How do selective herbicides work?

Most users of products chemicals for plants have at some point questioned how each of the selective herbicides works and how the weed selection process is done, leaving only useful plants. For this it is necessary to know the selectivity of the herbicides.

Selectivity of herbicides

2,4-D and MCPA are the most common phenoxy herbicides, which target selectivity to broadleaf weeds and act to hinder the balance of biochemical processes which are carried out in these weeds.

In this regard, the phenoxy herbicides emulate a naturally occurring plant chemical known as indole acetic acid (IAA). These generate the production of unregulated IAA in the plant, which causes an uncontrolled growth, torsion, elongation, thickening and, finally, death.

Another herbicides target the photosynthesis of the plant, which is the process where plants generate energy using sunlight. Generally, the inhibition of photosynthesis means a slow death for weeds as they would die of hunger; however, the effect of these herbicides is extremely rapid.

Many selective herbicides target the enzymes that reside within plant cells. Enzymes are proteins that work as catalysts in different processes within cells, resulting in complex chemical reactions occurring in conditions where they might not.

This enzymatic activity of the plants is generally carried out in a regulated equilibrium environment. When an herbicide attacks an enzyme, it blocks the sequence of complex chemical reactions and, in many cases, generates compounds of great toxicity in the plant, thus destroying the weeds.


Certain herbal herbicides are not herbicides when they are stored in their original container. The cause is because it is made as ‘proherbicides’ that are modified in the chemical formula which is activated once immersed in the weeds.

As an example of proherbicides are herbivorous herbicides which have in their composition diclofop-methyl, which is a proherbicide, not an herbicide. After applying this proherbicide, it is absorbed by the leaves of the weed and an enzyme in the plant cells neutralizes the methyl group of diclofop, it is there where the diclofop then destroys the weed by preventing the production of an enzyme that drives the synthesis of acids Fatty acids.

Fatty acids play a very important role in cell membranes, waxes and leaf cuticles, since by stopping the production of fatty acids, weeds dies because it can not continue to perform the biological processes essential for life.

So why do not selective herbicides destroy the useful plants that we are helping to grow? The main cause of this is because useful plants have greater ability to metabolize or eliminate the herbicide much faster compared to bad herbs, so they can survive. In other cases, such as 2, 4-DB, some useful plants do not possess the ability to break it down into the active form of 2,4-D so they survive while weeds having the ability to break down to 2, 4-D do not.

 Resistance to herbicides

The growth of herbicide-resistant weeds has generated a major problem that has had a worrying impact on sustainable control for over 20 years.

Resistance to herbicides is usually created when a plant that was previously controlled by an herbicide, survives the application of a quite lethal dose, and also the change that produced the resistance is heritable, so it can be transmitted to the next generations of weed by means of the seeds.

Unfavorably, when a weed creates resistance to an herbicide, it also creates resistance to other herbicides that are in the same action group. Sometimes, it can even generate resistance against all the herbicides in the same group.

Likewise, there is a possibility that a weed will cause resistance to several herbicides from more than one Mode of Action group. This is known as ‘multi-herbicide resistance’ and is really very tricky to handle.

A key technique to treat the resistance of weed is to keep a record of the use of the chemicals, in order to monitor the modes of action applied to the crop. It is necessary to create the specified records within 48 hours of using an agricultural chemical, and keep them for at least two years. This is implemented for each of the agricultural chemicals applied, including poisoned baits dedicated to the control of pest animals.

A sample of herbicide resistance is the annual Ryegrass (Lolium rigidum). This weed often has resistance to some or all of the herbicides belonging to group A. This implies that a thorough selection of herbicides from different groups of mode of action is necessary in order to achieve an effective chemical control of this herb.