Increased herbicide resistance in ryegrass (Lolium rigidum) weed populations continued to surface across southern-eastern Australia last year.

Herbicide resistance specialist Dr Peter Boutsalis explored the “significant increase” in glyphosate-resistant annual ryegrass at the 2021 GRDC Grains Research Update in Adelaide.

Dr Boutsalis, who is a University of Adelaide postdoctoral fellow, said that in the past five years, random weed surveys through GRDC investment have examined more than 1000 cropping paddocks in South Australia and Victoria to identify glyphosate resistance.

Glyphosate resistance was generally detected in about three per cent of these paddocks where ryegrass was collected in the surveys conducted by Dr Boutsalis from 2016 to 2020.

But he added that “in some regions, a much higher incidence of resistance was identified".

For example, glyphosate-resistant ryegrass was found in almost a third of paddocks in south-eastern SA and 40 per cent of paddocks in south-western Victoria, he said.

Dr Boutsalis also runs a commercial herbicide resistance testing service at Adelaide-based Plant Science Consulting (PSC). This testing is based on weed samples that growers collect from their paddocks each season. Growers can send samples of plants that have survived herbicide applications in early autumn to PSC for herbicide resistance testing using the Quick-Test method.

Dr Boutsalis said the commercial testing in 2020 showed more than half the ryegrass plants sent from paddocks were resistant to glyphosate.

“Glyphosate resistance was identified in 79 per cent, 70 per cent and 43 per cent of ryegrass samples collected by growers in New South Wales, Victoria and South Australia respectively,” he said.

“This highlights that, in most cases, glyphosate resistance has contributed to reduced ryegrass control in the paddock.”

Last year, the samples collected by growers originated from 83 cropping paddocks in NSW, 74 paddocks in Victoria and 37 paddocks in SA.

“The early break in the 2020 season across most southern cropping regions provided an opportunity for knockdown weed control. Multiple applications of glyphosate and paraquat herbicides were possible, targeting several flushes of weeds, particularly ryegrass, prior to sowing.”

Resistance rise

Dr Boutsalis said that in some regions, confirmed cases of glyphosate resistance have increased significantly in the past 10 years.

“However, it took about 20 years after the registration of glyphosate for the first case of resistance to be confirmed.”

He said the decades between when glyphosate was registered for use in Australia in the 1970s and when resistance was identified in ryegrass in a NSW orchard in 1996 indicates the “natural frequency of glyphosate resistance was initially extremely low”.

Dr Boutsalis said the increasing incidence of glyphosate resistance in ryegrass stems from a wide range of factors, often in combination. But two main causes are prominent: “Strong selection pressure for resistant ryegrass resulting from repeated glyphosate use, coupled with application under suboptimal conditions, have played a major role.”

In particular, spraying glyphosate at reduced rates can see ryegrass plants accumulate mechanisms for resistance that enable them to survive when the herbicide is applied at higher rates.

“University of Adelaide research shows that cross-pollinating ryegrass plants stacks glyphosate-resistance mechanisms, producing hybrids with stronger resistance,” he said.

Other important influences that can reduce glyphosate efficacy and, ultimately, promote glyphosate-resistant ryegrass populations are:

• using low-quality glyphosate herbicide and surfactant products;
• mixing glyphosate with many other active ingredients that can cause antagonism, particularly in low water volumes;
• using low-quality water, particularly hard water. Glyphosate is a weak acid that binds to positive cations, such as magnesium, calcium and bicarbonate, which are highly concentrated in hard water. Glyphosate also binds to soil particles in dirty water;
• applying glyphosate during periods of high temperature and low humidity can cause rapid glyphosate losses from leaf surfaces, reducing its absorption;
• applying glyphosate on to stressed plants can reduce translocation. Maximising glyphosate efficacy relies on its translocation to plant root and shoot tips. Glyphosate needs to translocate further to root and shoot tips in large plants compared with seedlings;
• shading effects can reduce spraying coverage on plant leaves, resulting in sub-lethal glyphosate applications;
• application of glyphosate on to dusty, soil covered leaves can reduce its efficacy since the herbicide strongly binds to soil particles; and
• variable application approaches, such as spraying speed, nozzle selection and spray boom height, can reduce glyphosate coverage on plants.

Integrated weed management

Dr Boutsalis urged growers to use glyphosate more efficiently as part of integrated weed management (IWM) strategies to help curb increasingly resistant ryegrass populations. “There are ways to optimise glyphosate efficacy, such as partnering it with other herbicides to improve weed control.”

For example, double knock weed control strategies that partner glyphosate and paraquat herbicide spraying. The ‘first knock’ with glyphosate, for instance, aims to control most of the ryegrass population, while a subsequent ‘second knock’ with paraquat aims to control any surviving ryegrass plants.

He said research by the University of Adelaide’s Professor Christopher Preston shows a double knock application of paraquat one to five days after glyphosate provided optimal control of resistant ryegrass.

“The time lapse between applications of glyphosate and paraquat depends on the weed size and growing conditions, with one to five days generally required to maximise glyphosate efficacy.

“In favourable growing conditions, seven days after glyphosate is applied, resistant ryegrass plants tend to become stressed, resulting in reduced absorption of paraquat and, in turn, less-effective weed control.”

Dr Boutsalis also emphasised the importance of glyphosate spraying in relatively cool conditions.

A 2019 trial showed applying glyphosate in cool conditions (following a hot period) improved ryegrass control by 65 to 80 per cent compared with applications in hot conditions.

“Addition of ammonium sulfate (AMS) is very important for conditioning water and for promoting plant uptake and translocation of glyphosate.”

The growth stage of ryegrass plants can also influence the effectiveness of glyphosate spraying, he said.

For instance, a trial found glyphosate effectively controlled ryegrass at early tillering, but control was ineffective at the one-leaf growth stage.

“Most glyphosate labels do not recommend application on one-leaf ryegrass seedlings because they still rely on seed reserves for growth. Consequently, less glyphosate circulates to the roots of seedlings,” he said.

More information on weed management is available from the GRDC-supported WeedSmart resource centre.

WeedSmart is a multi-channel platform for information on controlling cropping weeds and herbicide resistance in Australia.

More information: Peter Boutsalis, 0400 664 460, peter.boutsalis@adelaide.edu.au, Plant Science Consulting.