Herbicide effect and use
Simazine is a root-absorbed residual herbicide. It relies on rain to wash the chemical from the leaves into the soil, enough soil moisture for uptake by a growing plant and biological activity to remove the remaining herbicide from the soil. Simazine uptake is reduced under waterlogged conditions. (Dowling et al., 1997)
The herbicide restricts carbohydrate production, so weed growth only slows when plant reserves are exhausted. Therefore, larger plants take longer to show the effects of simazine uptake. Hard grazing before application is desirable to reduce the remaining dry matter and root mass and quicken the response.
The cleaning effect can be enhanced by the addition (to the herbicide mix) of a small amount of paraquat (such as Gramoxone®), a contact grass-selective herbicide with no residual activity. Paraquat ‘burns’ the leaves, which reduces the amount of material remaining. It is particularly useful when the target weeds are more mature.