Lessons from Australian cropping farmers show that herbicide resistance does not mean an end to arable farm profitability.
Addressing the Foundation for Arable Research (FAR) conference at Lincoln, Professor of Weed Management at the University of Adelaide Christopher Preston said resistance covers almost all the herbicides available for grain production.
“If you talk to Australian growers, they will tell you that weed resistance has made them better farmers.
“But it does mean that weed management becomes more complex and has to be more strategic,” he said.
In Australia there are 24 grass species and 28 broadleaf weed species with resistance to at least one herbicide mode of action.
The most important weed species with resistance is annual ryegrass, which has evolved resistance to nine different herbicide modes of action.
Annual ryegrass with resistance to Group 1 and 2 herbicides is widespread across the southern states of Australia and resistance to glyphosate (Group 9) is becoming common.
“Across Australia, one in six farmers now have glyphosate-resistant ryegrass. The only post-emergent treatment that reliably works is Paraquat (Group 22), so farmers are using a lot more of this,” Preston said.
“Weed control is a long-haul job. Farmers are thinking about control of weeds over the next five to 10 years.”
Australian grain farmers have adapted to post-emergent herbicide resistance by increasing their use of pre-emergent herbicides and using sequences of chemistry rather than relying on singular treatments.
Crop competition is another strategy.
“This doesn’t kill any weeds but reduces the seed set by up to 50%.”
Early-sown crops reach canopy closure quicker, shading out ryegrass seedlings.
Preston was involved in a nine-year integrated management trial for weeds on a farm site that was overrun with herbicide-resistant ryegrass.
This showed the effectiveness of changing crop rotation and double breaks, where two years of effective annual ryegrass control can be implemented to drive down the weed population.
The project also highlighted that the high-cost strategy for weed control was the most profitable.
“So, if you think you are spending too much on weed control you are probably not. There may be space to spend more. Do it right and get more profit.”
End-of-season weed control tactics such as hay crops are used to further reduce the amount of weed seed going back into the soil.
This includes the use of machinery such as an impact mill, which crushes weed seeds that enter the header during harvesting.
Future alternatives are electrical, laser and microwave weed control.
“However, as travel speeds are slow, these will require robots to drive.”
The WeedSmart Big 6 tactics developed in Australia to manage seed banks are: rotate crops and pastures, increase crop competition, mix and rotate herbicides, optimise spray efficacy, stop weed seed set and implement harvest weed seed control.
“The use of WeedSmart Big 6 tactics when used well keeps herbicide resistant weeds under control.
“Stacking of weed control tactics in each crop is essential to keeping weed populations low and allowing long-term profitable crop production,” Preston said.
As part of an AgResearch-led MBIE programme, Managing Herbicide Resistance, FAR has carried out weed surveys across New Zealand for the past five years and found widespread resistance in ryegrass, particularly to Group 1 and 2 herbicides.
No resistance to glyphosate has been found on arable farms.
FAR senior field research officer Matilda Gunnarsson said that the programme has helped remove the stigma from having herbicide resistance as farmers and the industry realise how widespread it is.
Resistance is a bigger problem in areas where grass seed crops are grown and rotation options are limited, with 71% of arable farm samples in South Canterbury showing resistance, compared with 35% in Mid Canterbury and 11% in the lower North Island.
Just five ryegrass plants per square metre can reduce wheat yield by 5%.
Yield loss is not the only reason to control weeds; seed certification and export requirements also demand good weed management.
Gunnarsson said advances in precision agriculture technologies such as remote sensing, GPS, and Artificial Intelligence can change weed management.
Using these technologies in conjunction with other weed management practices is important for the longevity of herbicides and slowing down the selection of resistant weeds.
“However these technologies can be very costly and difficult to access in NZ.”
The most sustainable weed management strategy involves integrating multiple control methods, which also minimises reliance on any single method.
Gunnarsson said to effectively manage weeds, it is essential to understand the biology and the cycles of the specific weed.
