A proposed new cotton Herbicide Resistance Management Strategy
A new Herbicide Resistance Management Strategy (HRMS) is coming to Australian cotton growers. The new strategy continues the key messages of zero tolerance for survivors of herbicide sprays, and the need for diversity in weed management. But it takes a broader view of the importance of herbicide susceptibility, beyond thinking just about glyphosate. The new HRMS will call for growers to utilise three herbicide modes of action in crop, and three different modes of action in fallows, aiming for six different modes of action every two years. The advice to aim for zero herbicide survivors remains absolutely paramount.
Why change?
The existing HRMS was designed for responding to increasing glyphosate resistance, in a system that relied very highly on glyphosate in cotton, as well as in other phases. It was devised during the heyday of Roundup Ready, when agricultural industries worldwide, including cotton farming in Australia, were discovering glyphosate resistant populations of a number of weed species on farm. As the most important herbicide in the system (by various ways of looking at it), glyphosate susceptibility needed protecting, and a suitable strategy was developed.
A few changes in the system in recent years have necessitated a shift in thinking, away from glyphosate as a single point of concern. Resistance to other herbicides is now more important than before, because:
- The introduction of XtendFlex cotton (with over-the-top applications for glyphosate, glufosinate, and dicamba) means more herbicides are available in crop. That makes them more useful and therefore more important, and they should be protected for as long as possible;
- Residual herbicide use has increased. They are a key part of our capacity to engage in diverse weed management, but only as long as they’re effective;
- Most of these key herbicides to be used in XtendFlex systems have substantial histories of use already in cotton fields. So selection for resistant biotypes in some paddocks will be much quicker than we saw for glyphosate resistance, if diversity is not practiced; and,
- Truly new chemical weed control methods (whether new modes of action or resistance-busting additives for existing herbicides) are still not on the table for the foreseeable future. We must preserve what we have.
The existing HRMS was released in 2014, the result of a CRDC project led by Annabel Guest. It has been a marked success:
- 80 per cent of irrigated growers, and 70 per cent of dryland growers, reported using at least two herbicides other than glyphosate in crop (Crop Consultants Australia Qualitative Survey 2022-23); and,
- 71 per cent of crop consultants recognised the importance of survivor control in the evolution of glyphosate resistance.
Although growers and consultants continue to report that glyphosate-resistant species are their greatest weed challenge, these populations are generally well controlled with a diversity of tactics. The new HRMS is an evolution of the existing guidelines that reflect the growing importance of, and risks to, herbicides other than glyphosate.
Is the HRMS mandatory?
Like the existing HRMS, the new HRMS will be a recommendation supported by industry, not a mandatory or regulated plan.
How were the new guidelines developed?
We used population genetics modelling to assess resistance risks in multiple herbicide modes of action against multiple weeds at the same time, to investigate what happens in complicated real fields. We took the results of several hundred simulations, across four locations and six different weed species, and identified high and low-risk situations and key species and system weak points. The data allowed us to generate rules of thumb that would protect multiple herbicides against excessively high selection pressure for resistance, simultaneously. These rules support the guidelines for the new HRMS.
Three modes of action in crop (including glyphosate), and three other modes of action in fallow, provided adequate control of resistant populations and generally slowed the development of new resistances. Greater diversity in one part of the rotation can offset lower diversity in other parts, but this must be managed carefully. In dryland systems with frequent summer fallows, in particular, there are opportunities to load higher diversity in the fallow, if needed. In irrigated farming with fallows only every four or five years, high herbicide diversity must be built into the crop phase. Glyphosate remains a central part of the weed management strategy in all parts of the rotation, supported by a diversity of other tactics.
As in the previous HRMS, zero tolerance for herbicide survivors is critical. Cultivation was found to be a useful protective tactic where available, especially against small-seeded species susceptible to seed burial such as common sowthistle and feathertop Rhodes grass. Simulations also assessed the usefulness of other survivor-controlling techniques, including chipping and spot spraying. Optical applications for patch and spot spraying in fallow were found to be highly protective against resistance, when maximum label rates were used.
Example HRMS-compliant strategies for a dryland cotton rotation with frequent summer fallows
Example HRMS-compliant strategies for an irrigated system with infrequent summer fallows
How will the new HRMS be implemented?
The guidelines for the new HRMS are simple in scope, but require interpretation in the field. Herbicides must be chosen that are appropriate for the onsite weed flora, including ensuring that there is a sufficient diversity of modes of action for both grass and broadleaf weeds. Herbicide use in crop must obey label requirements, particularly around crop growth stage, and drift management is critical.
Survivor control can occur in concert with applications for newly emerged weeds if the next herbicide in the program is an effective option, but the efficacy of any application on larger survivors of previous application must be gauged realistically. Direct control of survivors through rogueing or spot spraying are the most effective approaches. Scouting for survivors and monitoring post-spray efficacy are of critical importance, and growers should consider herbicide resistance testing for confirmed survivors of herbicide applications.
Rotate residual and spot-spray products carefully to reach at least six different MOAs per two years. Avoid using the same residual MOA in both planting and layby timings. Use different at-plant residual MOAs in successive years. Similarly, rotate products used for spot spraying between years.
The updated HRMS will be released in 2025, following industry consultation and approval. Keep an eye out.
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