Control patches of resistant weeds early to avoid farm takeover

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With annual ryegrass resistance on the rise and wild oats resistance now widespread in the northern grains region, growers are being urged to be vigilant about seeking out any small patches of resistant weeds on their property.

Tony Cook, technical weeds specialist with the New South Wales Department of Primary Industries, says: “It’s important to manage the small patches and contain the weeds so they don’t take over the farm.”

“Some growers think it isn’t economical to manage the small outbreaks of resistant weeds on their property, but some weeds can set between 50 and 1000 seeds each year and if not controlled will take over paddocks within five to 10 years.”

Mr Cook’s weeds work in north-western NSW has found patches of ryegrass resistant to glyphosate as far up as Moree. 

A photo of glyphosate-resistant weeds

Patches of glyphosate-resistant weeds.

“Resistant annual ryegrass weeds are on many properties in the Liverpool Plains; on farms, roadways and along waterways. There are also small isolated patches in the central west.”

Further south where winter cropping dominates, the annual ryegrass is not only resistant to glyphosate. Weeds are also becoming resistant to pre-emergence herbicides, which limits growers’ control options even further.

In comparison to annual ryegrass, resistant wild oats are already widespread throughout the northern region. 

“One in every two growers I see has wild oats resistance,” Mr Cook says. “They became resistant to Group A herbicides first [such as Topik®] and now they are becoming increasingly resistant to Group B herbicides [such as Atlantis® or Hussar®] and in some cases are now also resistant to Group Z herbicides [such as Mataven®].”

John Single, a cereal and sorghum grower operating 40 kilometres south-east of Coonamble in NSW, says he has herbicide-resistant wild oats and annual ryegrass on his property.

“In the days before herbicide resistance, we’d think about whether we needed to use herbicides on a cost-benefit basis. But all that thinking has gone; the focus is now about minimising our resistance levels and maintaining the efficacy of our existing chemistry,” he says. “The number of our herbicide applications has increased 40 per cent, but with the need to use more water with some herbicides and to be more precise with our applications our workload has increased by 60 per cent.”

Patches of resistant weeds can evolve near fencelines or tracks where glyphosate and other herbicides are used to keep the area clear of weeds. 

“People need to monitor their paddocks early and look for suspect patches of resistant weeds,” Mr Cook says. “Once they suspect a patch of weeds as being herbicide resistant, they need to get those plants tested.”

Mr Single agrees: “Our monitoring has become much more vigilant, so that we go over paddocks after herbicide has been used to see how effective it has been and to check for any resistance.”

Typically, with a resistant patch of weeds, growers will see dead plants mixed with live plants, which cannot be attributed to any specific spray pattern problem such as a blocked nozzle. 

A seed test at harvest will give results on whether the weeds are resistant to pre or post-emergence herbicides. It is impossible to get any pre-emergent results with live plants. However, tests can be done for post-emergent herbicide resistance.

Once a weed patch has been identified positively as having resistance, growers need to treat that patch separately from the rest of the paddock. 

“Growers need to take that patch out of production and promote weed growth and then stop the weeds from setting seed,” Mr Cook says. “This patch needs to be isolated from the rest of the paddock. It’s like treating a cancer in the fingertip; don’t cut off the whole arm but treat the tip separately.”

On-farm hygiene is critical. A large patch of resistant weeds can be initiated via wheels over farm tracks and access roads in wet conditions. Using quarantine can minimise the impact of seeds being spread to other areas. The aim is to keep farm tracks and fencelines as weed-free as possible.

But what should growers do if the resistant weeds have spread throughout their paddocks?

Mr Cook believes growers can get a “good bang for their buck” from growing a competitive crop that quickly forms a canopy, such as barley, oats or some wheat varieties. Different sowing rates and spacings can also have great impact.

Western Australian growers are promoting the Harrington Seed Destructor, which takes chaff out and pulverises any weed seed at harvest, but this can prove expensive. Others use a chaff cart to isolate the weed seed away from the paddock.

The Grains Orana Alliance has trialled another technique favoured by WA growers – windrow burning. This can be useful for growers in the central-west NSW region, but is less useful further north.

Growers with infestations of resistant wild oats will need to rely on pre-emergent chemistry or rotate out of winter cropping to a summer crop such as sorghum or sunflowers. Growers on the central-west slopes and plains region who cannot rotate into a summer crop can try two winter fallows, which can help clean up their ryegrass and wild oats problems. After two years of winter fallows, there will be less than one per cent of the weeds’ seedbank left.

Mr Single finds that growing a sorghum crop in summer can help reduce the seedbank of resistant weeds.

“We introduced a sorghum crop rotation in the 1980s to help us manage erratic rainfall, but this is also helping us to manage our resistant weed problems,” he says. “Rotating through different chemical groups is also important. In order to do that we include residual herbicides when appropriate. When conditions are right, we will double-knock the weeds with paraquat after using other chemicals.

“As far as we know there are no new herbicides in the science pipeline. Some believe science will help get them through, but we’re not relying on that. If herbicides are not applied strategically, we’ll get more resistant weeds, which make it difficult for everyone.”

Mr Cook says the common message to growers is to monitor and test any suspect population early. “Act soon and seek advice, and make a plan to see where you can attack the annual ryegrass or wild oats over the whole plant’s life cycle.”

More information:

GRDC Weedlinks

Australian Glyphosate Sustainability Working Group

Tony Cooke
0447 651 607

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