Riverina ryegrass resistance surprise in weeds survey

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Key points

  • Do not assume the resistance status of weeds; have samples tested and be certain
  • Survey finds high resistance in samples that were not expected to be resistant
  • Hussar® was active against samples resistant to Logran®, even though they are both sulfonylurea herbicides

A review of ryegrass and sow thistle in the western Riverina has revealed surprising – and slightly disturbing – results on the extent of herbicide resistance in the region.

Ag Grow Agronomy & Research’s Barry Haskins says floods in March 2012 dispersed seeds over a large area, changing resistance levels on many farms. 

Some grain growers and advisers felt they did not have enough knowledge to choose the right herbicides for effective weed control so a GRDC-funded Fast Track Project was initiated through the Low-Rainfall Regional Cropping Solutions Network (RCSN).

The fast-track project sought to map resistance to key pre-emergent and post-emergent herbicides, and measure the ability of growers and advisers to predict the resistance status of their samples.

A 4WD in a paddock with weeds

A recent survey of weeds in the Western Riverina has found more than 20 per cent of ryegrass is either resistant or developing resistance to glyphosate.

Seeds were collected by advisers and growers for 102 ryegrass and 31 sow thistle samples between Hillston, Lake Cargelligo, Ardlethan and Griffith, in New South Wales. These were grown out by John Broster at the Graham Centre for Agricultural Innovation resistance testing facility in Wagga Wagga, NSW. Samples were taken from expected resistant populations and non-resistant populations.

Resistance results for ryegrass were:

  • Roundup® (post-emergent application at 1.2 litres per hectare) – more than 20 per cent of samples were either resistant or developing resistance;
  • Logran® (pre-emergent, 35 grams/ha) – 95 per cent of samples were resistant but no resistance was found to new pre-emergent products Boxer Gold® (2.5L/ha) and Sakura® (118g/ha);
  • TriflurX® (pre-emergent, 1.7L/ha) – while a small number of samples were developing resistance there were still high levels of no resistance;
  • post-emergents Hoegrass® (0.75L/ha), Verdict® (0.075L/ha) and Axial® (0.3L/ha) – high levels of resistance in samples of 79 per cent, 71 per cent and 62 per cent respectively;
  • post-emergents Intervix® (0.75L/ha) and Hussar® (0.1L/ha) – still achieving control with only 42 per cent and 28 per cent resistant; and
  • Status® (post-emergent, 0.5L/ha) – least resistance at 11 per cent.

“Several ryegrass samples were resistant to one or more herbicides but the number resistant or developing resistance to all key post-emergent herbicides was disturbing,” Mr Haskins says.

“It highlights the need for non-herbicide weed control options to maximise the usefulness of the herbicides that are still effective, and using pre-emergent herbicides such as Treflan®, Boxer Gold® and Sakura®.”

He says resistance levels to more conventional herbicides, such as Logran® and Hoegrass®, were higher than expected.

This was also the case for the more recent herbicide Axial®.

“Resistance to Roundup® is much higher in the region than industry trends suggest, highlighting the need to use other knockdown herbicides, such as Gramoxone®, together with other non-herbicide knockdown tactics to extend the life of Roundup®. The same could be said for Axial® and Verdict®.”

Mr Haskins also says there were some resistance patterns emerging, in particular a high correlation of resistance to Roundup® in conjunction with other herbicides. For example, most samples – but not all – that were resistant to Hoegrass® were also resistant, or developing resistance, to Verdict® and Axial®.

“With both Logran® and Hussar® belonging to the sulfonylurea group, we expected a high correlation of cross-resistance but this was not the case. Hussar® was still active against 67 per cent of samples resistant or developing resistance to Logran®,” he says.

There was much less resistance found in sow thistle. For Ally®, 10 per cent of samples were resistant and 10 per cent were developing resistance while only one sample of 31 was developing resistance to Midas®.

Results showed that higher cropping intensity correlated with higher resistance to a larger number of herbicides.

The most surprising finding was levels of resistance measured in samples with no expected resistance. In many cases, these paddocks had never had an application of the herbicide that the ryegrass or sow thistle was resistant to and in some cases the paddocks had never had any herbicides applied to them.

Mr Haskins says the populations may be naturally resistant or resistance has crept into the population by seed movement via wind, water, stock, wild animals or potentially (but not likely) pollen movement.

Can you predict resistance?

The Fast Track Project found that about 25 per cent of the time growers and advisers try to predict herbicide resistance in a paddock, they are getting it wrong.

Barry Haskins says this was true in the survey even for growers and advisers with many years’ experience in herbicide use and resistance, and for samples that have had little or no exposure to the herbicides being tested.

When a sample was being collected for testing, growers and advisers were asked to state whether they thought it was resistant.

“Their accuracy of predictions, averaged across all herbicides, was 74 per cent for pre-emergents and 65 per cent for post-emergents,” he says.

“The fact that 26 per cent of the time the grower or adviser thought that a sample was not resistant when it actually was, may paint a picture of the level of failed herbicide applications that commonly occur in farming.

“This underscores the importance of seed testing for resistance and not assuming a weed is susceptible to all herbicides – because in many cases it won’t be.”

More information:

Barry Haskins
0427 007 418


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