Queensland grains and cotton producers are urged to review weed management practices after the state's first case of a glyphosate resistant weed was recently confirmed.
Barnyard grass, a common grass weed across Australia's north-east cropping region, has developed a resistance to glyphosate, the most widely-used herbicide for fallow weed control in broad acre farming.
According to Queensland Primary Industries and Fisheries, the glyphosate-resistant barnyard grass is currently restricted to areas on two farms in southern Queensland, as well as several in northern New South Wales.
"These barnyard grass populations were found in zero tilled paddocks that had a long history of glyphosate use for summer fallow weed control," Minister Tim Mulherin said.
As barnyard grass will be emerging in the next few months, Mr Mulherin said it was timely for growers to review their practices and assess their risk for glyphosate resistance.
QPIF weed sciences leader Dr Steve Walker said complacency about this problem could lead to a doubling in weed control costs and reduced crop yields.
"It is costing growers in northern New South Wales about $100 per hectare more to control these weeds," Dr Walker said.
"Barnyard grass weeds can reduce sorghum yields by 25-40 per cent.
"In general, weeds at high risk are in zero or minimum tilled paddocks that have had a long history of summer fallows and exclusive reliance on g lyphosate with no follow-up actions on survivors of glyphosate spraying.
"The crop rotations at most risk are those with predominance of winter cropping and summer fallows, as well as growing glyphosate tolerant cotton alternating with summer fallows.
"Growers are urged to monitor closely for evidence of weed survivors following glyphosate spraying in fallows and glyphosate tolerant cotton, and these weeds should be tested for resistance.
"Actions need to be taken to ensure weed survivors do not set seed and become the source of further problems in following seasons."
Dr Walker said herbicide resistance was an increasing threat facing both growers and agronomists across Australia's northern grain region.
"Already 10 cropping weeds have been confirmed as herbicide resistant in various parts of this region, and more have been identified at risk of developing resistance, particularly to glyphosate," Dr Walker said.
"These experien ces have shown that resistant weeds require more expensive and less robust herbicides for effective control, along with changes in management practices."
Chair of Australian Glyphosate Sustainability Working Group, Assoc Prof Chris Preston, said the national experience was that glyphosate resistant weeds developed where glyphosate was relied on exclusively for fallow weed control.
"Any cropping system with frequent summer fallows will be at risk of glyphosate resistance in summer weeds," he said.
To minimise the risk of glyphosate resistance, the Australian Glyphosate Sustainability Working Group encourages integrated weed management that combines herbicides with other cultural tactics for weed control, maximising the opportunity to prevent seed-set and to reduce the weed seedbank.
* For more information on glyphosate resistance or if you suspect glyphosate resistance in your crops, visit the Australian Glyphosate Sustainability Working Group website.
A fact sheet 'Testing for glyphosate resistance' is now available at the QPI&F websites.
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