Reducing the application rate of herbicides increases a weed's ability to evolve resistance and any saving in chemical costs is significantly outweighed by the risk of the low dose causing faster herbicide resistance evolution.
WeedSmart has produced a short video on why application rates should never be reduced from the recommended chemical label instructions and also points out that scientific studies have demonstrated resistance can rapidly evolve in weeds subjected to low doses of herbicide.
Some weeds can develop resistance within a few generations.
WeedSmart recommends when mixing herbicides, it is important that each product is still applied at the full label rate to ensure high mortality.
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. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control and always read the label.
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.
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. WeedSmart advises growers to get good advice on specific label requirements if there is any doubt about application rates.
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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