Resistance to pre-emergent herbicides in annual ryegrass confirmed
Author: Sharon Watt | Date: 27 Aug 2018
Resistance to pre-emergent herbicides from a total of three mode of action groups has now been confirmed in Australian annual ryegrass populations.
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.
Herbicide resistance expert Dr Chris Preston, of the University of Adelaide, says 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),” says 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 says it is not yet clear why resistance to the relatively new Group K herbicides has occurred so quickly, but it is 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 says 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 says 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,” Dr Preston advises.
“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/.
A Soil behaviour of pre-emergent herbicides in Australian farming systemsreference manualis available at www.grdc.com.au/SoilBehaviourPreEmergentHerbicides.
Chris Preston, University of Adelaide
Phone (08) 8313 7237 / 0488 404 120
Sharon Watt, GRDC
Phone 0409 675 100
GRDC Project code: UCS00024, U0A1711-002RSX
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