Unit 20: Herbicides Study Notes

Herbicides

Definition of Herbicides

  • A herbicide is a chemical specifically designed to kill weeds.
  • Herbicides are commonly employed to eliminate weeds in various environments, including:
    • Crop fields
    • Lawns
    • Along roadways
    • Other locations where weeds are present
  • Weeds are categorized as plants growing out of their intended place and exhibit a variety of forms depending on the region.
  • Alternative methods, such as mulching and crop cultivation, may sometimes be more effective and cost-efficient in controlling weed growth.

Types of Herbicides

  • Nonselective Herbicides
    • Definition: Kill all plants upon application.
    • Usage: Applied in areas where no vegetation is desired.
  • Selective Herbicides
    • Definition: Specifically kill certain plants while leaving others unharmed.
    • Functionality: Designed to eliminate weeds while preserving crop plants.

Effects of Herbicides

DIMENSION Herbicide
  • Type: Herbicide
  • Application: Must be applied before the weeds germinate.
  • Plant Behavior: Prevents photosynthesis, leading to the plant's starvation and eventual death.
2,4-D Herbicide
  • Type: Systemic herbicide
  • Application: Sprayed directly onto the foliage of the plant.
  • Plant Behavior: Causes abnormal growth affecting:
    • Respiration
    • Food reserves
    • Cell division
TREFLAN Herbicide
  • Type: Selective preemergence herbicide
  • Application: Generally diluted with water and applied as a spray.
  • Plant Behavior:
    • Not absorbed by leaves
    • Does not translocate through the plant
    • Kills sprouting seeds.

Summary of Herbicides

  • The discussed herbicides (DIMENSION, 2,4-D, TREFLAN) are primarily utilized by professionals rather than the average gardener.
  • It is essential to note that not all herbicides fit within these three classifications, but they represent a significant portion of commonly used herbicides.

A New Approach to Weed Killing

Overview
  • Innovative methods for killing weeds are emerging, including the use of:
    • A natural protein derived from soil fungi.
  • Mechanism: This protein causes weed cells to self-destruct within 24 hours by triggering their defensive response at an exaggerated rate.
  • Potential Application: This may represent the first viable herbicide for organic farmers seeking environmentally friendly options.
Biological Weed Control
  • Description: A nonchemical approach to weed management, commonly known as biocontrol.
  • Mechanism: Utilizes natural predators to control weed populations.
  • Advantages:
    • Cost-effective
    • Readily available for implementation.

Application of Herbicides

Overview
  • Herbicides can be:
    • Sprayed directly or mixed with fertilizers or granulated form.
  • Equipment: The same equipment used for insecticides can be employed for herbicide application.
  • Important Note: Sprayers should be thoroughly cleaned after using them for insecticides or fungicides to prevent cross-contamination.
Time of Application
  1. Preplanting: Herbicides are mixed into or sprayed onto the soil or seedbed.
  2. Preemergence: They prevent seeds from germinating or can eliminate small, tender weeds.
  3. Postemergence: Applications must be executed with great care.
Calibration of Sprayers
  • For Large Sprayers:
    • Measure and mark off an acre.
    • Spray the designated area with water and record how much water is required to refill the spray tank.
  • For Small Hand Sprayers:
    • Spray a 1,000-square-foot area and determine the volume of water needed for refill in pints.
Drift
  • Concern: Herbicides drifting onto nontarget plants can potentially damage or kill these plants.
  • Recommendations for Application:
    • Use spray nozzles that apply larger droplets rather than a fine mist to minimize drift.

Breakdown of Herbicides

Overview
  • Most herbicides are decomposed by soil microorganisms, eventually transforming into harmless substances.
  • Breakdown Duration: Varies from several weeks to several years, depending on various factors.
  • Testing Period: At least one growing season is typically necessary to verify breakdown effectiveness.
  • Important: Always consult herbicide labels for specific instructions regarding use and application.

Why Herbicides Sometimes Fail

Incorrect Selection
  • Unsatisfactory weed control may result from choosing a herbicide that does not effectively target the intended weed species.
  • Selecting the appropriate chemical is critical, which necessitates accurate weed identification.
Improper Application
  • Common reasons for failure include:
    • Poor application or improper timing
    • Uneven spreading of the herbicide
    • Inadequate rainfall or irrigation following application
    • Failure to mix the herbicide thoroughly into the soil post-application
    • Calibration errors with sprayers, either too high or too low per acre.
Environmental Variables
  • Factors that may affect herbicide effectiveness include:
    • Maturity of the weeds: Older, hardened plants typically require significantly more herbicidal chemical.
    • Rain conditions: Rain can benefit some preemergence chemicals but may adversely affect others, particularly with excessive rainfall.
    • Soil Conditions: Soil type and organic matter content can affect herbicide effectiveness, where soils high in humus may absorb and deactivate certain weed killers.

Summary

  • Herbicides, commonly referred to as weed killers, function through various mechanisms and can target multiple plants.
  • It is vital to select the correct chemical and ensure proper application techniques. A biocontrol herbicide has been identified as particularly efficient against broadleaf weeds.
  • Always adhere to labeling guidelines to ascertain appropriate herbicide use and application.