Fence lines have become a hot bed for generating resistance to the world’s most useful herbicide—glyphosate. About one quarter of glyphosate resistant populations within broadacre cropping situations across Australia come from fence lines and other non-cropping areas of the farm.
Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed.
Dr Chris Preston, Chair of the Australian Glyphosate Sustainability Working Group, said this is now a national problem and requires a fresh look at control options. “The control options used will generally be determined by whether the border in question needs to be kept bare or if vegetation is beneficial,” he said.
In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed.
Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak.
“One thing that is very clear is the need to move away from using herbicides to control weeds along the borders in spring,” said Dr Preston. “The weeds present in spring are too large to be effectively treated with contact herbicides. Using a slasher with a spray nozzle is a better option for these survivors.”
Dr Preston said there are more options available to growers to use herbicides in autumn but the practicalities are difficult, given the lack of time available in the lead-up to planting winter crops.
“There are a range of residual herbicide products suitable for application at sowing,” he said.
Another option is to apply a high rate of Spray Seed to small weeds, followed with paraquat 14 days later. While this is not a true double knock it is an example of an alternative knockdown strategy that works and gives glyphosate a break.
Dr Preston’s research demonstrated that an early treatment with a residual herbicide such as bromacil mixed with a knockdown such as paraquat has a beneficial effect on seed set. The trial results supported a successful application to have Uragan® registered for use along paddock borders.
“Weeds growing in and adjacent to head ditches and tail drains in irrigation areas are also of significant concern,” Dr Preston said. “Glyphosate resistant barnyard poses a high risk because the seed travels easily in irrigation water and can rapidly infest the crop area.”
In the northern growing region paddock borders are cleaned using a range of fallow management options.