Timing
Winter is the most common time for herbicide application because the target grasses have germinated, the clovers have at least three trifoliate (true) leaves and there is usually adequate soil moisture.
It is advisable to avoid the coldest period in winter (commonly July) because herbicide uptake, desirable plant recovery and herbicide degradation are all slower during this time.
Earlier treatment, favoured by an early autumn break, is possible if the ideal conditions can be met. This provides a longer period after treatment for more desirable plants to recover before the colder months, reducing the loss of winter feed and filling the gaps created by the removal of the annual grasses.
Early spring application can also be considered. This timing captures any grasses late to germinate and avoids suppressing winter growth, however, the treatment may not be as effective if the plants are larger or have matured early (Keys, 1998). Late application reduces the time more desirable species have to fill the bare patches created by removing the annual grasses. It can also negatively impact earlyflowering sub-clovers.
Avoid combining winter cleaning and spray-grazing (for control of broadleaf weeds). If both treatments are required, undertake the spray-grazing first when broadleaf plants are small. Allow at least three weeks before starting the winter cleaning treatment.
Winter cleaning is not recommended in newly established pastures (less than 12 months old).
Rate
The label rate of simazine is determined by soil type, with lower rates on light-textured soils and higher rates for soils with a higher clay content. Label rates for pastures may be lower than crop application for the same weed. This is to avoid lasting damage to the sub‑clover.
The addition of a small amount of paraquat to the herbicide mix is recommended when plants become more mature.
Adequate herbicide coverage is critical. Label directions for minimum water volumes are likely to be higher than amounts used for many other herbicides. A wetter is also recommended.
The impact of treatment with simazine and paraquat on sub-clover and grasses before (left) and 10 days following (right)