Resistance rising across Australia
Australia has the second highest number of herbicide-resistant weeds in the world – sitting only just behind the US. Twenty-five weed species have been confirmed resistant to one or more herbicides across Australia’s cropping regions
Herbicide-resistant weeds are on the rise across Australia, including an increasing number of cropping weeds with resistance to multiple herbicides.
In Western Australia, multiple resistance is now the norm for the state’s two worst cropping weeds – annual ryegrass and wild radish. Only two per cent of annual ryegrass populations remain fully susceptible to herbicide control, with all others resistant to one or more herbicide modes of action.
Fortunately atrazine, paraquat and glyphosate continue to provide good control on most annual ryegrass populations in WA, except for some populations across the southern coastal growing area (Figure 1).
A similar story exists for wild radish, with 93 per cent of populations resistant to one or more herbicides. A recent survey by the Australian Herbicide Resistance Initiative (AHRI) found that 84 per cent of 96 wild radish populations tested from Geraldton in the north to Esperance in the south had some level of resistance to the Group B herbicide chlorsulfuron (Glean®). Other herbicides such as glyphosate, atrazine and Velocity® (bromoxynil and pyrasulfotole) are still providing good control of wild radish.
Growers are being urged not to rely on herbicides alone to control cropping weeds. Non-herbicide methods for weed control, such as harvest weed seed control and crop competition, are being increasingly adopted across the WA wheatbelt and are proving highly successful at controlling weeds that escape pre-sowing herbicide control.
Across southern Australia, resistance is increasing in ryegrass and wild radish – mirroring the situation in the west, but lagging a few years behind because of a more diverse farming system. However, with the shift to a more intensive cropping system and higher herbicide use, it could only be a matter of time before the southern cropping states reach WA resistance levels. Resistance could be delayed in the southern region through adoption of the now widespread and highly successful harvest weed-seed control techniques used by WA growers to manage herbicide-resistant weed populations.
The level of chlorsulfuron (Group B) herbicide-resistant milkthistle across southern Australia now matches that of the northern cropping region, rendering the weed a national problem in cropping and fallow situations. Fortunately, switching to other mode-of-action herbicides is an effective management strategy, but the universal distribution of this species and widespread resistance to Group B herbicides is a future threat to cropping systems.
South Australian and Victorian surveys show the incidence of herbicide resistance is increasing in annual ryegrass populations in these states (Table 1). Seventy-five per cent of paddocks across south-east Australia have Group B chlorsulfuron-resistant ryegrass. A high level of ryegrass resistance to Group D trifluralin exists in SA and is emerging as an issue in Victoria.
Ryegrass with resistance to the Group A herbicide clethodim is also increasing across southern Australia. This is concerning because there are no highly effective alternatives to clethodim for controlling annual ryegrass post-emergence in canola and faba beans. With clethodim resistance, the most effective management may be to use an effective pre-emergent herbicide coupled with clethodim on young ryegrass plants and follow up by crop topping with glyphosate on faba beans. An application for crop topping canola with glyphosate was submitted to the Australian Pesticides and Veterinary Medicines Authority (AVPMA) in September 2012.
Herbicide-resistant wild radish is becoming a serious threat across the southern region, with more than 20 paddocks across Victoria and SA being reported to have populations with resistance. Of these, five populations are resistant to Group I herbicides (three in Victoria and two in SA) and one is resistant to Group B, Group F and Group I.
Since 2009, half of the 60 wild radish samples received for annual herbicide-resistance testing from growers across south-east Australia have been verified as resistant to Group B and Group I herbicides.
Glyphosate-resistant weeds in chemical fallows are threatening the viability of no-till cropping systems in the northern cropping region. Resistant annual ryegrass, barnyard grass and flaxleaf fleabane top the list. Glyphosate-resistant windmill grass, and to a lesser extent liverseed grass, are also increasingly being reported.
Glyphosate-resistant weeds are on the rise across Australia, with six species now confirmed with resistant populations – annual ryegrass, barnyard grass, liverseed grass, windmill grass, brome grass and fleabane (see Figures 2 and 3). Weeds with resistance to glyphosate have been found in every mainland Australian state. There are 347 documented glyphosate-resistant populations of annual ryegrass across Australia.
Of these cases, nearly half come from roadside verges and cropping fencelines – reflecting the long-term use of glyphosate to control weeds on property firebreaks and council verges. Australia has 612,000 kilometres of road considered at risk of developing weeds with glyphosate resistance, so the potential problem is huge.
In addition, market research has found many land managers are poorly prepared to deal with the looming crisis. The majority of the remaining cases stem from long-term use of glyphosate in broadacre cropping systems, particularly summer fallows. Glyphosate is an excellent herbicide that helps keep management costs down, but there are no easy replacement options available.
The rapid development of glyphosate-resistant weeds and species’ shift to glyphosate-tolerant status will have a large impact on the cost and ease of weed management in Australian cropping systems. It is critical that the life of this valuable herbicide is extended via integrated weed management.
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