14.3 Herbicide Effectiveness – Whole Plant

On a whole plant basis rather than at the biochemical level, a couple of points are important to note with respect to herbicide effectiveness. 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 particular tissue (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.

Selectivity

Herbicide selectivity refers to the differential effect of a herbicide when applied to a mixed population of plants. Selective herbicides are formulated to control specific plant categories such as grasses or broadleafs. For example, the selective herbicide 2,4-D controls broadleaf weeds, but not grasses. Nonselective herbicides are designed to control all weeds. Glyphosate is an example of a nonselective herbicide that was formulated to control both grassy and broadleaf weeds.  It should be noted that herbicides can damage desirable plants as well.  Some crops that would ordinarily be susceptible to a herbicide have been genetically engineered to withstand herbicide application (for example, Roundup Ready cotton will survive a glyphosate application, whereas conventional cotton will not).  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 plant species exist with respect to the first four items listed: Site of action sensitivity to a herbicide, absorption, translocation and metabolism of a herbicide. These mechanisms are characteristics of the plant rather than the herbicide. On the other hand, 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.