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Velocity resistance flags the value of herbicide resistance testing pre-harvest

If there were ever a herbicide silver bullet, it would be the commercial mixture of pyrasulfotole plus bromoxynil.

Formulated by Bayer and known as Velocity®, this product offered a pre-formulated mixture of two modes of action that came with a rare synergistic effect on wild radish.

For grain growers in Western Australia facing widespread resistance to multiple herbicide modes of action set to vastly reduce wheat yield, the release of Velocity in 2009 was a game-changer.

In 2019, a diligent agronomist noticed that this reliable herbicide seemed less effective on wild radish in a client’s field, so he collected samples for testing at the University of Western Australia (UWA) Resistance Testing Laboratory.

Two populations certainly stood out during the screening phase of testing, attracting the attention of the UWA testing service lead researcher, Dr Roberto Busi. After extensive greenhouse and field trials, high-level resistance to the pyrasulfotole component of Velocity was confirmed for the first time in wild radish.

The two populations were 5- to 8-fold less sensitive to pyrasulfotole than the susceptible control when applied at the 4-leaf stage. When tested against the pyrasulfotole plus bromoxynil formulation the resistance level was 4-fold, demonstrating that the bromoxynil [Group 6/C] component still provides some efficacy. When tested against other chemistries with the same Group 27 [H] mode of action as pyrasulfotole, up to 9- and 11-fold cross-resistance, respectively was measured to mesotrione and topramezone when applied postemergence.

This serious, although not-yet-widespread, situation had arisen following the field application of up to 12 applications of Velocity in wheat in successive years. This clearly demonstrates that all herbicides, even synergistic mixtures, must be incorporated into a diverse cropping program that enables the use of numerous herbicide modes of action, multiple use patterns, targeting the weed’s sensitive growth phases and applying cultural practices to suppress weed seed production.

Herbicide susceptibility testing is also invaluable, identifying modes of action that can curtail any low-level resistance that might be evolving in a paddock. In this instance, the resistance to pyrasulfotole plus bromoxynil (Velocity) or topramezone (Frequency) plus bromoxynil applied postemergence could be expected to respectively result in 21 and 13 per cent of the wild radish plants present in the field surviving and setting seed. Note that topramezone plus bromoxynil was applied twice. The population was also known to exhibit multiple resistance across three sites of action, including Groups 2 [B], 4 [I], and 12 [F] herbicides.

One hundred per cent effective field control of this multi-resistant wild radish population was achieved through separate or mixed application of postemergence herbicides including diflufenican, MCPA, bromoxynil, aclonifen, and pyroxasulfone. Mesotrione (Callisto) applied preemergence was also over 99 per cent effective in controlling this population.

The ‘Big 6’ WeedSmart tactics known to drive down weed numbers and drastically reduce the impact of herbicide resistance on Australian farms are 1. Rotate crops and pastures, 2. Mix and rotate herbicides, 3. Increase crop competition, 4. Use the double-knock, 5. Stop weed seed set and 6. Implement harvest weed seed control.

Three independent herbicide resistance testing services are available to growers in Australia:

Visit the website/s for details about the services offered, costs and specific instructions before submitting samples.

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