Australian Herbicide Resistance Initiative says dual ryegrass resistance a concern

The discovery was made last year in two paddocks which for the past eight years had been in a Round-up Ready canola and cereal rotation.

It was mainly found in wet areas along the fence line where the risk of bogging meant driving faster and a potentially lower dose rate.

It was also found in a separate wet area which was prone to continual late germination, poor crop competition and crop topping with glyphosate as a result.

Speaking at a WeedSmart webinar on the topic last Wednesday, Australian Herbicide Resistance Initiative (AHRI) senior research fellow Roberto Busi said the discovery of the dual resistance was very significant as there is no alternative option for pre-sowing knockdown.

"Glyphosate resistance in ryegrass has remained relatively low for many years, about 10 per cent, but paraquat resistance in ryegrass has very very rarely been documented," Dr Busi said.

With resistance to both herbicides, double-knock strategies - glyphosate followed by paraquat before sowing - could become ineffective.

"This would be a great problem and challenge to manage for many growers."

Work is currently underway to characterise and profile the herbicide resistance and so far it has been discovered one population was also resistant to clethodim, while the other was not.

To help with that process, two field trials will be conducted in 2022.

"Both sites have heterogeneous ryegrass distribution across the paddocks from a very thick infestation along the fence to much less dense distribution into the paddock," Dr Busi said.

"We aim to duplicate the work at two sites and complement field trials with glasshouse work to test many herbicide use patterns, combinations and mixtures.

"The grower needs an urgent answer and we have put ourselves on the front line of weed research to help."

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