🎯 Selectivity and Mode of Action of Herbicides

This lesson explains how herbicides act inside plants and why crop safety depends on more than just the chemical name.

---

What Is Mode of Action?

The mode of action of a herbicide is the sequence of biochemical and physiological events that begins after herbicide absorption and ends in weed injury or death.

Knowing the mode of action helps in:

• choosing the right herbicide
• interpreting symptoms
• avoiding repeated use of the same group
• managing herbicide resistance

---

What Is Herbicide Selectivity?

Selectivity is the ability of a herbicide to control weeds without causing serious damage to the crop when used correctly.

This selectivity may arise from differences in:

• plant morphology
• absorption
• translocation
• metabolism or detoxification
• growth stage
• placement of the herbicide

---

Main Mechanisms of Selectivity

1. Morphological selectivity

Plant structure influences how much spray is retained and absorbed.

Examples:

• narrow upright leaves may retain less spray
• thick waxy cuticle may reduce absorption
• protected growing points may escape contact injury

2. Physiological or biochemical selectivity

Some crops detoxify the herbicide faster than susceptible weeds.

This is one reason why a herbicide may be safe in one crop but harmful in another.

3. Positional selectivity

The herbicide may be placed where weeds absorb it but the crop escapes, such as certain pre-emergence or directed applications.

4. Temporal selectivity

Crop and weed differ in emergence time or growth stage, allowing herbicide application when weeds are vulnerable and the crop is less sensitive.

---

Major Herbicide Modes of Action

Different herbicides act at different target sites. For exam understanding, it is useful to recognize the major groups rather than memorize long lists blindly.

Growth-regulator herbicides

Examples:

• 2,4-D
• dicamba

These act like synthetic auxins and cause abnormal growth, twisting, epinasty, and death.

Photosynthesis inhibitors

Examples:

• atrazine
• metribuzin
• diuron

These interfere with electron transport in photosynthesis and commonly cause chlorosis followed by necrosis.

Amino-acid synthesis inhibitors

Examples:

• glyphosate
• metsulfuron
• imazethapyr

These stop synthesis of essential amino acids and gradually shut down growth.

Lipid or fatty-acid synthesis inhibitors

Examples:

• clodinafop
• fenoxaprop

These are especially important in grassy-weed control.

Cell-division inhibitors

Examples:

• pendimethalin
• trifluralin

These prevent proper root and shoot development in germinating seedlings.

Cell-membrane disruptors

Examples:

• paraquat
• oxyfluorfen

These cause rapid burn symptoms in plant tissue.

---

Symptom-Based Understanding

Mode-of-action knowledge is often supported by visible symptoms:

• twisting and epinasty: synthetic auxins
• yellowing then necrosis: photosynthesis inhibitors
• slow cessation of growth: amino-acid inhibitors
• poor seedling emergence and stubby roots: cell-division inhibitors
• rapid scorching: contact membrane-disrupting herbicides

---

Why Mode of Action Matters in Resistance Management

When the same herbicide or same mode of action is used repeatedly, the field experiences repeated selection pressure. Over time, resistant biotypes may survive and increase.

Therefore, resistance management requires:

• rotating herbicide groups
• mixing methods, not relying on chemistry alone
• controlling survivors before seed set

---

Sublethal Dose and Mode of Action

Sublethal or underdosed application may not kill the weed fully. This can:

• allow regrowth
• create apparent field failure
• strengthen resistance selection pressure

---

Management Implication

Selectivity and mode of action must be understood together. A herbicide recommendation is sound only when:

• the crop can tolerate it
• the target weed is susceptible
• the mode of action fits the objective
• resistance risk is considered

Summary Cheat Sheet

• Mode of action explains how a herbicide injures or kills a plant after absorption.
• Selectivity means the crop survives while the weed is controlled under proper use.
• Selectivity may result from differences in morphology, placement, absorption, translocation, and detoxification.
• Major herbicide groups act through growth regulation, photosynthesis inhibition, amino-acid inhibition, lipid inhibition, cell-division inhibition, or membrane disruption.
• Repeated use of one mode of action increases resistance risk, so rotation and integration are essential.

References

1 source • [1]

[1]

AGRO304 lesson source notes