Time to kick the herbicide habit
National weed-control agency Weedsmart has reported random weed surveys continuing to identify an increasing trend in herbicide resistance across Australia’s broadacre cropping regions.
It has identified the single cause of herbicide resistance in weeds as selection pressure through herbicide use.
Annual ryegrass leads the charge, with resistance to multiple herbicide modes of action, and demands a readjustment in weed-control strategies.
Dr Peter Boutsalis of Plant Science Consulting said the introduction of several new herbicides in recent years had provided options for controlling some resistant populations, particularly for Group 1 A and Group 2 B resistant ryegrass. But this alone would not halt resistance evolution in ryegrass populations across Australia.
“Simply changing to another mode of action when older chemistry seems less effective is not a long-term solution,” he said.
“Any herbicide has the ability to select for resistance, especially in a genetically diverse species such as ryegrass.
“The strategy needs to centre on increasing diversity in herbicides and non-herbicide tools, not just switching from an ‘old’ herbicide to a ‘new’ one.”
Grains Research and Development Corporation has invested in random weed surveys in different regions in New South Wales each year from 2015 to 2019.
These surveys have identified differences in the pattern of resistance between regions and states, but the trend toward multiple-resistance mechanisms and resistance to increasing application rates is undeniable.
Dr John Broster, Charles Sturt University, said most annual ryegrass populations in NSW were resistant to Group 1 A and Group 2 B herbicides with some variability between the surveyed sub-regions.
To date, no populations had been found that were resistant to newer preemergent herbicides, however resistance had been reported in other states.
“Of particular concern is the percentage of ryegrass populations sampled in the random survey in some sub-regions that are resistant to glyphosate,” he said.
“The extent of resistance in some areas was brought home strongly in the 2020 season when many growers were confronted with significant patches of ryegrass that clearly escaped pre-seeding glyphosate applications.”
The random surveys involved the collection and testing of 608 ryegrass populations, with the results showing five percent of these populations were resistant to glyphosate.
The highest level of resistance so far was found in 2019 results from the eastern NSW region alone, where 14 percent of populations were resistant to glyphosate.
A population is considered resistant to a herbicide when more than 20 percent of the plants grown from seed collected at a single site survive applications of registered rates of the herbicide in question.
In addition to the random sampling to provide the ‘big picture’ of resistance extent, Dr Boutsalis also has Quick Tests when growers and agronomists experience an apparent herbicide failure.
In 2020, he was sent 83 ryegrass samples from concerned growers in NSW and Quick Tests showed 79 percent of individual plants that survived
paddock treatments were resistant to glyphosate.
“This suggested that although glyphosate resistance is generally a significant contributing factor to weeds escaping herbicide treatment in the paddock, there are potentially other forces involved as well,” Dr Boutsalis said.
“Poor application technique or application onto stressed plants, incorrect timing, sampling plants that were not exposed to glyphosate, antagonistic tank mixes, inferior glyphosate formulation, poor water quality, incorrect adjuvants, or a combination of these can also result in poor weed control.
“To keep any herbicide as a longterm option it is essential that high-quality products are applied correctly and that survivor plants are prevented from setting seed. Switching products is a very short-term and inadequate solution. A better strategy is to implement a diverse program of both herbicide and non-herbicide tactics and be diligent.”
Other than confirming resistance, herbicide testing is a powerful way to identify modes of action that a resistant population is susceptible to.
Growers who are confronted with patches of ‘survivor’ weeds this season can send live plant samples in for the Quick Test to identify herbicide options that could be used to prevent seed set in the current season.
If the escapes are not seen until seed has set, seed can be collected and sent to either CSU or Plant Science Consulting for testing against a wider range of herbicides, including pre-emergent herbicides.
Testing of ‘suspect’ seed samples sent to CSU last year resulted in 30 percent of populations testing positive to glyphosate resistance.
Patch management strategies such as cutting for hay, spraying out with paraquat, or chipping can be effective in containing a potential blow-out.
The Weedsmart Big 6 strategies for integrated weed management can then be implemented to apply long-term downward pressure on weed numbers.
For more information about diverse weed control tactics, people can visit website, www.weedsmart.org.au.