Outsmarting weeds: beyond herbicides

Photo showing the effects of Feathertop Rhodes grass

Feathertop Rhodes grass has become an increasing problem over the past three years in the northern grain growing region, particularly on the Darling Downs, Queensland.

PHOTO:  Tom Dixon

As in other grain-growing regions across Australia, herbicide resistance is now a worsening problem in the northern region. Many northern farming systems rely on herbicides to control weeds, and herbicide resistance could hit hard.

Weeds in the northern region are developing glyphosate resistance at an alarming rate says Tony Cook, technical weeds specialist with the NSW Department of Primary Industries.

“Glyphosate became the north’s herbicide of choice with the move away from cultivation and a high frequency of rain on fallows,” he says.

“Fortunately, resistance has been slower to develop because of the diverse crop rotations in the north, allowing greater crop and herbicide choice, and swapping between summer and winter crops.”

But it seems that herbicide-resistant weeds are now here to stay, leaving growers to learn how best to deal with new cases of herbicide resistance. 

Problem weeds

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

Wild oats resistance is also now widespread in the northern grain-growing region.

“Every second grower I see has wild oats resistance,” Mr Cook says.

“Wild oats became resistant to Group A herbicides first (for example, Topik®), and are now becoming increasingly resistant to Group B herbicides (for example, Atlantis® or Hussar®) and in some cases are now also resistant to Group Z herbicides (for example, Mataven®),” Mr Cook says.

Glyphosate resistance is strongly suspected in sweet summer grass in Central Queensland and sowthistle in northern NSW, and samples of these are being tested for resistance levels. GRDC-funded researcher Greg Brooke has recently confirmed Group I resistant wild radish at Nyngan, NSW.

Feathertop Rhodes grass (FTR) has a natural tolerance to glyphosate and has also become an increasing problem over the past three years in the northern grains region.

FTR is controlled by conventional cultivation, but is becoming problematic in no-till systems.

Summer grass weeds confirmed as having glyphosate resistance include windmill grass, liverseed grass and barnyard grass says Dr Michael Widderick, principal research scientist at the Queensland Department of Agriculture, Fisheries and Forestry, in Toowoomba.

Dr Widderick says all of these grasses have different emergence patterns, so growers must know the appropriate time to apply a control method to treat the plants while they are at a suitable size, and to avoid them setting seed.

Double knock

Dr Widderick recommends growers use a double-knock strategy to overcome some of these resistant weeds.

“It is important to understand the weeds you’re trying to control, and part of this is knowing when they emerge,” he says.

“For example, in Central Queensland, growers have several opportunities through the year to apply a double knock to control FTR,” Dr Widderick says. “In southern Queensland and northern NSW it’s more of a summer problem, so growers have to target their double-knock application then.

“This is the same for other summer grasses. Liverseed grass tends to emerge in one large flush at the start of the season, whereas barnyard grass emerges numerous times throughout the season. When this emergence will occur largely depends on rainfall.”

Researchers are still looking at emergence patterns for windmill grass. These differences in emergence times mean that growers have a range of opportunities to combat these weeds.

For example, with liverseed grass, growers have only one opportunity for a double knock at the start of the season, but with barnyard grass they have several opportunities throughout the season.

Mix it up

Researchers recommend using a combination of tactics as the most effective herbicide-resistance management strategy.

Mr Cook says growers need to monitor their paddocks early and look for suspect patches of resistant weeds.

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

“Once you suspect a patch of weeds as being herbicide-resistant, you need to get those plants tested.”

A weed-seed test will give results on whether the weeds are resistant to pre or post-emergence herbicides, explains Mr Cook.

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 as having resistance, growers need to treat that patch separately from the rest of the paddock; whether by mechanically removing those weeds, or using a double knock to remove the problem weeds. It is vital that these weeds do not set seed.

An integrated weed management strategy is the most effective method of controlling problem weeds, and this can include:
  • harvest weed seed management;
  • crop sequencing;
  • strategic tillage;
  • double-knock treatment of weeds (chemical or mechanical); and
  • use of pre-emergent herbicides.

More information:

Michael Widderick (Queensland),
07 4639 8856,


Tony Cook (New South Wales),
02 6763 1250,



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