Research aims to stymie increasing resistance in annual ryegrass

Author: Sharon Watt | Date: 11 Aug 2015

Chris Preston

Researchers are investigating strategies to counter the “alarming” level of resistance to the herbicide clethodim which is being detected in annual ryegrass in the southern cropping region.

Weed surveys across the southern region show increasing levels of resistance to clethodim, which is the last Group A herbicide that provides effective control of herbicide-resistant annual ryegrass.

The loss of clethodim to resistance will make ryegrass management more difficult, according to weed control experts whose work is being funded by the Grains Research and Development Corporation (GRDC).

To address the issue, Dr Chris Preston, from the University of Adelaide, and a team of researchers have been looking at new alternative herbicides for canola as well as alternative herbicide strategies in break crops.

“We are currently working with some potential new ryegrass herbicides for canola,” said Dr Preston who has been speaking about the latest developments at recent GRDC grains research Updates.

“Most of the compounds we have been looking at are best used pre-emergent…and we have identified one molecule that has potential as a post-emergent herbicide for ryegrass control.”

Trials of alternative strategies have been conducted in faba bean and TT (triazine tolerant) canola and hybrid canola crops at Roseworthy in South Australia, as well as RT (resistant to both glyphosate and triazine herbicides) canola crops at Lake Bolac and near Minimay in western Victoria.

Dr Preston said last year’s trials in a crop of faba beans with clethodim-resistant ryegrass demonstrated that sole reliance on pre-emergent herbicides failed to adequately manage annual ryegrass and large yield penalties occurred.

“The trials showed that the most effective currently registered option for faba beans was the addition of Factor to clethodim post-emergent following a pre-emergent application of simazine,” Dr Preston said.

Canola trials at Roseworthy comparing TT open-pollinated varieties with hybrid varieties indicated that the latter offer greater competition which helps limit seed production from surviving ryegrass plants.

Pre-emergent herbicides performed better in the hybrid canola than in the open-pollinated canola due to the increased competition provided by the hybrid.

Dr Preston said Rustler pre-emergent with clethodim post-emergent was one of the better treatments despite resistance to clethodim being present.

Researchers have also been examining the potential for RT canola to be used as part of a strategy to control clethodim-resistant annual ryegrass in high rainfall areas, with trials at Lake Bolac and west of Minimay in Victoria.

“The strategy is comparing a TT option with an option including both glyphosate and triazine resistance and a third option including crop topping,” Dr Preston said.

“As the population at the Minimay site is resistant to clethodim, the TT strategy was the least effective option. Substituting Roundup Ready herbicide for clethodim reduced ryegrass numbers. Crop topping should have an additional effect on seed set.”

The trial has been sown to wheat this year, with several strategies used in an attempt to maximise the reduction in annual ryegrass populations.

Dr Preston said that the registration in 2014 of Weedmaster DST for use as a crop-topping application in canola had provided an opportunity to control grass weed seed set in canola crops.

“The registered uses are to apply over the top of the crop from 20 per cent canola seed colour change or under the windrower.

“To use Weedmaster DST effectively for crop topping, the correct timing, rate of herbicide, water volume and environmental conditions need to be followed.”

Before considering any recommended applications, grain growers are advised to refer to herbicide labels to ensure correct use.

Growers experiencing challenges with weed management within their farming systems are encouraged to consult with their adviser and seek more information on integrated weed management from the GRDC website at

Further information on long term sustainability of herbicide use in Australian agriculture is available at the GRDC-supported WeedSmart initiative website at

For Interviews

Chris Preston
University of Adelaide
08 8313 7237


Sharon Watt, Porter Novelli

0409 675100

Caption: Dr Chris Preston, from the University of Adelaide, and a team of researchers have been looking at new alternative herbicides for canola as well as alternative herbicide strategies in break crops. Photo: Peter Boutsalis

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