By Cindy Benjamin
Random weed surveys, funded by the Grains Research and Development Corporation, and commercial herbicide resistance testing results show similar trends for resistance levels to key herbicides used to control annual ryegrass in Australia.
Weeds researcher with Plant Science Consulting Dr Sam Kleemann, said the recent uptick in glyphosate and paraquat resistance needs urgent attention in broadacre cropping systems.
“The predictability of the herbicide resistance progression gives growers and agronomists opportunities to act early when a resistance issue is identified,” he said.
“Weeds such as annual ryegrass possess several characteristics which make the spread of resistance quite rapid, making it one of Australia’s most resistant weeds.”
Surveys and commercial testing have revealed resistance to glyphosate, and most recently paraquat, has entered a new phase with increasing numbers of broadacre populations identified as resistant to one or both of these knock-down herbicides.
“This has significant implications for the efficacy of the double knockdown tactic to control ryegrass before seeding crops,” Sam says.
“Growers who take early action and implement practices to reduce the spread of resistance genes can regain control. Using the WeedSmart Big 6 strategies to keep weed numbers low is the best antidote to herbicide resistance.”
What does the herbicide testing data say about glyphosate resistance?
Before 2010 glyphosate resistance in annual ryegrass was very rare.
The GRDC-funded random weed survey (RWS) in 2010–2014 detected less than 5 per cent of ryegrass populations with glyphosate resistance in broadacre cropping areas in NSW, Victoria, South Australia, and Western Australia.
This is a stark contrast to the 2020 RWS, which found that 12, 14, 22 and 23 per cent of ryegrass populations collected in WA, SA, NSW and VIC, respectively, were resistant to glyphosate.
Is paraquat-resistant ryegrass becoming more common?
Paraquat resistance evolution is relatively slow and is associated with production systems that rely on frequent applications of this herbicide.
For example, in a survey of annual ryegrass plants collected from 100 lucerne seed production paddocks in South Australia in 2022, 33 per cent of samples were resistant (>20% survival), and 22 per cent were developing resistance (1–19% survival).
In practical terms, half the paddocks sampled had paraquat-resistant ryegrass present, with as many as 15 per cent of these populations also cross-resistant to glyphosate.
Paraquat use is far less frequent in broadacre grain systems than in intensive systems.
As a result, there have been consistently low levels of paraquat-resistant ryegrass, less than 1 per cent, for the past 10 years of the broadacre cropping random weed survey.
Generally, it takes up to 25 years to evolve paraquat resistance if the herbicide is used annually. This is because the resistance gene is usually rare in nature, and in many cases, the inheritance of the resistance gene is either incomplete or only partially dominant.
The first cases of paraquat-resistant ryegrass were identified from lucerne fields in 2010, and the first reported broadacre case was confirmed in 2019. Since then, nearly 50 broadacre cases have been confirmed in submitted samples, not through random surveying.
The majority of these samples have also tested positive for glyphosate resistance.
How can I recognise paraquat resistance?
Many of the samples submitted for testing were plants that appeared to survive a double-knock treatment.
Within 48 to 72 hours of paraquat application, survivors exhibit strong chlorosis (yellowing) and necrosis (death) of the leaf tip. The rest of the plant remains largely unaffected, allowing it to recover quickly.
This suggests the herbicide is taken up by the plant leaves of a resistant plant, but only a limited amount of the active ingredient reaches the target site (chloroplasts).
The herbicide is effectively ‘quarantined’ in the cell vacuole, where it remains ineffective.
What is driving dual resistance to glyphosate and paraquat?
Fence line weed control, double-knocking, and crop-topping are common practices that involve applications of glyphosate and or paraquat.
While all could be contributing factors on different farms, the main selection pressure appears to be applied along fence lines.
Once resistance has evolved, it is very easy for the resistance genes to move into adjacent paddocks via pollen or across the farm as seed transported on machinery.
Once resistant individuals are identified, every effort should be made to prevent these plants from setting seed.
One grower, who sent in samples from two paddocks a few years ago, successfully managed the dual resistance problem using a Kelly chain to apply a third knock to kill any survivors of the traditional double-knock tactic.
Since the majority of the plants present in a paddock are still susceptible, the double-knock is still a valuable tactic, provided additional measures are taken to eliminate any survivor plants.
