Herbicide Classification Flashcards

Herbicide Classification

• Classification categorizes herbicides to understand their uses and the weeds they control.
• Herbicides are classified by their application method, control targets, and chemical characteristics.

Application Timing

• Preplant: Applied to soil before seeding or transplanting.
• Preemergence: Herbicide is applied prior to the emergence of crops or weeds, generally after planting.
• Postemergence: Applied after crops or specific weeds appear; can be categorized into early, late, or directed treatments.

Selectivity in Herbicides

• Selectivity: The ability of a herbicide to kill specific plants while leaving others unharmed.
• Non-selective: Culls all plant types at recommended rates, typically used for total vegetation control.

Methods of Selectivity

• Physical Methods: Utilizes carbon seeding; herbicide application protects germinating crop seeds from herbicide while unprotected weeds germinate.

Types of Herbicides

• Contact Herbicide: Target localized areas and do not move through the plant; effective on annual plants; requires good coverage (e.g., Gramoxone).
• Systemic Herbicides: Move within the plant's transport system; effective against perennial weeds and can be applied to foliage or soil (e.g., Roundup).

Movement Characteristics

• Symplast Movement: Herbicides that enter the symplast move both upward (acropetally) and downward (basipetally).
• Apoplast Movement: Herbicides that enter the apoplast primarily move upward (acropetally).

Persistence and Degradation

• Persistence: Measured by half-life, indicates the longevity of herbicide efficacy in the soil; moderate persistence typically has a half-life of around 45 days.
• Half-Life: Time taken for 50% of the herbicide to degrade, a crucial factor in understanding herbicide behavior.

Chemical Families

• Chemical Family: Group of herbicides with similar core structures that often act in similar biochemical ways.
• Family Variance: Selectivity and persistence can significantly vary within a chemical family due to small structural changes.

Mode of Action

• Describes the sequence of biological events leading to plant death after herbicide application, explaining the herbicide's effects on susceptible plants.
• Eight General Modes of Action:
  • Inhibition of lipid synthesis
  • Inhibition of amino acid synthesis
  • Growth inhibitors
  • Inhibitors of photosynthesis
  • Pigment inhibitors
  • Cell membrane disrupters
  • Seedling growth inhibitors
  • Undefined or unknown mechanisms

Site of Action

• Site-of-Action or Group #: The most precise classification method for herbicides based on the specific biochemical pathway targeted in a plant; there are 20 distinct sites of action.