers to ensure/promote more effective summer weed control using an integrated approach. These tactics include:

Use diverse rotations

  • Use rotation crops, fallow and pasture phases to drive the weed seedbank down over consecutive years.

Mix and rotate herbicides and tillage (2+2+0)

  • Rotate between herbicide groups.
  • Use different modes of action within the same herbicide tank mix.
  • Strategic use of tillage can complement a herbicide program and get on top of weeds in a reset situation.
  • In cotton systems, aim to target both grasses and broadleaf weeds using two non-glyphosate tactics for broadleaf and grasses in-crop and two non-glyphosate tactics during the summer fallow and always remove any survivors (2+2+0).
  • For cotton post-picking, ensure complete survivor control using all options, including at pupae busting for extra weed seed burial.

Crop competition in rotational grain crops

  • Adopt at least one competitive strategy (two is better) including reducing row spacing, higher seeding rates, precision seed placement, east-west sowing and competitive varieties.

Double-knock to stop survivors

  • Use two weed control tactics with different modes of action on a single flush of weeds to stop any survivors from the first application setting seed.
  • Also use non-herbicide tactics - cultivation can be the second knock.
  • When executed well, the double-knock tactic can provide 100 per cent control of target weeds.

Stop weed set in crop and in fallow

  • Aim for 100 per cent control of weeds and diligently monitor for survivors in all post-weed control inspections.
  • Crop-top or pre-harvest spraying in canola, pulses, feed barley and wheat in weedy paddocks.
  • Consider hay production, brown manure or long fallow in high-pressure situations.
  • Spray top/spray fallow pasture prior to cropping phases to ensure a clean start to any seeding operation.

Harvest weed-seed control in rotational grain crops

  • Capture weed-seed survivors at harvest using chaff lining, chaff tramlining, chaff carts or integrated weed-seed destruction.

More information: Paul McIntosh,, 0429 566 198.

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