CROPPERS’ increased dependence on Group J herbicides for the control of annual ryegrass has led to growing resistance issues and concerns that farmers may run out of control options.
University of Adelaide associate professor of weed management Christopher Preston said annual ryegrass populations with resistance to Group D and J pre-emergent herbicides would make management in cereal crops difficult.
“There’s growing resistance to Group J herbicides like Avadex, with the increasing populations causing real concerns,” he said.
Dr Preston said the problem had arisen partly because of resistance to post-emergent herbicides in grass weeds, particularly annual ryegrass, which meant there had been increased dependence on pre-emergent herbicides for weed control.
There has been resistance to trifluralin in SA for many years and since 2005 it has been widespread. This resistance led to the early adoption of Boxer Gold when it was released in 2008 and later Sakura in 2012.
While this Group J resistance was concerning, Dr Preston said there was an example on the Eyre Peninsula that was even more alarming.
In an EP annual ryegrass population with resistance to Group J herbicides, resistance had occurred across many herbicides with this mode of action. It also had resistance to the Group D trifluralin and there was also a reduction in susceptibility to both propyzamide and pyroxasulfone found in the Group K herbicide Sakura.
“This highlights the need rotate pre-emergent herbicides, to try and avoid issues like this,” Dr Preston said.
“Once you’ve lost the ability to use Groups D, K and J, you don’t have a lot of options left and it will make life a lot harder on-farm. That’s why it’s so important to make sure you don’t put lots of pressure on a single herbicide group.”
Indian hedge mustard was also showing concerning resistance in SA. The weed has shown resistance to Groups B, C, F and I herbicides, which greatly reduced control options within break crops. Dr Preston said the weed was a growing problem in the Mid North, Eyre Peninsula and the Wimmera/Mallee in Vic.
“You’ll often find it in alkaline soils, where it loves to grow,” he said.
There are some herbicide options that are still effective, in particular herbicide mixtures with bromoxynil, but this meant good control had to be achieved in the cereal phase.
Crop topping could be used to reduce seed set in canola and pulse crops.
Two weeds that are growing more problematic in cropping areas across SA are feathertop Rhodes grass and windmill grass.
Dr Preston said of the two grasses, feathertop Rhodes was more concerning.
“It was barely here in SA 10 years ago, now it’s on almost every roadside,” he said.
Both grasses are spring germinating species that have a natural tolerance to glyphosate.
But glyphosate can control both grasses if it is applied to small seedlings at robust rates.
“If you get windmill grass in a paddock, it’s harder to control than feathertop Rhodes grass, but you’re less likely to get it,” Dr Preston said.
Windmill grass is a short-lived perennial species, and while its seedlings are relatively easy to control, mature plants are very difficult to kill with herbicides.
The soil seed bank life of windmill grass is also longer than feathertop Rhodes grass.
Dr Preston said common pre-emergent herbicides used in SA were not effective at reducing feathertop Rhodes grass germination in spring, so control needed to occur as soon after harvest as possible, before weeds set seed.
The other major weed causing issues in SA was common sowthistle, with resistance surveys finding nearly as much resistance to imidazolinone herbicides (63 per cent of populations) as sulfonylurea herbicides (72pc).
