Australian farmers have long relied on what’s called a “double knock” approach to preventing herbicide resistance. Essentially, growers spray with Roundup and then come back right away and hit escapes with another form of weed control. Depending on the crop and time of the year, that might include tillage, burning or treatments with another mode of action.
Mostly, though, the system has evolved into applying Roundup and then coming back with paraquat.
The approach held up well for years but issues are now at hand. Researchers have confirmed cross-resistance to Roundup and paraquat among populations of tall fleabane. Essentially, fleabane slipped past the common double-knock approach – an initial glyphosate application followed by a paraquat spray 7 days later.
In a comparison, one of the biotypes was 4.9 times more resistant to paraquat than a susceptible biotype.
“While this level of resistance is generally considered ‘moderate,’ it is clear that resistance is building and must be taken very seriously, given the importance of the double-knock tactic in most cotton and grain production systems in Australia,” writes Cindy Benjamin on the Australian-based WeedSmart website.
After the initial Roundup application, herbicide injury was obvious on the resistant populations. That included narrowing of leaves and slow growth. But when the second application went on – the paraquat part of the double knock – those plants survived and fared well enough to produce seed.
The resistant populations were collected in Queensland and New South Wales — Australia’s cotton states — as part of a resistance survey funded by the nation’s cotton checkoff program. This is the first paraquat-resistant tall fleabane identified in the Australia, although it has also surfaced in Japan, Sri Lanka and Taiwan.
Plenty Of Seed To Go Around?
That significantly raises the stakes, says Md Asaduzzaman, a weed researcher with the New South Wales Department of Primary Industries.
“This species produces a large quantity of seed, germinates quickly and the seed can travel over 10 kilometers (6 miles) in the wind,” he points out. “So, dispersal of paraquat / glyphosate-resistance traits will be impossible to contain.”
Researchers in 2013 confirmed that a population of annual ryegrass also had developed resistance to both Roundup and paraquat.
“Having demonstrated resistance to the dual application of these herbicides in the otherwise effective double-knock tactic is cause for great concern,” Benjamin notes in her report. “Weed populations take longer to evolve resistance to paraquat and glyphosate (Roundup) compared to some other modes of action, but it will happen after years of regular applications without survivor control.”
No Survivors – That’s The Objective
Field crops can compete against fleabane, given the right circumstances, Benjamin writes, but the weed gains the advantage in thinnish wide-row crops like dryland cotton.
“Combating herbicide resistance and keeping weed numbers low will require the implementation of a wider range of weed control tactics rather than relying heavily on the double-knock tactic,” she adds.
The key to regaining control rests on monitoring and removing survivors, emphasizes Asaduzzaman. He points to the Australian cotton industry’s weed control strategy of “2+2 and zero” – going with 2 non-glyphosate tactics in the crop plus 2 non-glyphosate tactics in the fallow period and then ensuring zero survivors.