The world’s first cases of glyphosate resistant sowthistle (Sonchus spp.) have been confirmed in northern New South Wales. Sowthistle is a major summer growing broadleaf weed in many parts of the Australian cropping regions.
The two populations come from mixed cropping farms on the Liverpool Plains, the same area that produced the latest glyphosate resistant liverseed grass that was confirmed this January.
“Sowthistle and other surface germinating weeds are becoming bigger problems with the widespread adoption of reduced tillage agriculture and an over-reliance on glyphosate,” stated Tony Cook, Technical Specialist (Weeds) with NSW Department of Primary Industries and leader of the team that conducted the testing.
“Having glyphosate resistant populations is really going to make weed management more complicated, especially with each plant being able to produce thousands of wind-blown seeds.”
There is also widespread resistance to Group B herbicides such as chlorsulfuron and metsulfuron in the northern cropping zone, so this reduces the number of successful herbicide options. Antagonism between glyphosate and 2,4-D when tank mixed also reduces control of sowthistle.
At stem elongation-early flowering, which is when many growers spray sowthistle, 80 to 90 per cent of the resistant populations survive the application of 1.6 L/ha glyphosate CT.
“Just like the barnyard grass and wild radish stories, the size of plants treated and rate of herbicide applied have a big effect on the level of control. Importantly, the smaller plants are more easily controlled with glyphosate despite having some resistance,” said Tony Cook.
“As yet we are not seeing some of the extremely high levels of resistance seen in other species where herbicide rate has no effect, so this is good for managing sowthistle. The research shows that once a resistant plant grows beyond the rosette stage its ability to survive glyphosate increases rapidly.”
The following points will be critical to successful sowthistle management:
- Get sowthistle tested for glyphosate and Group B resistance.
- Spray weeds when rosettes are no bigger than the top of a drink can. This may mean spraying more often.
- Use full label rates when sowthistle is present and use appropriate water volumes for the herbicide.
- Have spray rigs properly calibrated to deliver the maximum amount of the herbicide to the weeds.
- ‘Double knock’ with another mode-of-action.
- Use other modes of action such as Group L, I, L+Q.
- Control large survivors by spot spraying which includes using the Weedseeker® or Weed-it® sprayers.
- Control sowthistle and other weeds around fences, buildings, roads, irrigation channels.
- Use targeted cultivation where appropriate.
- Stop all seed set.
“We have already collected nearly 50 populations of suspected sowthistle from around northern NSW which are currently undergoing testing. This is likely to be the start of a much bigger problem with the rapid spread of wind-blown seed.”
If you suspect glyphosate resistant sowthistle on your farm phone Tony Cook at Tamworth on 0447 651 607.
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