Controlling herbicide resistant ryegrass in canola crops?

with Gurjeet Gill, Associate Professor – Weed & Crop Ecology, University of Adelaide

The last in-crop herbicide option for the control of annual ryegrass in break crops is slipping away. Herbicide resistance testing in 2015 of weeds collected across the Eyre Peninsula in late 2014 has found that 7 per cent of the populations tested in the southern Eyre Peninsula are resistant to the Group A herbicide, clethodim.

Dr Gurjeet Gill, Associate Professor at University of Adelaide, says that the use of pre-emergent herbicides alone in break crops like canola is unreliable at best and the loss of the only post-emergent option would have dire consequences for break cropping in the southern and western regions.

Dr Gurjeet Gill, Associate Professor at University of Adelaide says that variety selection, crop competition, controlling survivors and spray application timing and technique all make a difference when it comes to extending the useful life of herbicides.

“Our Weed Science team has been investigating ways to protect this chemistry and extend its life,” he says. “Variety selection, crop competition, controlling survivors and spray application timing and technique all make a difference and need to be implemented as quickly as possible.”

“In a field where the ryegrass was known to be resistant to clethodim we tested three canola cultivars—an open pollinated cultivar, ATR-Stingray and two triazine tolerant hybrids—Hyola 559TT and Hyola 750TT against three herbicide treatments,” says Dr Gill.

“Where no herbicides were applied, the high biomass hybrid Hyola 750TT suppressed the number of ryegrass seed spikes by about 30 per cent through crop competition alone.”

When a regime of pre-emergent herbicide applications was applied, both hybrid cultivars suppressed spike production by about 50 per cent compared to the open-pollinated cultivar.

How does cultivar competitiveness help protect herbicide chemistry?

Short answer: It’s all about reducing weed seed numbers.

Longer answer: Crop competition minimises the survival of late germinating ryegrass and reduces seed production from surviving weeds. Choosing the most competitive cultivar available throughout the cropping sequence applies non-herbicide pressure to drive down the weed seed numbers. Early sowing of wheat with effective pre-emergence herbicide package increases wheat yields and reduces grass weed seed set, thereby reducing the pressure on grass control in the broadleaf crop phase.

How can I keep clethodim working and avoid resistance evolving?

Short answer: Use as many tactics as possible in each crop phase.

Longer answer: Clethodim is an effective herbicide in susceptible weed populations and provides another tactic to reduce seed set in-crop. Plan to use pre-em herbicides, choose competitive cultivars and implement one or even two harvest weed seed control tactics such as narrow windrow burning, crop topping or seed destruction to remove any weeds that escaped the clethodim treatment.

How can clethodim still be an effective control tactic in clethodim-resistant ryegrass populations?

Short answer: Application timing can have a significant effect on the efficacy of some herbicides, including clethodim.

Longer answer: Frost and weed size both affect clethodim efficacy. If resistance to clethodim is evident, applying the product to small (up to 3-leaf stage) annual ryegrass plants under warmer conditions improves the level of control. Wait for a couple of days after a frost for the ryegrass to be actively growing. Consider spraying when there is a cold front predicted as the cloud cover reduces the chance of frost. Combining clethodim and butroxydim and applying to small clethodim-resistant ryegrass plants can be an effective tactic. Note: frost does not affect the efficacy of clethodim in susceptible populations.

The timing of herbicide application influences the efficacy of herbicides—in this case, applying clethodim to ryegrass in frost-free conditions increased the efficacy of the herbicide on clethodim-resistant plants.

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