AHRI discovers wild radish resistance to Velocity

At the end of the 1990s, wild radish had almost beaten growers in the northern Wheatbelt.

The lupin:wheat rotation had been running for about 20 years, with little diversity in herbicide choice, resistance was rife and the crops were weedy.

However, growers adapted, more canola was sown, fewer lupins, and in 2009 a new herbicide from Bayer, Velocity, with a new mode of action, HPPD inhibitor, arrived.

It was the first new mode of action for cereals in 20 years in Australia and was just what graingrowers and agronomists needed, with clean crops the result.

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However, after 12 years, Velocity (pyrasulfotole + bromoxynil) resistant wild radish was confirmed by AHRI researcher Roberto Busi.

He said in one paddock the herbicide was used exclusively for wild radish control for 12 years in a row, resulting in wild radish with five to eight-fold resistance.

"Another paddock that had eight cereal crops in 12 years with eight shots of Velocity also tested as resistant," Dr Busi said.

"This paddock was in a canola:cereal:cereal rotation and there are plenty of paddocks with this rotation with an extensive Velocity history."

The research also showed there was cross-resistance to the similar herbicides Frequency (topramezone) and Callisto (mesotrione), however the latter applied pre-sowing appears to still work.

"We knew this day would come, I guess we hoped it would be after 20 years, not 12," Dr Busi said.

Bayer Crop Science Australia leader of integrated weed management Craig White said the development of resistance was unfortunately inevitable.

"There are other products on the market now which have a similar mode of action or active ingredients in them and looking at the AHRI research, there were other HPPD chemicals which also have levels of resistance developing," Mr White said.

"Growers need to involve the WeedSmart Big 6 and use a range of chemical, cultural and mechanical methods to make sure they stay on top of weed control.

"A dead weed is a dead weed, no matter how you achieve that weed control, so it's critical to match as many different tools together as possible and we will do what we can to help people understand how to optimise and use the chemistry the best way we can."

At the end of 2019, Dr Busi started a new herbicide resistance testing service based at The University of Western Australia.

Growers and agronomists sent in about 500 samples of ryegrass for testing, as well as 121 samples of wild radish.

Two of these wild radish populations, sampled by agronomists, had survivors to Velocity and Dr Busi embarked on extensive glasshouse and field testing.

A field trial was conducted last year at one of the sites where a sample was collected from.

"The results showed Velocity or Frequency applied post-emergent gave poor control, 79-87 per cent, due to resistance to these herbicides, but despite this, there are plenty of options that can achieve high levels of control," Dr Busi said.

"What we saw in the field reflected the cross-resistance profile detected in pots - resistance to Group 2 (Logran), 4 (2,4-D / MCPA) and 27 (Velocity), with no resistance to Group 12 (Brodal, Glocker) and therefore treatments based on Jaguar and Mateno Complete resulted in more than 98pc efficacy."

While Velocity resistant wild radish has arrived, with careful management, a healthy dose of the Weedsmart Big 6, and a take-no prisoners approach, there is a long future ahead for Velocity and other similar herbicides.

"The reason that we know about this resistance is because some agronomists took the time to test wild radish for their clients because they didn't like what they were seeing in the field," Dr Busi said.

"If you see weeds surviving a herbicide that previously has worked well for you, don't assume it was spray conditions or operator error.

"Assume resistance, take a test and find out - knowledge is power."