Agriculture notes prepared by IAS Topper for all agriculture based competitive exams-Agronomy-Herbicides

Herbicides

 Herbicides

Advantages:

• They control weeds even before emergence i.e. crop weed competition is nil
• Complete and long lasting control of weeds, which is not possible by other methods.
• It will reach where hand hoes can not reach
• Cheap method and easier to adopt

Disadvantages:

• Require technical know how about when and how to spray, precautionary measures to be adopted etc. Success of herbicides largely depends upon soil type, crop and other associated factors. Farmer has to understand these for a better result.
• Over dose may kill the crop
• Problem of soil residues and pollution
• Drifting of herbicides to nearby fields

 

Selectivity of Herbicides

Selectivity of herbicide means, differential response of plants to same herbicide. It is through differential absorption, translocation, deactivation, carbon metabolism and resistance to protoplasm. These many be furnished by morphological or physiological nature of plants.

Differential Absorption of Herbicides

1. Foliar Active herbicides: This is due to differential retention of herbicides on vegetation, which is depended upon orientation of leaf, waxiness, pubescence, corrugation, ridges, depression etc. Dicotyledanery weed leaves are flat and so retain more herbicides than monocots whose leaves are erect.
2. Soil Active Herbicides: Weeds germinate from top 3cm of soil whereas crop seeds germinate from the depth of 5cm or below because of larger seed size. Soil applied herbicides spread up to 3cm of soil. Therefore, weeds are killed. This is sometimes referred as depth protection.
3. Induced selectivity: Absorbents like activated charcoal are used along with the crop seeds, which absorb the herbicides present in the soil or coated along with crop seeds. This is how the selectivity is induced to have advantages for crops.

Differential translocation: Both weeds and crops may equally absorb herbicides, but they differ in the rate and level of translocation. Ex. Sugarcane is a poor translocater than beans.

Protoplasmic tolerance: Some plants can tolerate the deficiency or stress created by herbicides, some may not. Ex. Deficiency of pantothenic acid by Dalapon can be tolerated by wheat but not by weeds. Similarly trifluralin to mustard, ground nut and cotton.

Deactivation:

1. Metabolism: By chemical breakdown of herbicides into non toxic. Ex. Atrazin in Maize
2. Reverse Metabolism: Some herbicides are not as such toxic. Ex. 2,4 D and MCPB converted into toxic by beta-oxidation by specific enzymes which are not present in legumes but available in non-leguminous weeds.
3. Conjugation: The herbicides are conjugated by the plant compounds and thereby not available to act on the target site of the plant. Ex. Glucose of plant conjugates with chloraben to form N-glucosyl chloraben.

Herbicides: Mode of Action

Herbicides interfere into the physiological process of the plants in different way and affect the normal functioning of weeds.

Photosynthesis

Hill reaction: Photolysis of water is mediated by many enzymes and many herbicides inhibit these enzymes, thereby splitting of water in presence of water does not occur. Ex. Ureas and carbomates

Electron Transfer: Transfer of electron fro water to NAD in photo system II is affected. The herbicide either inactive or remove the carriers. Ex. Uracils, Ureas

Damage to photosystem: Formation of H2O2 produces free radicals and damages the photosystem. Ex. Bipyridilium accepts electron and in presence of nascent oxygen, produce hydrogen peroxide

Carbon dioxide fixation: The enzymes involved in carbon dioxide fixation are inhibited or denatured. Ex. Oxadiazon

Respiration

Oxidation of food leads to release of energy, which are trapped by ADP to form ATP (termed as oxidative phosphorylation). Herbicides like Dinitroanilines, Dinitrophenols anilides etc. affect conversion of ADP to ATP

Biosynthesis:

Inhibition of Protein synthesis:  Ex. Butachlor, alachlor

Inhibition of Lipid synthesis: Ex. Thiocarbomates

Vitamin synthesis: Ex. Dalapon (Pantothenic acid)

Cell division: Herbicides affect DNA synthesis, spindle fiber formation and precursor of proteins Inhibition of RNA synthesis: Ex. Trifluralin:

Germination: During germination, hydrolytic enzymes are produced like amylase, protease, lipases, phosphatase, esterases which breakdown complex molecules into simple sugars, amino acids etc. These enzymes are regulated by gibberelic acid.

Ex. Alachlor and Propachlor inhibit GA production

Abnormal tissue development

2,4 D, 2,4 DB, MCPA and MCPB are suspected to have 2 types of mode of action

• Interference with nucleic acid metabolism resulting in uncontrolled growth of cells leading to collapse of symplastic and apoplastic systems in plants.
• Rapid proliferation of roots and stems consumes most of photosynthates, nutrients thereby developing roots and leaves starve for food.