Sowthistle update on glyphosate resistance survey and overview of resistance testing and management options

Take home message

• Common sowthistle populations in the northern region have developed resistance to glyphosate.
• Survey results indicate that glyphosate resistant sowthistle populations are concentrated in northern NSW.
• An integrated approach to managing common sowthistle is recommended to prevent seed set and combat herbicide resistance.
• Herbicide resistance testing is recommended as part of an integrated weed management (IWM) strategy.

Survey of glyphosate resistant sowthistle in the northern region

In the northern region, glyphosate is a highly important herbicide for controlling common sowthistle in fallows. However, in 2013/2014, two populations of common sowthistle (Sonchus oleraceus) from northern NSW were determined to be glyphosate resistant. 

To establish how abundant and widespread this resistance is across the northern cropping region of Australia, the Queensland Department of Agriculture and Fisheries (DAF) are leading a glyphosate resistance survey of common sowthistle across the region. The research is being conducted in collaboration with the NSW Department of Primary Industries (NSW DPI) and grower solutions groups including the Northern Grower Alliance (NGA) and the Grain Orana Alliance (GOA).

Because the survey objective is to determine the abundance and distribution of glyphosate resistant sowthistle populations across the northern region, it was decided not to bias sampling by specifically targeting “problem” sites or survivors of glyphosate application, leading to an over-estimation of the incidence of resistance. To sample as many populations as possible, the decision was made to collect from sowthistle plants growing within both fallow and cropping situations.

Volunteers, mostly local agronomists and some farmers, have assisted in the survey by collecting seed samples, and this has been shown to be a very effective way of sampling the large study area.  To ensure that high quality sowthistle seed samples are collected, a “collection kit” was prepared by the research team, which was sent to each seed collector. This kit included a detailed sampling protocol, numbered calico sample bags, and postage-paid pre-addressed packages for sending the seed samples in for testing. Common sowthistle germinates in any season and can flower all year-round, provided there is adequate soil moisture. If there has been a significant rainfall event within recent months, seed producing plants are likely to be found in many crop and fallow paddocks. With their in-depth local knowledge and easy access to cropping properties, local agronomists and farmers can be on-the-spot to collect seed samples when the time is right.

In brief, the protocol for seed sample collection is as follows:

• Sites need to be cropping properties, located a minimum of 5 km apart.
• The paddock edges should be avoided in sample collection.
• Collect seeds from at least 20 individuals per site sampled.
• Record details of the sample population, including GPS location, on the field data sheet provided.

Once received for testing, the seed samples are stored in a dehumidified cold room at the Leslie Research Facility, so as to preserve seed viability until the time of their inclusion in screening tests. Before going into storage, they are sorted to assess quality, and to remove seed eating insects.

The diagnostic screening of samples to determine glyphosate resistance status is based on survival at 356.4 g a.i./ha of a commercial glyphosate formulation. Dose response experiments conducted at our research facility have determined that this dose accurately discriminates between resistant and susceptible populations. From a practical point of view, for the glyphosate product used in the screening, this dose is equivalent to the upper label rate to control sowthistle plants with five or less true leaves (or no more than 3cm in diameter/height), in a fallow situation or prior to planting a crop in the northern region.

In the screening tests, plants are sprayed at the two-to-four leaf stage and herbicide efficacy is assessed 21 days after herbicide treatment (increasing to 35 days in cold weather). Plants are considered survivors if the growing point (centre of the rosette) is green and/or if new leaves are present. A population is deemed “resistant” (R) if ≥20% of the plants survive the glyphosate treatment.

To date a total of 147 sowthistle seed samples have been submitted for glyphosate resistance testing. Of the populations tested, approximately 20% of the populations have shown resistance to glyphosate. For one seed sample, 93% of plants survived glyphosate application at the label rate for small plants. This sample was sourced from survivors of a glyphosate treatment in a fallow situation, located near Gunnedah, NSW.

The good news is that the survey so far indicates many populations remain susceptible to glyphosate, when treated at the small rosette stage and according to label recommendations. Through greater diversity in weed control as part of an integrated approach, it should be possible to retain the effectiveness of this key herbicide for control of common sowthistle.

Samples received to date provide a reasonable geographic coverage of the sampling area (Figure 1). Many samples have been sent in by growers and agronomists in northern NSW and the Southern Downs. However, additional samples are being targeted from central Queensland, the Western Downs, and Maranoa. Growers and agronomists in these areas are particularly encouraged to send in samples – if you can assist, please contact Annie van der Meulen (contact details are provided at the base of this paper).

Figure 1. Map showing the location of tested populations. Green tick = susceptible, Red triangle = resistant.

Sowthistle best management

Although glyphosate resistance in common sowthistle is of serious concern, glyphosate remains a viable control option for many populations. Where glyphosate resistance is confirmed, there are other effective options for controlling the weed.

Important control considerations for common sowthistle are as follows:

• It is important to know what herbicides will work. Glyphosate resistance is confirmed to be present in the northern region, and Group B resistance is reported to be widespread.  
• Aim for 100% elimination of seed set, including road verges and in channels.
• Maximise crop competition. Grow competitive crop species such as barley at narrow row spacing (e.g. 25cm), and in situations where common sowthistle is a persistent problem, avoid growing poorly competitive crops such as chickpea - this crop has high potential for sowthistle ‘blow out.’
• If relying on knockdowns in fallow, treat sowthistle while the plants are small, and double knock to control survivors.
• Apply residuals early in fallow. When using Flame® to control summer grasses, remember to partner it with an herbicide effective for controlling common sowthistle.

It is not advisable to rely on spray failure as an indication of herbicide resistance. Not only is spray failure a costly exercise, but there are multiple possible causes including poor application due to adverse environmental conditions and equipment failure. Herbicide resistance testing costs are typically around $125-$150 for a single herbicide, and $75-$95 for each additional herbicide tested. This relatively small expenditure could save considerable financial set-backs in lost production, wasted herbicide, and control costs of driving down a large resistant seed bank.

For specific details on testing options for common sowthistle, contact your local agronomist or a commercial seed testing centre:

Charles Sturt University, Wagga Wagga
Contact: John Broster, 0427 296 641, jbroster@csu.edu.au

Plant Science Consulting, Adelaide
Contact: Peter Boutsalis, 0400 664 460, info@plantscienceconsulting.com.au

Future research

A new GRDC-funded herbicide resistance surveillance project commenced in July 2015 and will involve resistance surveys of key herbicide groups commonly used for northern region weed species common sowthistle, flaxleaf fleabane, awnless barnyard grass and feathertop Rhodes grass. This project is led by Charles Sturt University, in collaboration with Queensland Department of Agriculture and Fisheries, University of Western Australia, and the University of Adelaide. In the northern region, the approach for testing and seed collection is currently being developed.

Acknowledgements

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.

Contact details

Annie van der Meulen
Crop and Food Science, Department of Agriculture and Fisheries, Queensland
PO BOX 2282 Toowoomba QLD 4350
Ph: 07 4639 8847
Email: annemieke.vandermeulen@daf.qld.gov.au