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Can ‘survivor’ weeds still be susceptible to glyphosate?

with Peter Boutsalis, Plant Science Consulting director and Adelaide University weed science team researcher

Annual ryegrass was the first cab-off-the-rank with resistance to glyphosate first recorded in Australia in 1996. In the last 20 years several other weed species have been added to the glyphosate resistant list but annual ryegrass remains the leader by far with over 600 resistant populations confirmed.

Dr Peter Boutsalis, Plant Science Consulting director and weeds researcher at Adelaide University has been in the thick of this unfolding dilemma, taking a leading role in testing for herbicide resistance and conducting random surveys across southern Australia to monitor the spread and evolution of resistance.

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“We now know that 15 years of consistent use of glyphosate, without alternative weed control measures, will invariably lead to glyphosate resistance in ryegrass,” he says. “There are confirmed cases of resistant populations under many land management systems but we see the largest number of cases in winter cropping paddocks, along fencelines and beside roads.”

One area of particular interest to Dr Boutsalis is the fact that even if resistance is confirmed at a particular rate, sometimes the resistant plants will be effectively controlled with a higher rate.

Dr Peter Boutsalis conducts random weed surveys across the southern states testing weeds for herbicide resistance under a range of management and land use conditions.

Dr Peter Boutsalis conducts random weed surveys across the southern states testing weeds for herbicide resistance under a range of management and land use conditions.

“This doesn’t mean that we can keep increasing the rate indefinitely,” he says. “It just means that if the population is susceptible to a higher rate, this information can be used to form part of an integrated management response to escalating resistance.”

How can a resistant plant still be susceptible?

Short answer: The resistance mechanism must work hard to ‘protect’ the plant but it can be overloaded.

Longer answer: The target site mechanism in annual ryegrass works by pumping the herbicide to the leaf tips and restricting translocation. A plant may be able to sustain this pump at one rate but at a higher rate there may be sufficient ‘leakage’ to kill the plant and stop it setting seed.

What is a good first response to confirmed glyphosate resistance?

Short answer: Glyphosate resistant weeds are often not good competitors when glyphosate is not used. Research has shown many instances of a ‘fitness penalty’ that makes these ‘weak survivors’ less competitive and so they set less seed.

Longer answer: If it is necessary to continue using glyphosate, apply a higher rate of glyphosate initially to reduce plant numbers and then use a different herbicide MOA to further reduce the number of potentially resistant plants setting seed. For best results with this double-knock, apply the second mode of action herbicide, e.g. paraquat, within three days of applying glyphosate.

What’s the best way to manage hot spots like fencelines to avoid glyphosate resistance taking hold?

Short answer: Don’t leave spraying fencelines until spring – try to get in early and treat weeds when they are small.

Longer answer: Resistant ryegrass is more sensitive to glyphosate when the conditions are optimal – aim for small, unstressed weeds, optimal spray coverage, including sufficient adjuvant, and spray in the cooler part of the day (below 30 degrees C). Resistant pollen can quickly spread at least 50 m into the paddock. Ryegrass cross-pollinates and there is a low, but increasing, incidence of populations possessing both target site and translocation resistance mechanisms.

The 2015 random weed survey 2015 revealed an incidence of 9 per cent glyphosate resistance in annual ryegrass in the Wimmera region of Victoria. It is not possible to diagnose herbicide resistance in weeds in the field. The only way to know what herbicides particular weeds are susceptible to is to undertake scientific testing. The ‘Quick Test’ can be done on living plants and provides timely feedback on suitable herbicide options.

The 2015 random weed survey 2015 revealed an incidence of 9 per cent glyphosate resistance in annual ryegrass in the Wimmera region of Victoria. It is not possible to diagnose herbicide resistance in weeds in the field. The only way to know what herbicides particular weeds are susceptible to is to undertake scientific testing. The ‘Quick Test’ can be done on living plants and provides timely feedback on suitable herbicide options.

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