Overview
Herbicides kill plants by interfering with essential plant processes. The more rapidly these plant processes are taking place the greater the negative effect on the plant if the process is impacted by a herbicide. An analogy would be that wearing a dust mask over your nose and mouth will restrict you more if you are running than sitting in a chair watching TV.
Plant processes are being carried out rapidly in actively growing weeds and therefore actively growing weeds are more susceptible to most herbicides than are plants that are growing slowly. In general environmental conditions including moisture stress and low temperature that reduce plant growth also reduce herbicide effect (Figure 1).
Figure 1. Roundup applied to foxtail growing under adequate soil moisture (left) and dry soil (right).
Herbicides kill plants by interfering with essential plant processes or biochemical pathways within the plant.This herbicide interference may result in the starvation of the plant for essential compounds such as amino acids (glyphosate and other amino acid synthesis inhibitors) or in some cases results in the production of toxic compounds (example; free radicals produced by cell membrane disruptors). Herbicides function at a specific point in a biochemical pathway referred to as the 'site of action'. We may apply herbicides to the soil in which plants are growing or directly to the plant. In either case the herbicide must be absorbed by the plant and move to the site of action in sufficient quantity before an effect will be produced.
On a whole plant basis rather than at the biochemical level, a couple of points are important to note with respect to herbicide effect. For a herbicide to kill a plant the herbicide must be present in the correct AMOUNT, at the correct TIME and in the correct LOCATION.
The herbicide must come in contact with the portion ( LOCATION ) of the plant through which it is absorbed. The herbicide must contact the absorptive portion of plant in the correct quantity ( AMOUNT ) or concentration. This exposure must occur at the correct physiological stage of the plant (TIME) for uptake to occur. Many cases of a plant surviving an otherwise lethal herbicide can be explained by one or more of these conditions not being met.
Herbicide selectivity refers to the differential effect of a herbicide when applied to a mixed population of plants. Selective herbicides kill weeds and leave desirable plants unharmed when applied to both. There are several mechanisms contributing to herbicide selectivity including differences between the weed and crop regarding:
Site of Action Sensitivity
Absorption
Translocation
Metabolism
Site of Uptake
Differences between species exist with respect to, 1. Site of action sensitivity to a herbicide, 2. Absorption, 3. Translocation, and 4. Metabolism of a herbicide. These mechanisms are characteristics of the plant rather than the herbicide. The site of uptake of soil active herbicides is a characteristic of the herbicide rather than the plant. In other words for a given herbicide the site of uptake can be the same across plant species.