Protect knock-down herbicides
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.
Independent Consultants Australia Network senior consultant Mark Congreve said the highly effective double-knock tactic, which combined an application of glyphosate followed by paraquat, was at risk if growers failed to 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 there is no room for complacency following a double-knock operation.”
Paraquat is a widely used herbicide, being an active ingredient in more than 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.
A moderate risk rating means resistance generally takes longer to occur, not that it will not occur.
“Paraquat resistance typically takes more than 15 years of consistent use before resistant weeds are noticeable in the field,” Mr Congreve 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 Victoria and southern NSW. While paraquat resistance is still relatively rare outside of lucerne systems, 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, Mr Congreve is concerned there are no new modes of action likely to be commercialised within the next 10 years or more, meaning farmers needed to protect what they had.
“It is essential farmers do everything in their power to preserve the effectiveness of the herbicide groups available,” he said.
“The key is to take a diverse approach to weed management and, 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.”
Checking steps
Mr Congreve suggested growers check the results of every spray application, looking for individual plants ‘surviving’ or ‘re-growing’ after a spray application that had killed adjacent weeds.
This might be a sign 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,” Mr Congreve 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.”
There are 10 weed species with confirmed resistance to paraquat – Group L – and 13 species resistant to glyphosate – Group M – in Australia.
• Information from Weedsmart, an industry-led project that aims to enhance on-farm practices and promote the long-term, sustainable use of herbicides in Australian agriculture. For more information about reducing the risk of herbicide resistance, visit the Weedsmart website: www.weedsmart.org.au.