Great Southern grain growers have been surprised to learn which herbicides are providing effective in-crop control for annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum), and which are failing. Growers are also tending to overestimate herbicide resistance levels in weeds, which are lower than those in the northern region, and this has implications for the efficacy of whole-year herbicide, rotation and weed-management strategies.
These are the findings of a localised annual ryegrass and wild radish herbicide-resistance survey as part of a GRDC Regional Cropping Solutions Network (RCSN) Albany Port Zone project.
The 2015-16 survey coordinated by the Nyabing Farm Improvement Group (NFIG) examined weed-seed samples from 44 paddocks on 32 properties between Lake Grace and Jerramungup. NFIG coordinator Fiona Hobley says knowledge of herbicide resistance in annual ryegrass and wild radish across the Great Southern region is lagging behind northern growing areas and needs to be improved through testing weed-seed samples.
“The RCSN-supported survey was designed to involve the region’s growers in the testing process and highlighted the value of checking weed seeds for herbicide resistance,” Ms Hobley says.
“Without identifying low levels of resistance, growers are at risk of exhausting herbicide options and wasting money on ineffective herbicides. Testing can also help to rule out other factors that might be affecting poor herbicide performance, such as application technique, weather conditions, soil moisture and compatibility.”
Ms Hobley says the RCSN project saw Charles Sturt University researchers assess eight commonly used herbicides on 37 annual ryegrass samples and five herbicides on 22 wild radish samples at half and full label-recommended application rates.
Figure 1 Annual ryegrass – percentage of samples with levels of herbicide resistance where herbicides were applied at half and fill recommended label rates (grams of active ingredients per hectare) in a Great Southern region survey in 2015.
She says the annual ryegrass testing, shown in Figure 1, found:
- where clethodim (Group A) herbicide was used at a full label rate, resistance was low (5 per cent of samples);
- where glyphosate (Group M) was used at a full rate, resistance was developing (5 per cent of samples); where clethodim and glyphosate were used at half label rates, resistance was developing;
- ryegrass had the highest resistance to imidazolinone Group B using Intervix with resistance (approximately 90 per cent of samples for both half and full label rates);
- Group A herbicide pinoxaden (AxialR) had only 10 samples tested and two samples had resistance at the full label rate; and
- where atrazine (Group C) and trifluralin (Group D) were used at full label rates, resistance was developing (small number of samples).
Figure 2 Wild radish – percentage of samples with levels of herbicide resistance where herbicides were applied at half and fill recommended label rates (grams of active ingredients per hectare) in a Great Southern region survey in 2015.
The wild radish testing, shown in Figure 2, found:
- where diflufenican (Group F) was used at full label rates, resistance was high (32 per cent of samples);
- where sulfonylurea active chemical (Group B) was used at full label rates, resistance was lower than expected (41 per cent of samples);
- where atrazine and glyphosate were used at full label rates, there was full susceptibility to the herbicide (100 per cent susceptible with zero samples developing resistance or resistant); and
- where MCPA amine (Group I) was used (at full rates), a low number of samples were developing resistance (two samples out of 22 samples tested).
Harvest weed-seed control (HWSC) tactics were used in half of the paddocks sampled.
Australian Herbicide Resistance Initiative (AHRI) communications leader Peter Newman says local growers were surprised clethodim resistance levels were not higher in the annual ryegrass samples tested through the project.
“But the fact that the resistance testing occurred in summer – under ideal, warm conditions for clethodim use – might explain this anomaly,” Mr Newman says.
“Clethodim doesn’t perform well in cold weather, so when it is sprayed in winter and there is low-level resistance in annual ryegrass, there will be many survivors, which is what growers are seeing in their paddocks.”
Mr Newman says a concerning finding of the survey was that several annual ryegrass samples showed developing levels of glyphosate resistance.
“But it was encouraging that growers in this region still have some great options for wild radish control,” he says.
Ms Hobley says more than half of the growers involved in the survey had never tested their weeds for herbicide resistance; 30 per cent incorrectly guessed which herbicides had resistant weeds; and there was a tendency to overestimate herbicide-resistance levels.
Note: Resistance is classified by Charles Sturt University as a population having more than 20 per cent survival, and ‘developing resistance’ is when 10 to 20 per cent of the population has survived.
Fiona Hobley, Nyabing Farm Improvement Group,
0448 880 585,
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