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Classification of agrochemicals: Time of application

Based on time of application, herbicides are classified as pre-plant, pre-emergence, and post-emergence.

Timing of herbicide treatments may depend on the chemical nature of the herbicide and its persistence, the crop species and tolerance, the age and characteristics of the weed species, plus other factors such as cultural practices, and climatic and soil conditions.

Pre-plant herbicides

Pre-plant herbicides – such as glyphosate - are applied before the crop.

This term is often abbreviated as PP in herbicide literature.

These products are usually used to kill weeds that have germinated before planting or those that were left from fallowing. These products, may or may not be residual. If they are not residual, then there is no point applying these products if the weeds have not emerged.

Similarly they are often not selective, killing all plants present.

An important factor to consider in the use of agrochemicals is the residual nature of the product – both in terms of its subsequent effect on crops planted in that field and on the environment. Some products break down very quickly and so become essentially inert to the environment. Others remain active even into the next crop, which can cause problems if the product is not recommended for the crop.

Pre-emergence herbicides

Pre-emergence herbicides – such as butachlor- are applied after the crop has been planted and before weeds emerge. Pre-emergence herbicides are usually applied to the soil surface.

This term is often abbreviated as PRE in literature on herbicides.

These products are used to prevent establishment of weeds right after planting. They often have some residual action but they must not be toxic to the crop.

Some care is required to ensure that the products are applied under the right conditions – read the label. For example, most products must be applied when the soil is moist, but some should not be applied to standing water. Others cause problems if they are washed into the root zone of the emerging crop.

Post-emergence herbicides

Post-emergence herbicides – such as 2,4-D - are applied after weeds have emerged.

This term is often abbreviated POST in herbicide literature.
These products are used to control weeds that have established in the crop. Products should not be toxic to the crop and usually need to have direct contact with the weed foliage to be effective.

Since these products usually require contact with the weed, it is important to make sure that there is sufficient time after application without rainfall.

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