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Competency Area 2: Weed Management

PO 20. Chemical – Know time(s) of application for different types of herbicides and how soil (texture, organic matter, pH) and weather (rainfall/soil moisture, temperature, etc.) affect herbicide performance.

Preplant- applied before crop is planted 
- Early preplant - surface applied before crop is planted
- Preplant incorporated
            - Some herbicides require mechanical mixing with soil because they are volatile or subject to
           photodecomposition.
            - Used to place herbicide near germinating propagules
            - Improve dependability of control
Preemergence - applied to soil surface prior to emergence of crop and weeds; dependant on rainfall or irrigation for incorporation and activation.
Factors affecting performance:
            - Amount of rainfall or irrigation
            - Herbicide solubility
            - Amount of herbicide adsorbed on soil affects application rates
            - Herbicide chemistry
            - Soil texture
            - Organic matter
            - Soil pH
Postemergence- applied after emergence of crop and weeds.
Factors affecting performance:

  • Time of application - should be applied when weeds are young and/or actively growing.

            - Summer annuals – apply to young plants in spring or early summer
            - Winter annuals – apply to rosettes in fall or early spring
            - Biennials – apply to fall growth or rosettes in fall or spring
            - Perennials – apply to bud stage in spring or summer or vigorous fall growth

  • Application factors

            - Spray volume
            - Pressure
            - Spray adjuvants - additives that aid or modify the action of the active ingredient(s)

  • Environmental factors

            - Light
            - Temperature
            - Relative humidity
            - Wind
            - Rain