Research targets clethodim resistant annual ryegrass
Trials have highlighted promising alternative practices – including the use of high vigour, hybrid canola varieties – which could help growers manage clethodim resistant annual ryegrass in canola crops.
They have shown that switching from an open-pollinated canola variety to a hybrid can reduce annual ryegrass seed set by 50 per cent, and that herbicide-tolerant RT® canola varieties offer additional options for control.
Chris Preston, of The University of Adelaide, told recent Western Australian Grains Research and Development Corporation (GRDC) Grains Research Update events that clethodim resistance in annual ryegrass was making this weed difficult to control in canola.
“As a result of extensive resistance to other post-emergent herbicides, clethodim has become the last effective herbicide for later control of annual ryegrass in non-RR® canola,” he said.
“However, annual ryegrass resistance to this important herbicide has been increasing across Australia in recent years.”
To address the problem, Dr Preston has led GRDC-funded short and long-term trials with the aim of developing alternative management practices.
While these trials have been conducted in South Australia and Victoria, he said the results were also relevant to WA growers.
Dr Preston said a 2015 trial at Roseworthy, SA, showed hybrid and high vigour canola varieties offered a simple opportunity to reduce annual ryegrass seed set by providing extra competition against the weed during the growing season.
“The results showed that hybrid canola, used in combination with pre-emergent herbicides and clethodim, can reduce seed set by 50 per cent,” he said.
In the trial, hybrid canola varieties Hyola® 559TT and Hyola® 750TT significantly reduced the number of annual ryegrass spikes at harvest compared with the open pollinated variety ATR Stingray, even in the absence of herbicides. However, the effects were much greater where pre-emergent herbicides were used.
A second trial at a high rainfall site at Lake Bolac, Victoria, investigated the use of RT® canola – which is resistant to both triazines and glyphosate - as a management tool, given triazine has a residual effect that can be used to reduce the impact of later germinating annual ryegrass.
The site – containing annual ryegrass resistant to all post-emergent herbicides including clethodim – was sown to Hyola® 525RT® canola in 2014 and subjected to low, medium and high intensity management strategies. In 2015, the site was sown to wheat.
“The high intensity management strategy was able to reduce annual ryegrass spike numbers by 70 per cent at the end of 2015 compared with the low intensity management, and by 50 per cent compared with the medium intensity management,” Dr Preston said.
“The ability to attack annual ryegrass at least three times during the canola phase is important for restricting the population size when only pre-emergent herbicides are available in cereals.”
Dr Preston said it was important to maintain effective control of annual ryegrass during the canola phase due to the lower number of weed management opportunities available in wheat.
“Pre-emergent herbicides, post-emergent herbicides, competition and seed set control are all crucial in achieving this,” he said.
The GRDC Grains Research Update paper ‘Managing clethodim resistant annual ryegrass’ is available here.
For information on herbicide sustainability, visit the WeedSmart information hub. It outlines a 10-point plan - including information on testing for resistance - to help you win the battle against weeds.
Useful information is also available in the Integrated Weed Management in Australian cropping systems manual, available here.
Information about sustainable integrated weed management (IWM) practices is available at the AHRI website.
Chris Preston, University of Adelaide
08 8313 7237 / 0488 404 120
ContactNatalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827
GRDC Project Code UCS00020