World's weed scientists unite against resistance

More than 300 international scientists gathered in Fremantle, Western Australia, in February to share their research and experience relating to herbicide-resistant weeds – one of agriculture’s most pressing global issues.

In Australia, herbicide resistance is now estimated to be costing grain growers at least $200 million a year in lost production and control measures. However, it is a global issue that is also beginning to seriously undermine food security.

As the world’s growers need to produce more food in the next 50 years than has been produced in the past 10,000 years, new and sustainable weed-management tools have become crucial.

The four-day conference in Fremantle, hosted by the Australian Herbicide Resistance Initiative (based at the University of WA), and supported by a number of sponsors including the GRDC, also attracted more than 100 growers and advisers keen to learn how science can help them tackle herbicide resistance evolution.

According to the International Survey of Herbicide Resistant Weeds (, Australia has the second-highest number of herbicide-resistant weed species in the world, with 62 species reported. It is worse only in the US, which has 142 resistant weed species. Canada has 59 resistant species.

A key announcement at the conference was a report that Monsanto researchers may have found a way to reverse glyphosate resistance using a non-GM spray-on application.

Dr Doug Sammons, a senior fellow at Monsanto in St Louis, Missouri, in the US, stirred considerable interest when he revealed a new technology called BioDirect®, which appears to have the potential to reverse weed resistance.

An example the company is exploring is the reversal of glyphosate resistance in Palmer amaranth (Amaranthus palmeri), a weed that has become a major problem in the US because of its ability to resist multiple herbicide groups. When glyphosate is applied to a susceptible weed it blocks the activity within the plant of a specific enzyme – known as 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) – without which the plant dies. In resistant Palmer amaranth more EPSPS enzyme is produced than normal, allowing the plant to survive typical application rates of glyphosate.

Dr Sammons said BioDirect® is a non-GM weed control technology based on the naturally occurring molecule ribonucleic acid (RNA). It delivers double-stranded RNA, a specific form of RNA, to a glyphosate-resistant plant via a spray application, disrupting EPSPS enzyme production.

“When you decrease the amount of EPSPS enzyme within this glyphosate-resistant plant, the ability to kill this undesirable weed with normal application rates of glyphosate returns,” Dr Sammons explained.

He said that when BioDirect® technology was combined with glyphosate and applied to glyphosate-resistant Palmer amaranth plants in pot trials, the resistant plants died after seven days.

Paddock trials have also been done on Palmer amaranth weeds that were able to tolerate up to 16 times the label rate of glyphosate in some highly resistant fields, and enhanced control of even these weeds was seen through the application of BioDirect® technology combined with glyphosate.

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