Herbicide resistance continues to increase in the southern high-rainfall zone (HRZ), highlighting the importance of using ‘stacked’ tactics to prolong the life of important chemistries in the ongoing battle against annual ryegrass.
Annual ryegrass is as competitive in the HRZ as it is in lower-rainfall environments, with each ryegrass plant reducing cereal yield by 0.3 per cent.
Data from random weed surveying and subsequent herbicide resistance screening conducted by the University of Adelaide with GRDC investment indicates there are almost no effective post-emergent herbicide control options left for south-western Victorian growers, while pre-emergent herbicide resistance to certain chemistries is increasing.
University of Adelaide researcher Dr Peter Boutsalis. Photo: University of Adelaide
University of Adelaide researcher Dr Peter Boutsalis says random weed surveys in Victoria’s south-west show resistance in annual ryegrass populations as high as 100 per cent for both FOP and sulfonylurea chemistries.
Dr Boutsalis says the high rates of ryegrass resistance to post-emergent herbicides mean there is a high reliance on pre-emergent herbicides and cultural tactics to keep the weed at bay.
However, results from the random weed survey conducted in 2020 indicate that resistance to some pre-emergent chemistries is also on the rise.
“We tested trifluralin, Boxer Gold®, Sakura®, propyzamide, Luximax® and Overwatch® in the 2020 survey. In south-eastern South Australia, we detected no resistance to Overwatch®, Luximax®, propyzamide or Sakura®,” Dr Boutsalis says.
“We found low levels of resistance to Boxer Gold® (three per cent), while resistance to trifluralin had jumped from 41 per cent in 2016 to 52 per cent in 2020, meaning that, in the lower south-east, trifluralin will only work in one in every two paddocks.”
In south-western Victoria, Dr Boutsalis and his team detected 21 per cent resistance to Boxer Gold® and 16 per cent to trifluralin.
However, the good news for growers is that extensive trials conducted in the HRZ over the past decade or more indicate that, with the right management, resistant ryegrass can be controlled.
Help in a handbook
The economical and sustainable management of herbicide-resistant annual ryegrass has been the focus of ongoing GRDC investments in the HRZ.
Led by the University of Adelaide’s Professor Chris Preston and conducted in conjunction with Southern Farming Systems and MacKillop Farm Management Group, research has delved into tactics such as pre-emergent herbicides, crop competition, crop rotations and harvest weed seed control (HWSC) to determine the best way to manage annual ryegrass.
This research is summarised in a GRDC publication, Managing annual ryegrass in the high-rainfall zones of Victoria, South Australia and Tasmania: An update on current research and practice. This publication is available free on the GRDC website.
The booklet highlights four major challenges in using pre-emergent herbicides in the HRZ: long seasons, high rainfall and waterlogging, high weed numbers and herbicide resistance.
Research trials in the HRZ have shown mixtures of pre-emergent herbicides often work better than the use of single products for the control of ryegrass.
Crop competition is an important non-chemical tactic in helping to control annual ryegrass. Maintaining high crop competition through sowing early, using adequate sowing rates and choosing competitive cultivars allows a high yield potential in the current year and can also protect the yield potential of a paddock for future years by suppressing ryegrass numbers.
Instead of relying on one tactic alone, growers are encouraged to ‘stack’ ryegrass control tactics where possible to get the best outcome.
An effective pre-emergent herbicide, followed by an in-crop tactic and a tactic to reduce seed returned to the soil, such as through hay, crop topping or HWSC, should be implemented in as many phases as possible.
In addition, the best outcomes for ryegrass management were found to occur where robust herbicide packages are paired with crop rotations that utilise double-break strategies effectively.
This is because it gives two consecutive seasons of being able to include multiple tactics for annual ryegrass control where a crop-topping application was added as another herbicide control tactic.
While HWSC is less effective in the HRZ than other environments due to the later harvest and, therefore, reduced weed seed capture, it is still a tactic that should be considered.
Research indicates that, in the long-term, using HWSC that removes 30 per cent of weed seeds from an intensive cropping system will stop the steady growth in annual ryegrass numbers that typically occurs, and may cause a decline in weed numbers.
Resistance testing
In order to gain an accurate picture of the weeds they are dealing with and their resistance status, Dr Boutsalis encourages growers to collect weed seed samples that can be tested for herbicide resistance.
“For factors such as glyphosate resistance, we test the seeds at several rates because, often, if you increase the rate you get better control, so it may be that the herbicide is still effective at higher label rates,” he says.
“The best time to collect weed seeds is just prior to harvest, or even after harvest, although we understand that is a busy time of year.
“Although we have detected no or low levels of herbicide resistance in other weeds such as wild radish, brome grass and wild oats, it is important that farmers still test those weeds so they can monitor the situation and any changes in resistance status.
“Some herbicides may be less effective on annual ryegrass due to resistance but more effective on other weeds, so it is important for growers to know whether they can still use a particular chemistry at an increased rate for ryegrass or other weed species.”
More information: Peter Boutsalis,
Useful resources: Managing annual ryegrass in the high-rainfall zones of Victoria, South Australia and Tasmania: An update on current research and practice; Harvest Weed Seed Control for the Southern High Rainfall Zone.
WeedSmart resource centre. WeedSmart has investment from the GRDC and commercial companies and delivers science-backed weed control solutions. GRDC is a Platinum Partner in WeedSmart.
Weed seed resistance testing services are available via: Peter Boutsalis, Plant Science Consulting, Adelaide, info@plantscienceconsulting.com.au, Planet Science Consulting.
John Broster, Charles Sturt University, Wagga Wagga,