Ioxynil: A Contact Herbicide for Broadleaf Weed Control

Definition:
Ioxynil is a selective, contact herbicide used primarily for post-emergence control of broadleaf weeds in cereal crops, pastures, and turf. It belongs to the nitrile class of herbicides and is known for its rapid weed knockdown effect.

Chemical Characteristics of Ioxynil:

• Chemical Name: 4-Hydroxy-3,5-diiodobenzonitrile

• Molecular Formula: C₇H₃I₂NO

• Molecular Weight: 370.9 g/mol

• Appearance: Off-white to pale yellow crystalline solid.

• Solubility: Slightly soluble in water; soluble in organic solvents.

• Stability: Stable under normal conditions; degrades under strong sunlight and microbial activity in soil.

Mode of Action:

Ioxynil acts as a contact herbicide by inhibiting photosystem II (PSII) in the photosynthetic pathway. This blockage disrupts electron transport in chloroplasts, leading to rapid chlorosis, necrosis, and plant death.

Uses and Applications:

1. Target Weeds:

   • Controls small-seeded broadleaf weeds such as chickweed, shepherd’s purse, charlock, field pennycress, and wild mustard.

   • Not effective on grasses or perennial broadleaf weeds.

2. Crops:

   • Commonly used in cereals (wheat, barley, oats), pastures, turfgrass, and non-crop areas.

3. Application Timing:

   • Post-emergence, targeting young, actively growing weeds at the 2–4 leaf stage for best results.

Benefits of Ioxynil:

• Fast-Acting: Rapid visible effects within a few days after application.

• Crop Safety: Selective action when used at recommended rates in cereals and grasses.

• Low Residual Activity: Minimal carryover risk to rotational crops.

• Useful in Cold Conditions: Effective even at low temperatures where systemic herbicides perform poorly.

Limitations:

• No Residual Control: Does not prevent new weed germination.

• Contact Only: Requires good spray coverage for effectiveness.

• Resistance Risk: Continuous use may contribute to PSII-inhibitor resistance.

Recommended Application:

• Dosage: Typically 0.25–0.5 kg active ingredient per hectare depending on weed density.

• Method: Foliar spray with adequate coverage.

• Precautions:

  • Avoid drift to broadleaf crops (e.g., vegetables, oilseed rape).

  • Apply during dry conditions and when weeds are young and actively growing.

  • Use protective clothing during handling and spraying.

Storage and Handling:

• Storage: Keep in tightly closed containers, in a cool, dry, and ventilated place.

• Shelf Life: 2–3 years if stored properly.

• Safety: Toxic to aquatic life; prevent contamination of water bodies.

FAQ on Ioxynil:

Q1: Does Ioxynil control grass weeds?
A1: No, it is effective only against broadleaf weeds and has no activity on grasses.

Q2: How quickly does Ioxynil work?
A2: Visible effects such as chlorosis and necrosis are typically seen within 2–5 days.

Q3: Can Ioxynil be mixed with other herbicides?
A3: Yes, it is often tank-mixed with grass herbicides or other broadleaf herbicides for broader weed control.

Q4: Is there a risk of carryover damage to rotational crops?
A4: No, it has low soil persistence and poses minimal carryover risk.