For 30 years northern growers and agronomists have relied on Group A herbicides to control wild oats in winter crops; however, the days are numbered for chemical control as herbicide resistance becomes more widespread across the region
Wild oats with resistance to Group A herbicides established inter-row in a wheat crop.
PHOTO: Emma Leonard
“Every second grower I see has wild oats resistance in the paddock,” says Tony Cook, technical weeds specialist with the New South Wales Department of Primary Industries (DPI). “It’s developing at an alarming rate.
“Wild oats first became resistant to Group A herbicides, such as Verdict® 520, and they’re now becoming increasingly resistant to Group B herbicides (for example, Atlantis® or Hussar®). In some cases they have also developed resistance to Group Z herbicides, such as Mataven®,” he says.
Wild oat plants are prolific seed producers. A single plant can produce up to 225 seeds, so even if only a small number of plants are allowed to survive and set seed, the seedbank can be quickly replenished.
The crop most under threat from wild oats is chickpeas, which is a notoriously poor competitor with weeds.
To work out the best herbicide strategy to control wild oats in chickpeas, the Northern Grower Alliance (NGA) conducted four small-plot replicated trials in 2014 to look at the potential of control with knockdown herbicides.
Haloxyfop, a ‘fop’ herbicide, is perhaps the most widely used Group A herbicide in the north for the knockdown of wild oats in chickpeas and other pulse crops. “Many agronomists concerned about the Group A resistance status of wild oats will mix haloxyfop with a ‘dim’ herbicide [most commonly clethodim],” Mr Cook says. “Other agronomists might use clethodim alone.”
Yet according to Lawrence Price, research manager with the NGA, resistance to haloxyfop is becoming increasingly frequent in wild oats.
Speaking at the Goondiwindi Grains Research Update in March, Mr Price said four NGA trials were conducted in commercial chickpea paddocks near Mungindi and Bowenville in Queensland and North Star and Edgeroi in NSW. The Mungindi and North Star sites were known to have wild oats with resistance to haloxyfop.
“We [the NGA] wanted to know if mixing haloxyfop with clethodim does give an advantage in wild oat control compared with either product applied alone, and whether we can get equal benefit by just increasing the herbicide rate,” he says.
“We also tested the effectiveness of butroxydim [for example, Factor®], another Group A ‘dim’ herbicide registered for wild oat control in many broadleaf crops but not widely used in northern NSW or southern Queensland.”
From the trials, the NGA team concluded that:
- Factor® (butroxydim) was not effective as a knockdown for wild oats in chickpeas by itself or when combined with Verdict® (haloxyfop);
- 500 millilitres/ha of Status® (clethodim) or mixtures of Verdict® (haloxyfop) plus Status® proved the most effective combination for knockdown control of wild oats; and
- Status® benefited from the addition of Liase® (ammonium sulfate).
Prevention better than cure
Mr Cook and Mr Price agree that the best investment a grain grower can make is to test samples of any weedy outbreak that is suspected of having Group A herbicide resistance. The choice of tactics available to growers in going forward can then be fully informed.
“Tests provide valuable information about the herbicides that don’t work and, more importantly, the herbicides that are effective,” Mr Cook says. “You may have resistance to one or more of the Group A herbicides, but other Group A herbicides may still work, such as resistance to haloxyfop but not to clethodim.”
An approximate cost of a broad-spectrum test is $400 to $500. This would include at least six to seven herbicides.
“This cost is rather insignificant compared to a widespread spray failure over 200ha that costs $30/ha in herbicides, totalling $6000 in wasted herbicide, not to mention crop yield losses and a potentially much larger weed-seed burden in coming years,” Mr Cook says.
“If resistant weeds are found, an integrated weed management approach is needed to ensure growers tackle the weeds using the most effective methods available, applying both chemical and non-chemical weed management tactics in combination.”
Mr Price suggests that if growers have a known wild oat problem in a paddock being planted with chickpeas, they should use a pre-emergent herbicide to reduce the population by 80 per cent. When informed by a resistance test, growers can then use an effective Group A to control any escapees.
“Long-term though, you also need to think about crop rotation. A common practice in the north is to switch to a sorghum crop where wild oats can be controlled by a combination of glyphosate or cultivation in the winter fallow, and atrazine in the sorghum crop.”
Another option, he says, would be to use Group B products in the cereal phase and save the Group A herbicides for the chickpeas and other broadleaf crops.
“The key point is that different populations of wild oats have different resistance profiles,” he says. “Some Group A products can work when others don’t, similar with the Group B products. If you find out which products might work before you spray the whole paddock, you can save yourself a lot of time and heartache.”
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