Read time: 5 minutes

Full label rates, always

With Delta Agribusiness senior agronomist, Tim Condon

Reducing the application rate of herbicides has proven to increase selection pressure on a range of weeds.

Delta Agribusiness senior agronomist, Tim Condon, said any saving in
chemical costs is significantly outweighed by the risk of the low dose causing faster
herbicide resistance evolution.

“Scientific studies have demonstrated that resistance can rapidly evolve in weeds
subjected to low doses of herbicide,” he said. “Some weeds can develop resistance within
a few generations.”

Low dose applications of some herbicides have been proven to cause rapid development of herbicide resistance in some weeds. Photo: Andrew Storrie.

When used at low rates on Lolium rigidum (annual ryegrass) Hoegrass®, glyphosate and Sakura® have been found to result in the rapid evolution of resistance to both the herbicide selected as well as herbicides that have yet to come into contact with the targeted population.

“When mixing herbicides it is important that each product is still applied at the full label rate
to ensure high mortality,” said Mr Condon.

Mr Condon said applying different chemicals in one mix can provide an additive
advantage. “It is important to understand the mode of action of each herbicide on the plant
when preparing a herbicide mix,” he said. “This is just as important for pre-emergent grass
weed mixes as it is for post-emergent mixes aimed at broadleaf weed control.”

Having dual mode of action does not negate the need to change herbicide products and rotate modes of action.

Can mixing herbicides help slow the rate of herbicide resistance?

Short answer: Yes.

Longer answer: Surrounding weed seeds with a combination of pre-emergent herbicides
with different modes of action can give a high level of control and help extend the useful
life of all the chemicals used. The high level of control must be supported with additional
control measures for all survivors. All products with different modes of action must be
applied at full label rates for this to be an effective strategy.

Can I reduce the rates if the herbicides in the mix have the same mode of action?

Short answer: No.

Longer answer: Mixing two chemicals with the same mode of action can achieve some
additional efficacy, however the mix should deliver the combined full rate to ensure a lethal
dose. The amount of stubble present and crop safety are all important considerations
when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice.

If a herbicide product has more than one mode of action can I cut the rate or not rotate chemicals?

Short answer: No.

Longer answer: Many herbicides on the market are a combination of two or more modes of
action within the one product. These products must be applied at the full label rate to be
effective. Having dual action does not negate the need to change herbicide products and
rotate modes of action. Repeated use of any single strategy will reduce the effectiveness
of that strategy over time.

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