Herbicide Selectivity

Definition; Herbicide selectivity = the ability of a herbicide to kill weeds while leaving the crop relatively unharmed.

Role of Selective Formulations

• Designed for crop safety: Some formulations are fine-tuned to affect weeds more than crops.
• Slow release formulations: Reduce the risk of crop damage by providing prolonged action on weeds.
• Oil-based or water-based forms: Can change how quickly herbicides penetrate leaves.

Example: Atrazine is formulated to be selective in corn but lethal to broadleaf weeds.

Role of Adjuvants in Selectivity

• Definition: Substances added to herbicides to enhance performance.
• Functions:
  • Improve spray coverage on leaves.
  • Enhance absorption into weed tissue.
  • Increase translocation to growing points.
  • Modify droplet spread, sticking, or penetration.
• Outcome: Helps herbicide act more selectively on weeds, reducing accidental crop damage.

Example: Surfactants added to glyphosate improve weed control but must be chosen carefully to avoid harming sensitive crops.

Herbicide Resistance

Definition; Resistance = inherited ability of weeds to survive herbicide application that would normally kill them.

• Natural resistance → weeds are inherently tolerant (never affected).
• Acquired resistance → weeds evolve due to repeated herbicide use.

Mechanisms of Herbicide Resistance

1. Target Site Resistance; Genetic mutation changes herbicide binding site (enzyme/protein). Herbicide cannot bind → pathway continues normally. Example: ALS-inhibitor resistance in Amaranthus weeds.
2. Enhanced Metabolism; Weeds produce detoxifying enzymes (e.g., cytochrome P450s, GSTs). Herbicide is broken down before it reaches toxic levels. Example: Ryegrass metabolizing ACCase inhibitors.
3. Reduced Translocation; Herbicide absorbed but fails to move to growing points. Weeds survive because lethal dose never reaches the target tissues. Example: Glyphosate-resistant horseweed with reduced phloem translocation.

Link Between Selectivity and Resistance

• Selectivity is crop-focused: why crops survive but weeds die.
• Resistance is weed-focused: why weeds survive herbicides meant to kill them.
• Selectivity depends on formulation, crop metabolism, and physiology.
• Resistance arises from weed evolution under selection pressure (overuse of same herbicide).
• Challenge in Weed Management: Overreliance on selective herbicides can accelerate resistance evolution in weed populations.