Rotate Broadleaf Herbicides

26 May, 2004 10:00 PM
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THE rotation of herbicides is a crucial tool in the fight against herbicide resistance in weed populations.

The message has been reinforced by Agriculture Department weeds research officer Peter Newman, who said it was crucial that farmers did not stick with the same herbicide regimes.

Mr Newman's work in herbicide resistance has included investigation into the development of herbicide resistance in broadleaf weeds, and in particular wild radish.

Herbicide resistance to wild radish is becoming increasingly common with herbicide-resistant wild radish populations growing in number.

Among other management strategies, Mr Newman is encouraging farmers to think about herbicide rotations as one of their tools to slow the development of herbicide resistance.

However Mr Newman urged farmers to make sure they were confident of the efficacy of the alternative herbicides they planned to use.

"It makes sense that when farmers are using a particular broadleaf spray, they want to continue to use it," he said.

"When a herbicide brew is working for farmers, it is important that they look for alternative herbicides to rotate into their weed control program."

One approach to ensuring confidence in alternative herbicides is for farmers to familiarise themselves with the new herbicide the year before they plan to use it over a large area of their cropping program.

Mingenew farmer Jamie Greaves is an advocate of this approach.

Mr Greaves' approach to rotation of herbicides is to buy a drum or two of an alternative and use it in a paddock alongside his current herbicides.

This enables Jamie to learn a little more about the rate, speed of kill of weeds, weed symptoms, weed spectrum and crop safety on his own farm.

Some of the alternative herbicide mixes for broadleaf weeds recommended by Mr Newman include mixes of Group I + Group B, Group I + Group C, Group F + Group C and Group I + Group G chemicals.

Mr Newman said herbicides belonging to the same herbicide group have the same mode of action on the weed.

"It is therefore important to remember to rotate with herbicides from different herbicide groups," he said.

"Switching from a regular herbicide performer to a new brew may be a little risky if you have had little experience with the new brew, hence the need to become familiar with a chemical prior to broad scale adoption.

"At present, most broadleaf herbicide strategies tend to include a lot of MCPA (Group I), which is a phenoxy herbicide, as is 2,4-D.

"Resistance of wild radish to this group of herbicides has been observed in WA over the past two growing seasons.

"The continued overuse and heavy reliance on Group I herbicides will only exacerbate this problem in future years."

Mr Newman recommends a few key management strategies that will help minimise phenoxy resistance:

p Use full rates of phenoxy herbicides

p Use full/lethal rates of mixing partners

p Include a non-crop phase in the rotation

p Include harvest management of weed seeds

"Wild radish that are phenoxy-resistant are late flowering and set seed later in the season than non-resistant wild radish," Mr Newman said.

"This characteristic facilitates the removal of herbicide-resistant seed at harvest time.

"In addition to herbicide-resistant radish populations, there is also potential for doublegees to become resistant to metsulfuron methyl [Ally].

"Killing weeds such as doublegees with more than one group of herbicide will make it much harder for these weeds to develop resistance."

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