Herbicide resistance update – North East Victoria and southern NSW
Take home messages
- Herbicide resistance in ryegrass, wild oats, brome and sowthistle has been confirmed in southern NSW and northern Victorian random weed surveys.
- A multiple tactic approach is required to combat resistance including seed set control and seed capture techniques.
- Resistance testing identifies effective herbicide opportunities.
- Over relying on Group J and K herbicides for ryegrass control increases selection for resistance.
Resistance to pre-emergent herbicides
Shifts in resistance of major weed species have been monitored since 2005 across southern Australia via random weed surveys. These surveys are funded by GRDC and have been conducted annually since 2005 by the University of Adelaide, Charles Sturt University and the University of Western Australia.
The methodology involves collecting weed seeds from paddocks chosen randomly at pre-determined distances prior to harvest. Weed seeds are tested in pot trials the following growing season. In northern Victoria and southern NSW, annual ryegrass resistance to in-crop selective post-emergence Group A and B herbicides continues to increase (Table 1). However, resistance to Group D and J herbicides was lower than in most other survey regions in NSW, Victoria and SA. The use of diverse cropping rotations and strategies such as hay production and grazing are contributing factors.
Table 1. Incidence of herbicide resistance in southern NSW* and northern Victoria# in annual ryegrass identified in GRDC random weed surveys. Paddocks were scored as resistant if the seeds collected exhibited >10%-20% survival in a pot test conducted the following season.
*Data courtesy of John Broster, CSU; #Data courtesy of P. Boutsalis, University of Adelaide.
Resistance has also been confirmed in other weed species in northern Victoria and southern NSW such as wild oats and sowthistle (Table 2). In wild oats resistance is restricted to the Group A herbicides and in sowthistle to the Group B herbicides. No resistance in wild radish or brome has been detected in the random weed surveys, even though resistance has been confirmed by commercial testing.
Table 2. Incidence of herbicide resistance in northern Victoria and southern NSW identified in GRDC-funded random weed surveys. Results determined by pot trials the following autumn.
Group A Fop
Group A Dim
- no weeds detected in that survey.
Resistance in wild oats remains relatively low in southern Australia compared to northern NSW. However, due to limited alternative herbicide options, resistance to Group A herbicides can restrict cropping options. Differences in efficacy between Group A herbicides in controlling Group A resistant wild oats occur. Resistance to the Fops herbicides used in cereal crops can often be controlled with Fops only used in broadleaf crops. Additionally, differences in efficacy between Fops, Dims and Axial® can occur in Group A resistant wild oats. Resistance testing can help identify the effectiveness of individual Group A herbicides. Although Group B herbicides were effective in both survey regions, resistance to sulfonylureas and sulfonamides has been detected in other regions. Relying on one mode of action increases the chance of resistance.
Brome is a competitive species with limited control options in cereals particularly since its peak germination occurs after sowing (low temperature vernalisation requirement) and after the seed has been buried (dark requirement). Pre-emergent herbicides therefore, provide limited brome control. The most effective herbicides for brome control are the imidazolinone herbicides in Clearfield® crops and Group A Fop herbicides in broadleaf crops. Various Group B sulfonylurea and sulfonamide herbicides are only registered in wheat. Further increases in resistance in brome is therefore a concern due to the limited herbicides available.
Once a weed of predominantly northern zones, sowthistle has spread to most cropping regions. It germinates all year-round if moisture is present and is quite drought tolerant as plants mature. A single plant can produce over 10,000 seeds that can germinate immediately under favourable growing conditions. The high incidence of resistance to Group B herbicides including imidazolinones complicates control in Clearfield® crops also. Additionally, biotypes resistant to glyphosate and Group I herbicides have been identified in nearby regions.
Resistance to pre-emergence herbicides
No resistance to Group K herbicides and limited resistance to Group D and J pre-emergence herbicides has been detected in southern NSW and northern Victoria. In other survey regions, such as the Wimmera and most SA cropping regions where continuous cropping is more common, the incidence of resistance to these pre-emergent herbicides is significantly greater.
Resistance occurs when individuals survive post herbicide application and set seed. Many tactics exist to control weeds and reduce the onset of herbicide resistance. These tactics include use of knockdown herbicides, pre-emergence herbicides (new mode of action pre-emergence herbicides are in development), selective post-emergence herbicides, seed set control and weed seed destruction. Increased adoption of these strategies will reduce herbicide resistance. During random weed surveys, the density of ryegrass and other species encountered was generally low.
Seed is collected from these plants and tested for resistance in pot trials. In some cases, the plants present at the end of the season are the ones that drive resistance further. If the density of weeds late in the season is low, late season tactics can be overlooked. However, the adoption of late seed-set control or weed seed destruction techniques can eliminate surviving herbicide resistant individuals and reduce herbicide resistance. A comprehensive list of these techniques can be found on the AHRI website.
Benefits of herbicide resistance testing
Establishing a baseline of herbicide resistance can help maximise weed control. What the random weed surveys highlight is that there are many opportunities to use older chemistry effectively where resistance is not present. This can result in a significant cost saving. In the latest surveys in northern Victoria and southern NSW, resistance to Axial® was detected in 60% and 20% of the ryegrass populations collected, respectively. Therefore, in 40% and 80% of cases, respectively, Axial® would be effective. Directed resistance testing can identify opportunities to use effective herbicides. This involves growers sending seed prior to harvest, or plants growing in the paddock during the growing season to enable a herbicide resistance Quick-Test to be conducted.
Some growers find it convenient to assume they have resistance to the older chemistry without ever having it tested. Our testing has shown there are often differences within a single mode of action group. For example, ryegrass resistant to chlorsulfuron is not always resistant to Hussar®. Similarly, ryegrass resistant to haloxyfop is not always resistant to Axial®. Wild oats can be resistant to Axial® and Achieve® but not haloxyfop. There are also cases where ryegrass has been identified as resistant to sulfonylurea herbicides and cross-resistant to imidazolinone herbicides without prior exposure to the latter chemical group. Less resistance exists to the newer products, such as Arcade®, Sakura, Boxer Gold® and Butisan®. Not only are these products often more expensive. but weed populations are unnecessarily being exposed to strong selection pressure. Resistance to Group J herbicides has already been confirmed in ryegrass biotypes with resistance to other chemistries under investigation.
Herbicide resistance testing can identify situations when a herbicide failure was not due to resistance. Some of these factors can be rectified, such as improving spray techniques. Ensuring optimum coverage and sowing speed is essential to maximise placement of pre-emergence herbicide in the close proximity to weed seeds. To maximise coverage and herbicide performance, it is important to spray during ideal weather conditions with the correct nozzles, speed, water volume, water quality and herbicide quality.
With literally thousands of herbicide products available today (for example for glyphosate alone there are approximately 500 registered products) certain cheaper formulations can also result in reduced control. Herbicide efficacy can also be reduced under certain adverse environmental conditions (for example, drought, frost and temperature extremes) that cause weeds to stress. Processes such as uptake, translocation and metabolism of herbicides can be disrupted thereby reducing weed efficacy.
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the authors would like to thank them for their continued support.
University of Adelaide, Waite Campus, Glen Osmond SA 5064
GRDC Project code: UCS00020, UA00158
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