Herbicide resistance isn’t a new challenge for Australian cotton, but the weeds, tools and farming systems we’re dealing with continue to change.
Whatever new herbicides, traits or technologies become available, preventing weed escapes from setting seed remains one of the most important things growers can do to slow the development and spread of herbicide resistance.
The weed spectrum is changing
Glyphosate resistance continues to be a major challenge globally and in Australia. More than 50 weed species worldwide are now resistant to glyphosate, with more than 20 resistant species identified in Australia. The weeds growers are encountering have also shifted. Where broadleaf weeds once dominated industry surveys, reduced tillage and increased reliance on glyphosate have favoured surface-germinating species including fleabane, sowthistle and feathertop Rhodes grass.
Grass weeds are an increasing concern, with feathertop Rhodes grass emerging as a particular threat. The panel heard that recent survey populations collected for testing were effectively 100 per cent resistant to glyphosate. Its capacity to produce large amounts of seed also means a small number of escapes can quickly become a much bigger problem, and those weeds aren’t necessarily starting in the cotton field.
Look beyond the crop
One of the strongest themes from the discussion was the importance of looking beyond cropped areas when developing a weed management strategy. Irrigation channels, drainage lines, roadways, fence lines, field edges and machinery yards can all act as reservoirs for problem weeds and potentially provide a pathway for resistant weeds to move into paddocks. With in-crop weed control generally effective, these areas represent an important opportunity for future weed management.
Good farm hygiene therefore remains critical. That means paying attention to machinery movement and cleaning, managing field edges and other non-crop areas, and controlling escapes before they have an opportunity to set seed.
Growers are using more tactics
There are encouraging signs that Integrated Weed Management is becoming more widely adopted.
Recent grower survey results presented showed 60 per cent of growers using pre-plant residual herbicides and 45 per cent using residual herbicides at planting. Residual herbicide adoption had increased by 10–15 per cent over the previous five years, while the use of more than three non-glyphosate weed-control tactics had increased by around 20 per cent. It’s an important shift away from relying too heavily on a single herbicide or mode of action.
Residuals bring opportunities and trade-offs
Residual herbicides were a major point of discussion, particularly in relation to forced fallows in northern NSW and Queensland. Growers want effective residual weed control, but they may also want to retain the flexibility to plant an opportunistic crop. Longer residual activity can mean longer plant-back periods, so the two objectives don’t always fit neatly together. The panel’s message was that there is no perfect residual herbicide that provides long-term control, short plant-back periods and complete cropping flexibility. S-metolachlor and Trifluralin were discussed as commonly used options that can retain some flexibility, but product selection needs to take the individual farming system and future cropping plans into account.
Precision spraying still requires precision thinking
Camera and optical spot-spraying technologies provide growers with another valuable tool, particularly for managing weeds across fallows and non-crop areas. The panel also raised an important caution around their use with residual herbicides. Where a sprayer repeatedly targets weeds in the same area, application can potentially be concentrated. The discussion highlighted the possibility that some areas of poor crop establishment could be associated with excessive residual herbicide application in repeatedly sprayed patches. The takeaway? Precision technology doesn’t remove the need to carefully consider product choice and application rates.
New traits help, but diversity remains essential
The introduction of XtendFlex cotton provides growers with additional weed management options. The panel highlighted the opportunity to use dicamba pre-plant to target difficult weeds such as fleabane, as well as glufosinate as an additional knockdown mode of action. These options may be particularly useful in dryland systems where cultivation is limited, but additional herbicide tolerance shouldn't be seen as a reason to step back from IWM. The panel noted that dicamba is a Group 4 herbicide and resistance already exists to other Group 4 herbicides such as 2,4-D. As with any herbicide, allowing survivors to remain and set seed increases resistance risk.
Don’t wait for a new herbicide to solve the problem
While additional herbicide registrations for cotton are being investigated, the panel cautioned against assuming there is a large pipeline of new active ingredients waiting to solve emerging resistance problems. Products including Reflex, Balance and Overwatch were discussed, along with the increasing regulatory and environmental requirements involved in bringing additional herbicide options to market. Similarly, questions were raised about how the traditional double-knock approach may need to evolve if paraquat becomes less available or useful in the future.
All of this reinforces the importance of managing weeds across the whole farming system, rather than relying on the next herbicide, trait or technology.
Six things growers can do now
The discussion ultimately came back to some straightforward principles:
- Control weeds before they set seed.
- Use multiple weed-control tactics, rather than relying on one mode of action.
- Use residual herbicides where appropriate as part of an integrated program.
- Maintain good farm hygiene to minimise weed-seed movement.
- Pay attention to field edges, channels and non-crop areas, not just the cotton field.
- Don’t rely on any single herbicide mode of action, even when new traits and technologies provide additional options.
If you wish to watch the recording visit: Eric Koetz - Snapshot on Key Weeds & Trends in Herbicide Resistance - YouTube
To read more about Integrated Weed Management(IWM) by CottonInfo Weeds Tech Lead Dr Graham Charles visit: Weed management | CottonInfo