Plant-Back Periods for Fallow Herbicides in Southern NSW
Herbicide plant-back restrictions should be taken into account when spraying fallow weeds prior to sowing winter crops in southern NSW. Many herbicide labels place time and/or rainfall restrictions on sowing certain crops and pastures after application, due to potential seedling damage. Crops such as canola, pulses and legume pastures are the most sensitive to herbicide residues, but cereal crops can also be affected.
When treating fallow weeds, especially in late summer or autumn, consideration must be given to the planned crop or pasture for the coming year. In some cases, the crop or pasture for the following year may also have an influence on herbicide choice.
Most herbicide residues are broken down by microbial activity in the soil. The soil microbes require warm, moist soil to survive and “feed” on the chemical. Degradation of chemical residue is slower when soils are dry or cold. Soil type and pH also have an influence on the rate at which chemicals degrade.
The following points are especially relevant:
Phenoxy herbicides such as 2,4D Ester, 2,4D Amine and Dicamba, require 15mm of rainfall to commence the plant-back period when applied to dry soil.
Group B herbicides such as Ally, Logran and Glean, break down more slowly as soil pH increases. Recently applied lime can increase the soil surface pH to a point where the plant back period is significantly extended.
Lontrel, Grazon and Tordon products break down very slowly under cold or dry conditions, which can significantly extend the plant-back period.
Keeping accurate records of all herbicide treatments and planning crop sequences well in advance, can reduce the chance of crop damage resulting from herbicide residues.
The below table provides indicative plant-back intervals for a selection of relevant herbicides:
* 15mm rainfall required to commence plant-back period ** Period may extend where soil pH is greater than 7 # Assumes 300mm rainfall between chemical application and sowing NS Not Specified