Frost puts freeze on effectiveness of ryegrass herbicide

Dr Gurjeet Gill, University of Adelaide Associate Professor in weed and crop ecology, speaking at a GRDC Grains Research Update in Campbell Town, Tasmania, on the topic of controlling herbicide-resistant annual ryegrass in canola crops.

Grain growers in the southern cropping region are being advised to avoid spraying annual ryegrass with clethodim herbicide when frost is forecast.

Researchers say poor performance of clethodim on annual ryegrass – a weed becoming increasingly resistant to herbicides – can be associated with cold and frosty conditions.

Trials undertaken by the University of Adelaide and funded by the Grains Research and Development Corporation (GRDC) have shown that frost treatment, particularly before clethodim application, reduced the herbicide’s activity in susceptible annual ryegrass populations.

Clethodim efficacy was further reduced in resistant annual ryegrass populations regardless of whether the frost event was prior to or post clethodim application.

However, the impact was much greater when frost occurred before clethodim application.

“Frosty conditions make it even harder to control annual ryegrass when clethodim resistance is present in the population,” said Dr Gurjeet Gill, University of Adelaide Associate Professor in weed and crop ecology.

“Considering these findings and several other field observations by agronomists and researchers, growers are advised to carefully review weather forecasts for their district and spray clethodim when cloud cover is present and the risk of frost occurrence is low,” Dr Gill said.

He advised growers not to spray annual ryegrass with clethodim two to three days before or after a frost event.

“Ideally, spray small ryegrass plants under warmer and frost-free periods for the best results with clethodim on ryegrass,” said Dr Gill when addressing a GRDC Grains Research Update in Campbell Town, Tasmania, on the topic of controlling herbicide-resistant annual ryegrass in canola crops.

Dr Gill’s message is particularly pertinent to areas prone to frost at this time of year, when many canola crops are likely to be sprayed for ryegrass control.

In July last year, for example, minimum temperatures recorded at the Cressy Research Station near Campbell Town were sub-zero on 14 occasions in that month alone.

“Stress imposed by frost is a highly significant factor affecting clethodim efficacy on annual ryegrass. We want the ryegrass to be actively growing and stress-free for clethodim to do the best job, so growers need to watch the weather forecasts closely to determine the best time to spray,” Dr Gill said.

Clethodim resistance has been increasing in the southern cropping region (Tasmania, Victoria and South Australia) at a steady rate, to now be at a level of 10-15 percent of ryegrass populations.

“In most cases, clethodim still provides fairly good ryegrass control provided weeds are sprayed early and at a time when frost is unlikely to occur two to three days before or after spraying,” Dr Gill said.

“The use of weed-suppressive hybrid canola can considerably boost clethodim efficacy on ryegrass. Crop competition should be viewed as an ally in the control of herbicide-resistant annual ryegrass.”

Dr Gill said that In order to extend the effective life of clethodim, it was important to maintain ryegrass populations at a low level by integrating pre-emergence herbicides and other tactics such as crop-topping, narrow windrow burning and other forms of harvest weed seed management.

More information on research into clethodim resistance can be found in Gurjeet Gill’s GRDC Grains Research Update paper.

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