RESISTANCE to pre-emergent herbicides from a total of three mode of action groups has now been confirmed in Australian annual ryegrass populations.

The University of Adelaide’s Dr Chris Preston. Photo: Alistair Lawson.

National weed resistance surveys of growers’ paddocks have identified populations resistant to the Group D, Group J and Group K herbicides and combinations of these herbicides.

University of Adelaide herbicide resistance expert, Chris Preston, said the resistance situation is complex, involving a variety of patterns of resistance.

“The resistance surveys have found resistance to trifluralin (Group D), Avadex Xtra (J), Arcade (J), Boxer Gold (J-K), Butisan (K) and Sakura (K), but not yet to propyzamide (D),” Dr Preston, who is involved in Grains Research and Development Corporation (GRDC) herbicide-resistance research investments.

“However, there is some low-level resistance to propyzamide in some annual ryegrass populations with resistance to multiple pre-emergent herbicides.”

Dr Preston said it was not yet clear why resistance to the relatively new Group K herbicides has occurred so quickly, but it was likely to be cross-resistance from some other herbicide selection pressure.

“This may be selection pressure through a post-emergent herbicide rather than one of the other pre-emergent herbicides.”

Because the patterns of resistance across the pre-emergent herbicides are so variable, even between nearby paddocks, Dr Preston said resistance testing will be essential in informing growers’ weed management decision-making.

“Testing for resistance to trifluralin, propyzamide, Avadex® Xtra, Boxer Gold®, Butisan® and Sakura® should be considered to understand which herbicides will still work on that population in the future.

“Seed tests are required for identifying resistance to pre-emergent herbicides.”

Dr Preston said not all pre-emergent herbicide failures will be due to resistance, as environmental conditions can greatly affect performance of pre-emergent herbicides, and include insufficient moisture, high stubble loads or later germinating weeds.

However, some of these failures will be the result of resistance and all relevant factors should be considered when assessing pre-emergent herbicide performance.

“Mixtures and rotations of pre-emergent herbicides should be used, where practical, to reduce annual ryegrass numbers in crop and delay the onset of resistance to individual pre-emergent herbicides.

“It is important that multiple tactics that reduce annual ryegrass seed set, such as crop topping and harvest weed seed control, are also used.”

More information on managing herbicide resistance in annual ryegrass can be found via the GRDC’s Integrated Weed Management Hub at https://grdc.com.au/resources-and-publications/iwmhub and via the GRDC-supported WeedSmart resource centre at https://weedsmart.org.au/.

Source: GRDC