What is Herbicide selectivity? 13 factors of selectivity

Herbicide selectivity referred to the phenomenon where the chemicals kill the target plant species in a mixed plant population without harming or only slightly affecting the other plants. The differential response of the plant to herbicide is called selectivity of herbicide.

In other words, herbicides harm or kill weeds whereas crop plants are not affected due to selectivity. The susceptibility or tolerance of different plants is called selectivity. Many plant injuries occur wheat and barley tolerate growth regulator herbicide such as 2, 4 – D, and MCPA.  

Factors affecting herbicide selectivity

1.      Chemical and physical properties of the herbicide
2.      Formulation
3.      Time of application
4.      Method of herbicide application
5.      Rate of application
6.      Plant morphology
7.      Stages of development
8.      Cultivation practices.
9.      Absorption
10.  Translocation.
11.  Physiological difference of plant species.
12.  Metabolism
13.  Environmental factor

1. Chemical and physical properties of the herbicide

Herbicides of one chemical family differ from another chemical family. The herbicide is characterized by different physical and chemical properties. This includes water-solubility, absorbability forces, vapor pressure, etc. which determine the phytotoxicity of a chemical.  

2. Formulation

Formulation affects the solubility, specific gravity, and even phytotoxicity of a herbicide. A wide variety of herbicide formulations is design to suit a particular method of application 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, the 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 for soil or foliage depends on weed – crop situation, type of herbicide, herbicide efficacy, and cost of application.  

5. Rate of application

A herbicide that is selective at a lower rate may become non – effective when applied at a higher rate. For every herbicide, there is an optimum rate at which it maintains 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 determining 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, waxiness of leaf, and thickness of cuticle. A thick cuticle prevents herbicide absorption and a thin cuticle which permits greater herbicide absorption.  

7. Stages of plant development: Stages of plant development affect the ability of herbicides to enter the plant. Plant in the germination or young seedling stage is 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 brings out the dormant weed seed roots and stolon to the topsoil layer and exposes them to the sunlight resulting in germination. Thereby increasing their susceptibility to an herbicide applied to the soil or foliage.  

9. Absorption:  Once a herbicide is applied its entry into the plant is facilitated by absorption through the root, shoot, and stem. Absorption is influenced by the morphological characteristics of plants. The rate of absorption and amount of chemical which entered the plant varies with plant species and stages of plant growth. This differential absorption from one plant species to another determines herbicide selectivity.  

10. Translocation:  When a herbicide enters the plant it must be translocated to the site of action for it disrupts the metabolic activities of the plant. The translocate herbicide moves from the site of entry to the site of action via the phloem and xylem, but contact herbicide moves very little or not of all from the point of entry. Many physiological and environmental factors of plants cause differential movement of herbicide to the site of action. This germinates the selectivity of herbicides.  

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 depends upon the genetic makeup of the plant taxonomy, morphotoxicant and phosphological characters.  

12. Metabolism:  When any herbicide enters the plant its metabolism depends on the physical and chemical properties of the plant. The capacity of the plant to affect the 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:  The 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.

Leave a Comment