1. Chemical and physical properties of herbicide2. Formulation3. Time of application4. Method of herbicide application5. Rate of application6. Plant morphology7. Stages of development8. Cultivation practices.9. Absorption10. Translocation.11. Physiological difference of plant species.12. Metabolism13. Environmental factor
Chemical and physical properties of herbicide:
Herbicide of one chemical family differ from another chemical family. The herbicide are characterized by different physical and chemical properties. This include water solubility, absorbility forces, vapour pressure etc. which determine the phytotoxicity of a chemical.
Formulation affect the solubility, specific gravity and even phytotoxicity of a herbicide. A wide variety of herbicide formulation is design to suit a particular method of applications and to achieve increase selectivity and efficacy.
3. Time of application:
Time of application is determined by weed species, time of germination of weed and crop plants and growth stage of weed. Generally time of application falls into three categories – pre planting, pre – emergence and post emergence I relation to crop and weed.
4. Placement of herbicide:
Herbicides are soil applied and foliage applied. Soil applied herbicides are applied at pre – planting and pre – emergence while foliar applied herbicides are applied at post – emergence. The choice of selecting herbicide to soil or foliage are depend on weed – crop situation, type of herbicide, herbicide efficacy and cost of application.
5. Rate of application:
A herbicide which is selective at a lower rate may become non – effective when applied at higher rate. For every herbicide, there is an optimum rate at which it maintain its selective characteristics and this rate varies from one weed or crop species to another.
6. Plant morphology:
Plant morphology has a prominent role in determine herbicide selectivity. The amount of spray retained by the foliage influence the amount of chemical available for entry into the plant. The various morphological characters of plant that affect herbicide entry. The morphological characters are leaf size and shaped leaf arrangement, waxness of leaf and thickness of cuticle. Thick cuticle prevent herbicide absorption and thin cuticle which permits greater herbicide absorption.
7. Stages of plant development:
Stages of plant development affects the ability of herbicides to entry the plant. Plant in germination or young seedling stage are more susceptible to soil applied herbicides than the later stage. Similarly perennial weeds are more sensitive to foliage applied herbicide during the active growth period than before or later stage.
8. Cultivation practices:
Various pre – planting and post – planting cultivation practices followed before herbicide application could make tolerate plant sp. Susceptible to a particular herbicide after application. Tillage practice bring out the dormant weed seed roots and stolon to the top soil layer and expose them to the sunlight resulting in germination. Thereby increasing their susceptible to herbicide applied to the soil or foliage.
Once a herbicide is applied its entry into the plant is facilitated by absorption through root, shoot and stem. Absorption is influenced by morphological characteristics of plant. The rate of absorption and amount of chemical which entered the plant vary with plant species and stages of plant growth. This differential absorption from one plant species to another which determines herbicide selectivity.
When a herbicide enter the plant it must be translocate to the site of action for it disrupt the metabolic activities of the plant. The translocate herbicide move from the site of entry to the site of action via the phloem and xylem, but contact herbicide move very little or not of all form the point of entry. Many physiological and environmental factor of plant cause differential movement of herbicide to site of action. This germinate the selectivity of herbicide.
11. Physiological difference of plant species:
After reaching the site of action a herbicide affects one or many metabolic activities related to plant growth and survivability. The physiological tolerant of a plant species variety largely depend upon the genetic makeup to the plant taxonomy, morphotoxicant and phosphological characters.
When any herbicide enters the plant its metabolism depend on the physical and chemical properties of plant. The capacity of the plant to affect metabolic conversion of the applied chemical molecules. The herbicide metabolism and its differences among plant species from the basis of herbicide selectivity.
13. Environmental factor:
Selectivity of a herbicide is also influenced by the environmental factor under which the plant grows. These environmental factors are light, temperature, rainfall, humidity, wind etc.