Weed issues and action items

Take home message

  • The incidence of glyphosate resistance in common sowthistle is increasing. However no cases have currently been identified on the Eastern Darling Downs
  • A range of double knock treatments which did not include glyphosate provided excellent control of small and large sowthistle plants
  • Strategic tillage can reduce the emergence of common sowthistle, but some forms of tillage will bury weed seeds and increase their persistence
  • There are a range of residual herbicides that provide effective control of feathertop Rhodes grass, but the efficacy can differ as a result of environment (eg rainfall)
  • Using Group A herbicides for fallow control of summer grasses is a risky practice and poses a resistance and crop damage threat

Sowthistle resistance and management

Sowthistle glyphosate resistance survey

In 2014, researchers at the NSW DPI, Tamworth Agricultural Institute, confirmed glyphosate resistance in two populations of common sowthistle from Northern NSW.  Identification of resistance followed application of glyphosate at 720 g.a.i. per hectare to sowthistle plants that had developed beyond the rosette stage, when the plants had commenced bolting or stem elongation. Since that time, researchers within the Queensland Department of Agriculture and Fisheries (DAF) have confirmed glyphosate resistance at the upper label rate (354 g.a.i. per hectare) for control of common sowthistle at the small rosette stage (less than 5 true leaves or ≤3cm diameter).

To benchmark how widespread this new resistance problem is, DAF is undertaking a random survey of common sowthistle growing in cropping fields across the northern growing region, as part of a GRDC funded project. The survey has been promoted through the grower solutions networks, including NGA, GOA and CQGS, and has involved the voluntary participation of agronomists and growers in collecting samples of common sowthistle for glyphosate resistance screening.

As shown in the map below, samples have been received over much of the northern cropping region. However, to date few samples have been obtained from cropping areas of Central Queensland, the Western Downs and Maranoa. In order to address these areas, we would particularly welcome sowthistle seed samples from growers and agronomists in these regions.

Figure 1. Map of the northern grin region showing the location () of common sowthistle samples for glyphosate resistance testing.

Figure 1. Map of the northern grin region showing the location (tick) of common sowthistle samples for glyphosate resistance testing.

Glyphosate resistance screening of the sampled sowthistle populations will be ongoing until early 2016, but preliminary results indicate the development of a resistance problem in the Liverpool Plains. To date, there have not been any populations from the eastern Darling Downs identified as having glyphosate resistance. However, it is not a question of if this will occur, but when. The choice of weed control options for control of your common sowthistle is critically important in ensuring glyphosate is retained as an effective herbicide for this weed.

If you are interested in becoming involved in the survey by sending in samples for glyphosate resistance screening, please contact Annie van der Meulen. Annie will provide information on collection methods, forms for gathering necessary information, and individually numbered bags to ensure correct identification of samples. You will receive a report on the resistance status of the sample(s) you provide (assuming good seed viability). Collecting samples is easily done when you are ‘out and about’ in the field, and samples will be accepted up to the middle of March 2016.

Sowthistle management

Due to the identification and increase in glyphosate resistance in this common weed species, alternative chemical and non-chemical options are required for its management. By including different management tactics as part of your weed management approach, the risk of resistance to glyphosate and other modes of action will be reduced, and management of already resistant populations will improve. Two such tactics are the double knock and strategic tillage.

The double knock tactic is a common tactic for the control of other weed species including flaxleaf fleabane and awnless barnyard grass. This tactic involves the sequential application of two different weed management tactics, where the second tactic is applied to control survivors of the first.

In a 2013 field trial located near Cecil Plains, double knock treatments were the most effective fallow treatments, and were equally effective on both small (<10cm diameter) (97-100% control) and large (>10cm diameter to elongating) (95-100% control) sowthistle plants. Most double knock treatments provided 100% control, thereby preventing any weed seed production.

Roundup® alone also provided very good control of both small and large sowthistle plants in this field. This helps explain why farmers continue to rely heavily on glyphosate for fallow control of common sowthistle. None of the single knock treatments were as effective as the double knock treatments in the control of large sowthistle plants, even at higher rates. Increasing the rate of herbicides did improve the control of small plants for Spray.Seed®, Tordon™ 75D + Roundup, Starane™Advanced + Roundup and Alliance®. However, even at the higher rates, these treatments only provided 88 – 97% control, thereby allowing survivors to grow and set seed.

By including a double knock as part of your fallow management approach, survivors will be controlled and seed set will be reduced. This will reduce weed density, and thereby the impact of weeds on crop production.

