Most cropping systems rely fairly heavily on a small group of non-selective or ‘knock-down’ herbicides. Since the widespread adoption of zero and minimum tillage, these herbicides have provided effective control of many grass and broadleaf weeds – but these useful herbicides could be lost to the industry if steps are not taken to increase the diversity of weed management tactics used.
Mark Congreve, ICAN senior consultant, said the highly effective double knock tactic, which combines an application of glyphosate followed by paraquat, is at risk if growers don’t remain vigilant and ensure removal of any surviving plants.
“The double knock strategy of glyphosate, plus a Group I herbicide for weeds such as flaxleaf fleabane, followed by paraquat has provided excellent control of weeds that are difficult to kill with glyphosate alone,” he said.
“Recent confirmation of a fleabane population that is resistant to paraquat, found in a New South Wales vineyard, is a clear warning to grain producers that there is no room for complacency following a double knock operation.
“In addition, an annual ryegrass population from a West Australian vineyard was confirmed in 2013 to have resistance to both glyphosate and paraquat.
This shows that a single plant can develop resistance to both of the main non-selective knockdown herbicides used in Australian grain production.”
Paraquat is a widely-used herbicide, being an active ingredient in over 100 herbicide products registered for use in broadacre cropping. It is a group L herbicide and as such is considered a ‘moderate risk’ for herbicide resistance.
Having a moderate risk rating means that resistance generally takes longer to occur, not that it won’t occur.
“Paraquat resistance typically takes over 15 years of consistent use before resistant weeds are noticeable in the field,” Mark said. “This critical period has now elapsed on many farms where paraquat is used in cereals and broadleaf crops, and for general weed control around the farm.”
Paraquat resistance has been present and widespread in barley grass in lucerne production systems for many years in southern NSW and Victoria. While paraquat resistance is still relatively rare outside of lucerne systems, very high level resistance to paraquat was confirmed in three weed species (crowsfoot grass, blackberry nightshade and cudweed) taken from sugarcane and tomato blocks around Bundaberg in 2015.
In the event of widespread resistance to paraquat, Mark is concerned that there are no new modes of action likely to be commercialised within the next 10 years or more, so protection for existing methods is paramount.
“It is essential that farmers do everything in their power to preserve the effectiveness of the herbicide groups currently available,” he said.
“The key is to take a diverse approach to weed management and, most importantly, remove weeds that survive herbicide applications. This is the best way to keep weed numbers low and when numbers are low, resistant weeds can be controlled more effectively. It’s a numbers game.”
Mark suggested that growers check the results of every spray application, looking for individual plants ‘surviving’ or ‘re-growing’ after a spray application that has killed adjacent weeds.
This may be a sign that the surviving plants carry the genetic mutation that ‘protects’ them from the herbicide’s mode of action.
“If this is observed, the first step is to remove those individual plants before they shed seed,” he said. “It is recommended to have the plants, or their seed, tested to confirm resistance and determine what herbicides those individuals are still susceptible to.”
A second warning sign is when a higher rate of a herbicide is needed to have the same effect as achieved on the target weed in previous years.
Mark called this ‘rate, or dose, creep’ and said that it is the most common sign of resistance to herbicides like paraquat.
“Paraquat resistance primarily occurs as a result of a plant having the ability to re-direct the herbicide molecules away from the chloroplasts in the cell and into the cell vacuole, where the herbicide has no effect,” he said. “If you are finding that you now need to use a higher rate of a herbicide such as paraquat, it is time to change how you manage those weeds.”
The Weedsmart website provides many practical tools for farmers wanting to make their weed control program more diverse and robust to delay herbicide resistance on their farms.
Non-crop areas around farms are often treated with paraquat annually. This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed.
Herbicide resistance frequently occurs first along fencelines, roadways and irrigation channels where herbicide use tends to be the same year in year out and less attention is paid to survivor weeds or poor herbicide efficacy.
Currently there are 10 weed species with confirmed resistance to paraquat (Group L) and 13 species resistant to glyphosate (Group M) in Australia.
