📈 Herbicide Selectivity and Mode of Action

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

---

Herbicide Selectivity and Mode of Action

The mode of action (MoA) describes how a herbicide disrupts a specific physiological process in the target weed. Understanding MoA is essential for selecting herbicides, managing resistance, and rotating chemical groups.

ALS/AHAS Inhibitors (Group 2)

Acetolactate synthase inhibitors block the enzyme responsible for synthesis of branched-chain amino acids (valine, leucine, isoleucine):

• Chemical families: sulfonylureas, imidazolinones, triazolopyrimidines
• Examples: metsulfuron-methyl, chlorimuron-ethyl, imazethapyr
• Effective at very low doses (2-10 g/ha for sulfonylureas)
• Selectivity: crops metabolise these herbicides via cytochrome P450 enzymes
• Most resistance-prone group globally

ACCase Inhibitors (Group 1)

Acetyl-CoA carboxylase inhibitors block fatty acid synthesis in grasses:

• Chemical families: aryloxyphenoxypropionates (FOPs), cyclohexanediones (DIMs)
• Examples: fenoxaprop, clodinafop, sethoxydim
• Grass-specific — safe on all broadleaf crops (soybean, cotton, pulses)
• Used to control Phalaris minor and Avena fatua in wheat

Photosystem II (PSII) Inhibitors (Group 5/6/7)

These herbicides block electron transport in Photosystem II of chloroplasts:

Chemical Family	Examples	Crops
Triazines	Atrazine, simazine	Maize, sugarcane
Ureas	Isoproturon, diuron	Wheat, cotton
Nitriles	Bromoxynil	Wheat, barley

• Symptoms: chlorosis followed by necrosis starting at leaf margins
• Selectivity based on root uptake depth and metabolic detoxification in crops

EPSPS Inhibitors (Group 9)

5-enolpyruvylshikimate-3-phosphate synthase inhibitors block the shikimate pathway, preventing synthesis of aromatic amino acids:

• Glyphosate is the sole commercial EPSPS inhibitor
• Non-selective, systemic — kills virtually all plant species
• Widely used for pre-plant burndown, zero-tillage systems, orchard weed control
• Glyphosate-resistant (Roundup Ready) crops were developed via transgenic introduction of a resistant EPSPS gene

Synthetic Auxins (Group 4)

Auxin mimics cause uncontrolled cell elongation in broadleaf weeds:

• Examples: 2,4-D, dicamba, picloram, fluroxypyr
• Symptoms: stem twisting (epinasty), leaf curling, swollen nodes
• Selective in grasses — monocots rapidly deactivate synthetic auxins
• 2,4-D is among the oldest and most widely used herbicides in India

Other Important Groups

• PPO inhibitors (Group 14) — oxyfluorfen, fomesafen; cause rapid membrane disruption
• Microtubule inhibitors (Group 3) — pendimethalin, trifluralin; prevent cell division in root tips
• Glutamine synthetase inhibitors (Group 10) — glufosinate; causes ammonia accumulation

Rotating herbicides across different MoA groups is the cornerstone of resistance management.

---

Summary Cheat Sheet

Topic	Key takeaway
Main focus	Herbicide Selectivity and Mode of Action — ALS inhibitors, ACCase inhibitors, PSII inhibitors, EPSPS inhibitors, and auxin mimics.
Section context	Revise this lesson with the rest of Weed Management for stronger conceptual continuity.