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
- Target Site Resistance; Genetic mutation changes herbicide binding site (enzyme/protein). Herbicide cannot bind → pathway continues normally. Example: ALS-inhibitor resistance in Amaranthus weeds.
- 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.
- 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.
