WITH glyphosate prices at rock-bottom levels, the temptation for Australian growers is to keep pouring on the popular herbicide rather than rotate chemical modes of action and use more expensive options.
According to leading international herbicide researcher Harry Strek, leader of Bayer Crop Science’s integrated weed management and weed resistance biology team in Germany, it could be the most expensive cheap option they have ever taken.
He said the issue of herbicide resistance, across most of the major chemical groups, was already a big issue and was only going to get bigger, citing research showing that by 2018, 50 per cent of agricultural weed species will be glyphosate resistant.
And while Australia has yet to have significant problems with glyphosate resistance, there are already significant problems with Group A ‘fops and dims’ and Group B sulfonylurea resistance and emerging problems with trifluralin resistance meaning Australia has some of the highest instances of weed resistance in the world.
The ryegrass resistance to a range of herbicides that is an issue through many cropping zones in Australia was an example of Australia’s resistance problems, Dr Strek said.
“You look at the work of Stephen Powles at the University of Western Australia on resistant ryegrass, he found that it was ideally suited to becoming resistant, as it was planted as a fodder crop over many acres and when the land was converted to grain production, there was a genetically diverse population with lots of outcrosses, so there was the resistant genetic material and through pollen, it travelled widely.”
Dr Strek said resistant weeds were one of the biggest threats to cropping in the 21st century, especially in no-till systems becoming increasing popular in Australia, where there are few other options other than herbicide applications to control weeds.
“Weeds present a higher global threat to yield loss than even fungus and insect infections.”
He said with issues of resistance in mind, a solid chemical rotation and an integrated weed management (IWM) strategy were critical in keeping chemical modes of action effective.
“Farmers must be smart about both their crop rotations, and by extension, their chemical rotations and using different chemistries.
“The surest way of getting herbicide resistant weeds is to continue to rely on the same mode of action for year after year.
“It’s natural for farmers to keep using the same product if it is effective, but the key message is that there need to be different modes of action.
“You look at the example of glyphosate in the US, it’s a tool that farmers love, but it is trouble because of that popularity and its overuse.”
He said along with using different chemicals, a ‘double knock’ approach of using the usual herbicide along with another mode of action to ensure control of the population becoming resistant to the major group, was an effective strategy.
Crop rotations, timing of cropping and less fashionable management options such as cultivation were other methods to minimise issues.
However, Dr Strek was pessimistic about stopping the onslaught of resistant weeds altogether, even with sound management strategies.
“We used to talk about stopping resistance, now we are talking about slowing it down.
“Rotations will work well in slowing resistance down, and keeping that selection pressure, but there will be issues no matter what.”
He said growers needed to understand how resistance worked, with most weeds controlled by the herbicide, but a small amount each generation that were not.
Each generation through the selection process, there were more of the plants with the resistance trait.
Dr Strek said along with management techniques, the hunt was on for new chemical modes of action, but warned the low-hanging fruit had well and truly been picked and it was a struggle to develop new lines.
“There have been no significant new modes of action developed for 20 years, you may get chemistry that is new to a particular crop or country, but nothing huge, and that is reflected by the fact six herbicide classes are responsible for 75pc of worldwide herbicide sales, and just three groups, the As, Bs and glyphosate represent 50pc of sales, and these three groups are under strong resistance pressures in many areas.”
However, he said Bayer was continuing to invest in research.
“The success rate isn’t what it was, but our research is more targeted, it is no longer a numbers game, its smarter and more focused and we believe we have some work that will yield something.”
But, ultimately, Dr Strek said the frontline in battling resistance would be sound decision making from growers.
“IWM strategies, where crops and herbicides are rotated and there are different crop management strategies, whether it be cultivation, early or late sowing or fodder making, will be the best defence against resistant weeds.”
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