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Alternative second knock herbicides for broadleaf weeds in fallow

The sequential application of two separate herbicide treatments has become the most common ‘double knock’ approach used in weed management. Unfortunately, these approaches have added cost, complexity and scheduling issues to weed management programs but have been required for two main reasons:

1. To control herbicide-resistant weed populations, that may have been selected by prolonged use of a similar mode of action chemistry; and

2. Control of weed species or stages that are unsuccessfully controlled with single herbicide applications.

Paraquat has been the key active ingredient used in the second knock situation and can provide effective management of a wide range of grass and broadleaf weeds. However, it is clear we require other options to use in this management window to:

1. Avoid the more rapid selection of paraquat resistance; and

2. Provide options that may improve weed control in situations where paraquat efficacy is not adequate.

Since winter 2016, NGA has been screening a range of herbicides, to identify options that have the potential for this usage pattern. The two key broadleaf weeds being targeted are common sowthistle (Sonchus oleraceus) and flaxleaf fleabane (Conyza bonariensis).

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