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Fighting weeds with plants in the west

AHRI's Dr Mike Ashworth, left, and Dr Hugh Beckie in a canola trial in Cunderdin, Western Australia, that has demonstrated seeding rate is the most important factor in increasing crop competitiveness against annual ryegrass for both hybrid and open-pollinated varieties.
Photo: Jo Fulwood

Trials have highlighted canola's ability to fight weeds in the right conditions.

A robust and competitive crop early in the season can potentially be the cheapest and most effective way to combat herbicide resistant annual ryegrass, with new research illustrating the value of agronomic strategies in the fight to control weeds.

Trials in Western Australia in 2018 and 2019 have demonstrated that a vigorous canola crop can go a long way towards winning the battle against ryegrass if the right agronomy, such as selecting the largest seed size possible and establishing enough plants by keeping the seeding rate high, is applied.

Dr Mike Ashworth, from the Australian Herbicide Resistance Initiative (AHRI), says canola can be a very competitive crop when established well, allowing it to out-compete ryegrass and significantly reduce the seed set of problematic wild radish.

"Canola is a part of the Brassica family, which is known for its rapid growth rates, strong competitiveness and ability to compensate and yield well if conditions allow," Dr Ashworth, who is managing the trials, says.

"So it is a natural fit for us to consider a canola crop as an effective weed control method on its own.

"Conversely, due to its small seed size, canola can also be vulnerable at early plant growth stages due to weed competition.

"So strong early establishment is not only critical to yield and profitability, but also important to reduce the competitive impact weeds have on canola yield and weed seed production."

It is a natural fit for us to consider a canola crop as an effective weed control method on its own. - AHRI weed researcher Dr Mike Ashworth

Dr Ashworth says small seed and low seeding rates can result in canola crops being extremely herbicide dependent.

"There has been a lot of research indicating the benefits of using wider crop row spacings and very low seeding rates of hybrid cultivars due to seed cost," he says.

"However, this comes at the cost of crop competitiveness against weeds, which places the herbicides used under intense pressure for resistance."

A new direction for AHRI

The trials form part of the new focus of AHRI under the leadership of Canadian-born Dr Hugh Beckie, who says developing agronomic solutions is now a research priority for the initiative, to provide growers with non-herbicide solutions to combat resistant weeds.

Dr Beckie, who was a research agronomist for more than 30 years before moving to Australia to head-up AHRI, believes the future of resistant weed control lies in agronomic strategies.

While he says the vision of AHRI hasn't changed, it has expanded to acknowledge the potential for reducing herbicide dependency, not only given increasing grain buyer demands and declining global acceptance of herbicide usage, but also focusing on the need to improve grower profitability through a reduction in weed control costs.

"A reliance on herbicides is only creating increased resistance issues, so we must also now look at other ways to ecologically manage this major inhibitor to profitability," Dr Beckie says.

On the ground

The trials are part of a GRDC investment aiming to quantify the effect of combinations of crop competition factors on weed seed-set and crop yield.

Other trials in both WA and South Australia are also part of the GRDC investment, which is led by the University of Adelaide.

Dr Ashworth says his work was inspired by the findings of research from 2012 to 2014 by Rohan Brill, from the NSW Department of Primary Industries, showing larger canola seed size resulted in better crop establishment and early seedling vigour - which resulted in higher yields.

"Following on from Rohan's research, we wanted to put some competition value to this data - to see what we could achieve in maximising the competitiveness of canola against ryegrass and wild radish," Dr Ashworth says.

"Canola is such a herbicide reliant crop, and we don't know what herbicides will be allowable, or useful, on these crops in the future.

"So we want the crop and the agronomy to do more of the heavy lifting."

Canola trials in Cunderdin and Mingenew in 2018, followed by trials in York and Kojonup in 2019, were planted to assess the impact of variety, seeding rate and row spacings on crop establishment and weed competition, in treatments with both herbicide applied, and no herbicide applied, to assess the effect of the agronomic strategy on both the canola yield and quality, and also the weeds present.

The two varieties of canola, Trophy - a hybrid mid-maturity - and ATR Bonito (PBR) - an open-pollinated triazine tolerant mid-maturity - were compared at seeding rates of 50, 35 and 25 seeds per square metre, with every variety and seeding rate treatment seeded at the row spacings of 25 centimetres and 50cm.

Key findings

Australian Herbicide Resistance Initiative canola trials in Kojonup, Western Australia. PHOTO AHRI

Australian Herbicide Resistance Initiative canola trials in Kojonup, Western Australia. Photo: AHRI

Results from the AHRI trials suggest growers should look to maximise their crop competitiveness using:

  • larger canola seeds, either by variety choice or seed rating; or
  • higher seeding rates that are appropriate for the rainfall zone.

"What we found was that it's all just a numbers game," Dr Ashworth says.

"Canola can yield at low plant densities, but you still need enough plants to out-compete the weeds."

The ultimate goal is to achieve a sustainable, less herbicide dependent cropping system, Dr Ashworth says, and growing a competitive crop is central to this.

Canola can yield at low plant densities, but you still need enough plants to out-compete the weeds. - AHRI weed researcher Dr Mike Ashworth

Annual ryegrass and wild radish plant establishment, biomass, canopy light infiltration and weed seed set were assessed throughout the season.

Within each trial, there was a clear trend demonstrating the seed production of annual ryegrass was reduced as canola establishment increased from 25 to 35 and up to 50 plants per square metre, with hybrid varieties providing an improved competitive ability compared to open-pollinated variety tested.

However, when canola seed of a larger diameter was used, the competitive effect of each seeding rate was greatly increased, resulting in a further reduction in ryegrass seed production.

Ryegrass biomass and seed production were highest in the treatments where the lowest canola seeding rate was sown, with narrower row spacings having a limited effect on either weed biomass or seed production.

Interestingly, maximising crop competitiveness in canola did not appear to affect the establishment or biomass of wild radish. However, despite this, the wild radish grown in the midst of a highly competitive canola crop did produce far fewer pods.

The recommendations

In simple terms, Dr Ashworth says, to maximise crop competitiveness against weeds, canola crops should be grown wherever possible using the largest seed available and using the highest practical seeding rate for the rainfall zone.

When any one of these factors is compromised, focus on another to compensate.

Regardless of whether you grow hybrid or OP varieties, the effect of using canola seed that is greater than 2mm in diameter and increasing seeding rates to the highest possible and profitable rates, will see weed control benefits.

Dr Ashworth says further trials throughout SA and WA are considering the competitiveness of wheat, barley, canola and faba beans on ryegrass, wild radish and brome grass.

GRDC Research Codes UOA1707-005RTX

More information: Dr Mike Ashworth,; GrowNote, canola seeding rate calculator,

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