Herbicide resistance is quietly increasing in weeds growing along paddock borders such as fence lines, roadways and irrigation channels.
Dr Sally Peltzer said that approximately 25% of glyphosate-resistant populations within broadacre cropping situations across Australia come from fence lines and other non-cropping areas of the farm.
“These non-crop areas are not subject to the pressures of crop competition and are usually given lower priority than the paddocks when it comes to weed management,” explained Dr Peltzer. “The over-reliance on glyphosate along borders with no control of survivors is high risk and herbicide resistance under these circumstances is inevitable.”
“Also, weeds on paddock borders are typically not sprayed until later in the season when many of the plants are large and much less susceptible to herbicide,” she said. “It is also difficult to get good spray coverage on these large plants.”
Once herbicide resistant weeds establish on paddock borders the seed can easily spread into the paddock with wind, water or machinery.
Dr Peltzer is encouraging growers to treat weeds growing along paddock borders as a high priority job that requires good timing and a variety of management tactics, including non-herbicide options. “New populations of herbicide resistant weeds growing along paddock borders are being identified all the time,” she said. “The problem is already widespread and demands a serious re-think of how growers manage these areas around their farms.”
What is the first step if I think some of my paddock border weeds are not dying after being treated with herbicide?
Short answer: Get samples tested.
Longer answer: There are two tests available, a ‘quick test’ conducted on plant samples and a traditional test conducted using seed samples. Testing is needed to establish the level of resistance and to identify herbicide groups that are still effective on the sampled plants.
Are there new treatment strategies specifically for weeds growing along paddock borders?
Short answer: Yes.
Longer answer: Over the last few years field trials have helped identify some tactics that have the potential to improve weed control in non-crop areas. Spraying weeds on paddock borders in July and August is probably too late. Spraying in May when the weeds are smaller is more effective and a second treatment later in the season is commonly required to treat late germinations. The later treatment will also have a beneficial effect on managing summer weeds.
Other than glyphosate, what chemical options are available for treating weeds along paddock borders?
Short answer: A residual herbicide followed by a knockdown.
Longer answer: Weeds growing along paddock borders require targeted management tactics aimed at preventing seed set. Field trials in Western Australia have shown that an early treatment with a residual herbicide such as triazine or bromacil and a knockdown such as paraquat has a beneficial effect on seed set.
Other options for fence line control, including non-herbicide options, are available here.
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