The impact of different forms of tillage on seed burial and subsequent emergence is being investigated in 4 field and 2 pot experiments. In all field experiments, harrows had the least seed burial (majority at 0-2 cm) and one-way discs had the most seed burial (majority buried below 5 cm). Glasshouse pot experiments have shown that emergence of sowthistle is greatest when seed is sown on the soil surface, and less when sown at 2 cm. For sowthistle, off set discs and one-way discs reduced emergence compared with zero tillage in all field trials (Figure 2), but in field trials 2 and 3, harrows and chisel ploughs increased seedling emergence. The timing and amount of rainfall during the trial can partly explain these differences.

Figure 2. Impact of different forms of tillage on the emergence of common sowthistle as % of emergence in zero tilled plots.Figure 2. Impact of different forms of tillage on the emergence of common sowthistle as % of emergence in zero tilled plots.

Residual herbicides for feathertop Rhodes grass management

Feathertop Rhodes grass (FTR) is a difficult weed to control with knock down herbicides, especially once it grows past the early tillering stage. While the double knock tactic can provide useful control in fallow, the inclusion of residual herbicides can provide additional control in crop and fallow. Currently the only residual herbicide registered for fallow control if FTR is Balance®. However, other residual herbicides used in the cropping systems of the northern region may provide useful control of FTR.

The Northern Grower Alliance (NGA) and DAF have conducted multiple field trials to explore the efficacy of a range of residual herbicide for FTR with the aim of identifying herbicides to pursue for registration. Results differ across sites and seasons. However, results show that in addition to Balance, several other herbicides are consistent in their effective control of FTR.

Group A herbicide plantbacks

Typically, Group A, grass selective herbicides are designed for use in broad-leaf crops to selectively control grass weeds. However, the recent occurrence of difficult to control summer grass weeds including FTR and awnless barnyard grass (particularly with glyphosate resistance) has resulted in some growers choosing to apply Group A herbicides in fallow. Applying group A herbicides in fallow is a high risk practice for two reasons:

1)      Previous research has shown that it only takes 6-8 years of repeated Group A use before resistance to this important herbicide group appears. Once resistance is present, the herbicide will no longer be effective as an in-crop herbicide and there are few alternative options. It is therefore important that Group A herbicides be preserved for their intended in-crop use.

2)      There are plant back restrictions for cereal crops following the application of Group A herbicides. As such, Group A herbicides applied in fallow can result in crop damage.

Currently there is a minor use permit (Permit 12941), valid in Queensland only, allowing the application of Verdict™  and other registered products containing 530 g/L haloxyfop for the control of FTR in fallow. The permit stipulates that Verdict  and other registered products containing 530 g/L haloxyfop can be applied once per season in fallow preceding a mungbean crop  followed within 7-14 days by a treatment of paraquat applied at a minimum rate of 1.6 L/ha (using 250 g/L paraquat product).

Many Group A herbicides can persist in the soil and cause damage to monocot crops such as wheat, barley, cereals, sorghum and maize. As such, many Group A herbicides include plantback warnings on their labels (Table 1).

Table 1. Summary of the currently available crop rotational recommendations as per registered labels (at August 2014).


Crop rotational constraints

Butroxydim (e.g. Factor®)

Do not plant cereal crops for 4 weeks after application.

Clethodim (e.g. Select®)

(No crop rotational information is provided on the label.)

Fluazifop (e.g. Fusilade®)

Do not plant cereal crops for 12 weeks after application.

Haloxyfop (e.g. Verdict®)

Cereal crops or grasses planted within 12 weeks of application may be damaged.

Quizalofop (e.g. Targa®)

Do not plant cereal crops into the treated area for 18 weeks after application.

Table extracted from the publication ‘Group A herbicide in fallow – GRDC Fact sheet’ and used with permission from authors.

However, limited data exist on the persistence of Group A herbicides for the northern region conditions and proposed application practices. The NGA have done some work in this area and the DAF weed science team currently have two field trials exploring the impact of a range of Group A herbicides on wheat growth and yield. Results are not yet known.

Further information/resources:





The research undertaken as part of these projects is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

The authors acknowledge those who have been involved in the sowthistle survey collection. Your participation is very important for the success of this survey, and for improving our ability to manage the development of glyphosate resistance in this species.

Many thanks to the Northern Grower Alliance for providing trial data for inclusion in this paper.

Contact details

Michael Widderick
Department of Agriculture and Fisheries (DAF) Queensland
Leslie Research Centre, 13 Holberton Street, Toowoomba
Ph: 07 4639 8856
Email:  Michael.Widderick@daf.qld.gov.au

Annie van der Meulen
Department of Agriculture and Fisheries (DAF) Queensland
Leslie Research Centre, 13 Holberton Street, Toowoomba
Ph: 07 4639 8847
Email:  Annemieke.vanderMeulen@daf.qld.gov.au

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