10.2 Herbicide Classification, HRAC Groups & Sites of Action

10.2 Herbicide Classification, HRAC Groups & Sites of Action

Introduction to Herbicide Classification

Herbicides are the cornerstone of modern weed management in conventional agricultural systems. However, the reliance on chemical control necessitates a rigorous understanding of how these molecules function at the biochemical level. For a Certified Crop Adviser (CCA), mastering herbicide classification is critical for designing effective weed control programs, avoiding crop injury, and, most importantly, mitigating the evolution of herbicide-resistant weeds. The Herbicide Resistance Action Committee (HRAC) and the Weed Science Society of America (WSSA) classify herbicides based on their specific Site of Action (SoA). The Site of Action is the exact biochemical pathway, enzyme, or protein within the plant that the herbicide binds to and disrupts. This is closely related to, but distinct from, the Mode of Action (MoA), which refers to the overall sequence of events—from absorption to plant death—that ensues following the initial disruption.

Key HRAC/WSSA Groups

Understanding the major herbicide groups is essential for rotating chemistries and managing resistance. The following represent the most agronomically significant groups utilized globally.

Group 1: ACCase Inhibitors

Site of Action: Inhibition of Acetyl-CoA Carboxylase (ACCase). ACCase is a crucial enzyme in the first committed step of lipid (fatty acid) biosynthesis. By inhibiting this enzyme, Group 1 herbicides prevent the production of cell membranes in newly dividing cells, primarily in the meristematic regions.

• Selectivity: ACCase inhibitors are highly selective for grasses. Broadleaf plants (dicots) possess a different, insensitive form of the ACCase enzyme, making them naturally tolerant. Thus, Group 1 herbicides are widely used as post-emergence grass killers in broadleaf crops like soybeans, cotton, and canola.
• Symptomology: Symptoms appear slowly, taking 1-3 weeks. The youngest leaves at the growing point cease growth, turn yellow (chlorosis), and eventually brown and necrotic. The growing point can easily be pulled out and will appear mushy and brown at the base.
• Chemical Families: Aryloxyphenoxypropionates (FOPs), Cyclohexanediones (DIMs), and Phenylpyrazolins (DENs). Examples include clethodim, sethoxydim, and fluazifop.

Group 2: ALS Inhibitors

Site of Action: Inhibition of Acetolactate Synthase (ALS), also known as Acetohydroxyacid Synthase (AHAS). The ALS enzyme is responsible for synthesizing three essential branched-chain amino acids: valine, leucine, and isoleucine. Without these amino acids, protein synthesis halts, and the plant rapidly starves.

• Selectivity: ALS inhibitors can be selective for grasses, broadleaves, or both, depending on the specific active ingredient and crop metabolism. They are used in a vast array of crops, including corn, soybeans, wheat, and rice.
• Symptomology: Growth stops almost immediately, though visual symptoms may take days or weeks to manifest. Symptoms include severe stunting, yellowing of meristematic tissue (the "yellow flash"), shortened internodes, and distinctly purplish or reddish veins on the underside of leaves due to anthocyanin accumulation.
• Chemical Families: Sulfonylureas (SUs), Imidazolinones (IMIs), Triazolopyrimidines. Examples include imazethapyr, nicosulfuron, and chlorimuron.
• Resistance Issue: Group 2 herbicides are notoriously prone to resistance. A single target-site mutation can render a weed completely immune.

Group 4: Synthetic Auxins

Site of Action: Mimic the natural plant hormone indole-3-acetic acid (IAA). Synthetic auxins overload the plant's hormonal balance, causing unregulated, rapid, and disorganized cell division and elongation. The plant essentially grows itself to death, leading to vascular tissue collapse and blocked transport of nutrients and water.

• Selectivity: Group 4 herbicides are primarily selective for broadleaf weeds, leaving grasses unharmed. They are extensively used in grass crops (corn, wheat, sorghum, pastures) for broadleaf control.
• Symptomology: Epinasty is the hallmark symptom—twisting, curling, and cupping of stems and leaves. Stems may become swollen, cracked, and brittle. Symptoms appear rapidly, often within hours of application.
• Chemical Families: Phenoxys (2,4-D), Benzoic acids (dicamba), Pyridines (clopyralid, fluroxypyr). Examples include 2,4-D, dicamba, and MCPA.

Group 9: EPSPS Inhibitors

Site of Action: Inhibition of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). The EPSPS enzyme is a critical component of the shikimate pathway, responsible for producing the aromatic amino acids tryptophan, tyrosine, and phenylalanine. Without these amino acids, protein synthesis is impossible.

• Selectivity: Glyphosate is the only significant herbicide in this group. It is naturally a broad-spectrum, non-selective herbicide, killing both grasses and broadleaves. Its selective use in agriculture relies entirely on the genetic engineering of crop plants to express an insensitive form of the EPSPS enzyme (e.g., Roundup Ready crops).
• Symptomology: Glyphosate translocates extensively to the growing points and roots. Symptoms take 7-14 days to appear, progressing from yellowing of new growth to widespread necrosis and plant death.

Group 14: PPO Inhibitors

Site of Action: Inhibition of Protoporphyrinogen Oxidase (PPO). The PPO enzyme is involved in the synthesis of chlorophyll. When inhibited, a highly reactive intermediate molecule (protoporphyrin IX) accumulates in the cells. In the presence of sunlight and oxygen, this molecule reacts to form destructive lipid radicals that literally tear apart cell membranes.

• Selectivity: Used primarily for broadleaf weed control in soybeans and cotton, though some have activity on grasses. They act primarily as contact herbicides (causing rapid "burn-down") but some offer residual soil activity.
• Symptomology: Very rapid onset, requiring sunlight for activation. Tissues exhibit rapid "water-soaking" appearance, followed by bronzing, speckling, and necrosis (browning) within 1 to 2 days of application.
• Chemical Families: Diphenylethers, N-phenylphthalimides, Triazolinones. Examples include fomesafen, flumioxazin, and sulfentrazone.

Group 15: VLCFA Inhibitors

Site of Action: Inhibition of Very Long Chain Fatty Acid (VLCFA) synthesis. The exact target enzyme remains definitively elusive, but these herbicides disrupt the synthesis of long-chain fatty acids critical for the formation of cell membranes and the waxy cuticle in emerging plant shoots.

• Selectivity: Used primarily as pre-emergence, soil-applied herbicides to control annual grasses and small-seeded broadleaf weeds in corn, soybeans, and cotton.
• Symptomology: Because they are root or shoot absorbed as the seedling germinates, weeds often fail to emerge. If they do emerge, grasses exhibit "buggy-whipping" (leaves fail to unfurl from the whorl), and broadleaves show crinkled, "drawstring" leaves. They do not control emerged weeds.
• Chemical Families: Chloroacetamides. Examples include S-metolachlor, acetochlor, and pyroxasulfone.

Importance of Rotating Sites of Action

Relying continuously on a single Site of Action imposes immense selection pressure on weed populations. In any field, there are naturally occurring biotypes with random genetic mutations that confer resistance to a specific herbicide. Continuous use eliminates the susceptible population, allowing the rare resistant individuals to survive, multiply, and rapidly dominate the seed bank. A core competency of a CCA is designing programs that utilize multiple, effective Sites of Action (tank mixtures and sequential applications) to delay or prevent the onset of herbicide resistance.