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Glyphosate over-use a risky business

The risks of glyphosate resistance vary so widely on Australian cotton farms that growers need to closely assess their management practices and the weed species present on each farm, according to weed researcher, Dr Jeff Werth.

Dr Werth, Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) says a new risk assessment framework enables growers to tailor their weed management to focus on those species that are at a high risk of evolving resistance.

“Growers will need to use effective alternatives to glyphosate which, when targeted at their at-risk weed species, will help to ensure glyphosate’s long term sustainability,” he said.

“Glyphosate resistance will have a major impact on current cropping practices in glyphosate-resistant cotton systems.”

He says the risk assessment framework will aid decision-making for resistance management.

“We developed this framework and then assessed the biological characteristics of 65 species and management practices from 50 cotton growers,” Dr Werth said.

“This enabled us to predict the species most likely to evolve resistance, and the situations in which resistance is most likely to occur.”

Dr Werth says species with the highest resistance risk were sweet summer grass (Brachiaria eruciformis), flaxleaf fleabane (Conyza bonariensis), liverseed grass (Urochloa panicoides), feathertop Rhodes grass (Chloris virgata), sowthistle (Sonchus oleraceus) and awnless barnyard grass (Echinochloa colona).

“The summer fallow and non-irrigated glyphosate-resistant cotton were the highest risk phases in the cropping system,” he said.

“When weed species and management practices were combined, flaxleaf fleabane in summer fallow and other winter crops were at very high risk.

“Sowthistle had very high risk in summer and winter fallow, as did feathertop Rhodes grass and awnless barnyard grass in summer fallow.”

He says the assessment confirmed previous perceived risks and demonstrates its usefulness but the overall risk is highly dependent on the weed species present in the different phases of the rotation.

“The average risk for ‘other’ winter crops was approximately half of that for summer fallow and non-irrigated glyphosate-resistant cotton.

“However, when flaxleaf fleabane was present in the ‘other’ winter crops phase, individual risks were the same as for the summer fallow, and higher than non-irrigated glyphosate-resistant cotton.”

Eleven of the 50 surveyed growers indicated that they grew non-irrigated glyphosate-resistant cotton. In this phase, the risks for flaxleaf fleabane, liverseed grass and awnless barnyard grass were high.

“Currently there is only one confirmed resistant awnless barnyard grass population in a non-irrigated glyphosate-resistant cotton system, but the number of cases is likely to increase.

“It is concerning that there were growers, who indicated they did not control survivors of glyphosate application, despite the requirements to do so.”

Dr Werth says these growers are likely to have thought that glyphosate provided sufficient control negating the need for further action.

The individual responses of non-irrigated glyphosate-resistant cotton growers did, however, indicate that they all used an alternative to glyphosate at some stage, and so no grower in this survey relied on glyphosate only for weed control, he says.

“The risk scores for the summer fallow were consistently high. Glyphosate has been relied upon for weed control in the summer fallow phase for several years.”

He says research is now concentrating on strategic use of residual herbicides and tillage to reduce the reliance on glyphosate. In fallow situations, Group A herbicides (ACCase inhibitors) are starting to be used.

The WeedSmart campaign brings together industry organisations including the Grains Research and Development Corporation (GRDC), research providers and major crop input firms to deliver the message that herbicide resistance is a difficult but not insurmountable problem – but changes need to occur on-farm.

Funding for the risk assessment and online toolkit was provided by the Cotton Research and Development Corporation (CRDC).

For more information on herbicide sustainability practices, visit

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