Herbicide Classification Based on Timing of Application

Soil-Applied Herbicides

Soil-applied herbicides generally affect seed emergence or the growth of weed seedlings and must persist in the soil to be effective. When applied before the crop is planted, these herbicides are referred to as preplant herbicides. Some preplant herbicides must be incorporated into the soil to be effective and are referred to as preplant incorporated (PPI) herbicides. Preplant herbicides are applied from a few days to several months before crop planting, depending on their soil persistence, weeds controlled and tolerance of the crop to be planted. Herbicides applied at planting or within a few days before crop emergence are referred to as preemergence (PRE) herbicides. In row crops, preplant and PRE herbicides can be applied in a band over the crop row to reduce herbicide costs, especially if cultivation will be used to control weeds between the rows. A soil-applied herbicide, in some cases can also be applied after the crop is established (POST) to lengthen residual weed control in the crop. Soil-applied herbicides are important as assurance that weeds will not emerge with the crop and be too large to control with the first foliar-applied application. Soil-applied residual herbicides can help delay herbicide resistance from evolving by adding an additional SOA into your herbicide program.

Foliar-Applied Herbicides

Foliar-applied herbicides are applied to weed foliage, with or without contact of the spray with the crop, and are effective generally against young weed seedlings. POST herbicides are generally considered to be those applied after crop emergence. The spray can be applied broadcast over the crop and weeds, directed to the weeds at the base of the crop if there is limited crop selectivity, or applied under shields if there is no crop selectivity. Foliar sprays also are used for controlling emerged weeds present at planting in conservation-tillage systems, referred to as burndown herbicides. Foliar-applied herbicides are referred to as contact herbicides when only the treated part of the plant is affected and are called systemic or translocated when the herbicide enters the plant and moves within it to the site of herbicide action. Translocation can be either through the phloem, which carries the herbicides to aboveground and belowground growing points, or through the xylem, where they move with the transpiration stream and accumulate at leaf margins.