Resistant weeds spur research into new technologies
Global cropping systems are at a crucial point for sustainable weed control due to rapidly evolving weed resistance to glyphosate and other valuable herbicides.
But the good news is that these herbicide resistant weeds are reinvigorating interest and efforts to discover new weed control technologies.
United States weed and crop scientist Jerry Green, of DuPont Pioneer, delivered this message to the Global Herbicide Resistance Challenge conference in Fremantle, Western Australia, earlier this year.
The conference was an international, multidisciplinary research event addressing herbicide resistance hosted by the Australian Herbicide Resistance Initiative (AHRI) and supported by major sponsor the Grains Research and Development Corporation (GRDC).
Dr Green said the conference was timed perfectly to address the issue of herbicide resistance.
“We are now at a critical juncture for sustainable weed control and herbicide resistant weeds have put us in that situation,” he said.
“The use of glyphosate in glyphosate resistant crops (such as Roundup Ready® soybeans, corn and cotton) in the US was great and worked well for more than a decade, but the weeds have adapted and now we need to change our weed control tactics.”
Dr Green said using glyphosate in glyphosate resistant crops had been the most effective weed control option for many growers in the US since they were first introduced in 1996.
“The first glyphosate resistant crops worked wonderfully - you could apply glyphosate at almost any time and get perfect weed control,” he said.
“They dramatically improved agricultural productivity and became the most rapidly adopted technology in the history of agriculture.
“In just seven years glyphosate resistant soybean crops were adopted by 80 per cent of US soybean growers.
“Growers urgently needed these glyphosate resistant crops because weeds were becoming widely resistant to the commonly used selective herbicides, thus making weed control too complicated and time consuming for large-scale crop production systems.
“The ability to use glyphosate at a wide range of application timings changed agronomic practices and made weed control easier and more efficient than the weed control systems that were being replaced.
“But in answer to the question of whether the extensive use of glyphosate alone in glyphosate resistant crops was sustainable, the answer is no.
“Last year 49 per cent of Canadian and US grain growers surveyed reported that they believed they had glyphosate resistant weeds on their farm.
“We have reached a tipping point and it is not just a glyphosate resistant weed problem, it is a problem with weeds being resistant to multiple herbicides.
“It is taking everything we can throw at them to manage these weeds.
“Growers are now being forced to use combinations of partially effective chemical, cultural, biological, mechanical and sometimes even manual weed control practices.”
But despite this, Dr Green said the crop protection industry remained healthy and he was optimistic that there was renewed interest in the development of new weed control technologies.
He said there was no silver bullet herbicide to offer growers to combat herbicide resistant weeds, as there were too many weeds with multiple herbicide resistance.
However, he said it was fortunate that most selective herbicides that were available before the introduction of glyphosate resistant crops, were still registered and their utility could be renewed.
“But weed resistance to many of these older herbicides already exists so new weed control practices are desperately needed,” Dr Green said.
He also said that an array of new herbicide resistant crops, with stacked traits, were being developed that would enable broader use of existing herbicides.
“At least for soybeans, corn and cotton, the days of a single herbicide resistant trait crops are over,” Dr Green said.
He said other options for controlling herbicide resistant weeds included new mechanical technologies to cultivate out weeds and destroy weed seeds.
Dr Green said the development of new weed control systems, including crops with stacks of herbicide traits, would not replace the need to discover herbicides with new modes of action (MOAs).
“It has been more than 20 years since a new MOA came onto the marketplace and the need is great,” he said.
Dr Green said this ‘famine’ for new MOAs was due to different reasons including fewer scientists working on herbicide discovery, and increased costs associated with the development of new herbicides.
“But nevertheless I am confident that we will see the announcement of new herbicide MOAs in coming years,” he said.
Gene silencing technology
Global Herbicide Resistance Challenge conference convenor and AHRI director Stephen Powles said Dr Green was one of more than 300 scientists from 30 countries who attended the conference.
Also present was Monsanto Company’s Doug Sammons, who discussed the development of RNA interference (RNAi) technology to help glyphosate and potentially other herbicides.
Professor Powles said that under this technology, which is in the early stages of development, a mirror copy is made of a weed’s DNA in which targeted genes can be turned on or off.
“Monsanto utilised precise RNA segments directly able to inhibit normal production of the EPSPS protein in plants,” he said.
“EPSPS is the enzyme that glyphosate inhibits. “Thus RNAi technology (branded BioDirect™ by Monsanto) is a naturally occurring way of improving a herbicide.
“Particularly interesting and important is that under experimental conditions it appears that BioDirect™, when combined with a herbicide, can reverse resistance.
“It is possible to specifically target the RNA to a resistant weed.” Professor Powles said the potential for BioDirect™ was a subject of considerable hallway conversation at the conference.
“Whether or not BioDirect™ as a herbicide will come to commercial fruition is very difficult for me to judge, but I hope fervently that it does because we need all the tools we can muster to battle weeds and herbicide resistance,” he said.
“However, even if comes to commercial fruition it will be years away and if commercialised for weed and resistance control it may not be as good as glyphosate.
“Thus, I am reminded of an expression my mum often used – ‘a bird in the hand is worth two in the bush’.
“We have glyphosate now and given the absence of new herbicides we need to do everything we can to help preserve glyphosate.
“Keep glyphosate sustainable through diversity is my mantra.”
Global effort to address a global problem
Professor Powles said there was no doubt that the evolution of herbicide resistant weeds was a major threat to global food production, which needed to be able to feed nine billion people in coming decades.
“This conference brought together scientists, technicians, chemical company people and biotechnologists to really focus on the problem of how to produce food for nine billion people while battling weeds,” he said.
“We can do it, but we’ve got to preserve the precious herbicides we’ve got.
“The only way to preserve them is to diversify the forms of weed control - including using herbicides as infrequently as possible and using non-herbicide methods – and in that way we can keep the great herbicides like glyphosate working on our Australian farms.
“We only need to look to what’s happening in the US where there has been a massive over-use of glyphosate, to take lessons from that and keep glyphosate working in Australian agriculture.
“We can do it if we diversify our weed control systems.”
Professor Powles said that in Australia, herbicide resistance was a major problem in cereal cropping systems across the country, especially in ryegrass.
“We must win the battle against herbicide resistance, just as medical science must win the battle against antibiotic resistance,” he said.
“To do so we need good science coupled with good agronomy and good agricultural engineering.
“We need well trained people at all levels from high science, right through to dedicated farmers implementing a range of practices.”
Professor Powles said research was learning how to manage a number of aspects of herbicide resistance, and there were successes including in the area of harvest weed seed control systems in Australian cropping systems.
The Global Herbicide Resistance Challenge conference saw 100 speakers and 66 poster presentations addressing all areas of the science of herbicide resistance in weeds, in global food production.
Major issues discussed included the threat of herbicide resistance and its impact on global grain production, alternatives to herbicide weed control and the latest gene modification advances.
For more information about managing herbicide resistance visit the AHRI website at www.ahri.uwa.edu.au
Through an ongoing partnership, AHRI is supported by the GRDC, which invests $8.2 million annually in weed control research nationally.
Regionally-specific information about weed control can also be found at the WeedSmart campaign hub at www.weedsmart.org.au
Launched at the Global Herbicide Resistance Challenge, the WeedSmart industry initiative provides a toolkit of resources for Australian advisers and growers to keep weeds – and herbicide resistance – at bay.
GRDC Project Code UWA000124