Triple knockdown resistance highlights need for diversity
GroundCover™ Issue: 116 | Author: Peter Newman and Stephen Powles, Australian Herbicide Resistance Initiative
- Research has confirmed resistance to glufosinate, glyphosate and paraquat in a crowsfoot grass (also known as goosegrass) population in Malaysia
- The population, also resistant to some grass-selective herbicides, has the highest level of glyphosate resistance documented in any weed species on the planet
- The lesson is to add diversity to weed-management systems, rather than simply relying on herbicides alone to control weeds
Relying on herbicides alone to manage weeds is not the answer, new research showsThe first confirmed case of resistance to the three main knockdown herbicides – glufosinate, glyphosate and paraquat – in a population of crowsfoot grass in Malaysia highlights the need for Australian grain growers to use diverse weed-management tactics.
Australian Herbicide Resistance Initiative (AHRI) PhD student Adam Jalaludin recently documented the case in a palm nursery. He also found the crowsfoot grass population was resistant to some grass-selective herbicides.
Not only is the population resistant to four groups of herbicides, it also demonstrated the highest level of glyphosate resistance ever recorded.
What mistake was made? It is the same story: no diversity. When one herbicide failed the nursery management simply switched to another herbicide.
What can Australian grain growers learn from this? As resistance to important herbicides such as glyphosate evolves, the answer is more than simply switching to paraquat.
Mr Jalaludin, who is originally from Malaysia, was among the first in the world to report on glufosinate (Basta® and Liberty®) resistance in 2010.
Glufosinate is a globally significant herbicide because it is known as the third knockdown (non-selective) herbicide behind glyphosate and paraquat.
Although Australian grain growers do not commonly use glufosinate, it is alarming to learn that a single weed population can evolve resistance to the three main non-selective herbicide groups.
Crowsfoot grass is a plant that is 98 per cent self-pollinating. In terms of glufosinate (Basta® and Liberty®), the 14-fold resistance described in this study was similar to previous studies by Mr Jalaludin and others working in the area.
For glyphosate, the study revealed 144-fold resistance. This is the highest level of glyphosate resistance ever documented in any weed species.
Fifty per cent of the population tested survived more than 47 litres per hectare of glyphosate 450 (the standard glyphosate that has 450 grams/L of active ingredient). The mechanism of this resistance is under investigation.
The only glimmer of good news is that the population appeared to show a fitness penalty. This is where the plant does not grow quite as well as those that are susceptible to the herbicide. However, the plants are very fit when susceptible plants are killed by the herbicide, reducing competition.
The study showed the crowsfoot grass population had low-level (three-fold) paraquat resistance. Paraquat was only used for a few years, which meant the crowsfoot grass had lower resistance levels.
Resistance to Group A (ACCase) herbicides was also detected, with about 50 per cent survival of the resistant population to paddock rates of the herbicides fluazifop (Fusilade®), haloxyfop (Verdict®) and butroxydim (Falcon®).
Gene sequencing showed that a single target site mutation (2027) was likely responsible for this Group A resistance.
The population was susceptible to sethoxydim (Sertin®) and clethodim (Select®).
The population was found in a palm nursery in Malaysia where palm oil growers buy plants for their plantations. The palm plants are lined up in rows in polyethylene bags. People working on foot spray herbicides around the bags at least once a month so the area is free of weeds for buyers to inspect the plants.
The climate in Malaysia is hot and wet. Tropical rain on a daily basis with warm, humid conditions are perfect for crowsfoot grass plant to thrive. Consequently, there are germinations of crowsfoot grass all year round.
Although there were poor spray records for this population, discussions with workers at the nursery identified the following sequence of events:
- 1970s – ‘fop’ herbicides (Group A, ACCase) applied roughly once a month until resistance evolved;
- early 1980s – paraquat applied once a month (this practice was only used for a few years);
- 1980s and 1990s – it is likely that glyphosate was applied at least once a month for about 15 years until resistance evolved; however, there are no records to confirm this (the high level of glyphosate resistance implies there must have been a significant period of glyphosate use); and
- mid-1990s to 2010 – glufosinate applied at least once a month until resistance evolved.
The nursery management had to work hard for the crowsfoot grass to evolve these levels of resistance – at least 12 sprays per year for 15 years with no other form of weed control. It is remarkable the herbicides lasted as long as they did. Crowsfoot grass mostly self-pollinates, which would have slowed the evolution of resistance.
If we exposed annual ryegrass (which mostly cross-pollinates) to that much selection pressure, it is likely resistance would have evolved much sooner.
Although Australia is different to Malaysia, the same weed management principles apply.
Glyphosate resistance is rearing its ‘ugly head’ in Australia. If the only strategy to manage this is to switch to paraquat, then paraquat resistance will soon follow.
However, if diversity is added to the system as new herbicide groups are rotated, Australia will give itself a fighting chance.
When resistance to important herbicides such as glyphosate is found, it is worth rotating to alternatives such as paraquat. At the same time, war needs to be declared on the weed seedbank by using tactics such as crop competition, pre-emergent herbicides, seed-set control and harvest weed-seed management.
You cannot make the same mistake twice. The second time you make it, it is no longer a mistake. It is a choice. We all have choice to add diversity to weed management as resistance evolves.
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