Herbicides – classification, selectivity and resistance, Allelopathy

AGRON – 111 : Fundamentals of Agronomy

Herbicides – Herbicides are the chemicals used for weed control and which suppress or destroy the growth of weeds.

Herbicides is a Latin word.

Histories of herbicides/chemical weed control
• The use of chemical weed control started with inorganic copper salts e.g CuSO4 for
broadleaf weed control in cereals in Europe in 1896.
• Other inorganic salts that were tested between 1900-1930 included nitrates and
borates.
• In 1912, sulphuric acid (H2SO4) was used for selective weed control in onions and
cereals. In 1932, the first organic herbicide, Dinitro-ortho Cresol (DNOC) was
introduced.
• In the 1950s triazine was introduced. In 1974, Glyphosate, frequently sold under brand
name Roundup for non-selective weed control was introduced.
• Agriculture witnessed tremendous changes through the production of organic
herbicides, which came at a time when field workers were reducing, high cost labour
and productive cost of production.

Thus, farmers in advance countries almost depended on herbicide because it met their production challenges in agriculture and relatively ignored other methods of weed control.

Herbicides – classification

Herbicides are classified according to their:-

1. Selectivity
2. Mode of action
3. Timing of application.
4. Chemical group
5. Mechanism of action

Herbicides classification on Selectivity

1. A non-selective herbicide kills or damages all plant life in a treated area (e.g.,
   Roundup).
2. A selective herbicide will kill weeds in a germinating or growing crop without harming
   the crop beyond the point of recovery (e.g., 2,4-D used to control broadleaved weeds in
   a grass pasture; Avadex to control wild oats in cereals.)

Herbicides classification on mode on action

Contact Herbicides

• Kill plant parts covered by the herbicide
• Directly toxic to living cells
• Little or no movement through the plant
• Effective against annual weeds
• Only “burn off” tops of perennial weeds, similar to mowing
• Can be selective or non-selective:
• Selective: Torch (bromoxynil) kills broadleaved weeds in cereals without harming the crop
• Non-selective: Gramoxone (paraquat) kills any green plant material

Systemic Herbicides

• Absorbed by roots or above-ground parts
• Move or are translocated within the plant
• Exhibit a chronic effect (full effects may show after a week or more)
• Overdose on leaves can prevent translocation
• Can be selective or non-selective:
• Selective: 2,4-D, MCPA, Banvel, Tordon
• Non-selective: Roundup

Soil Sterilants

• Prevent growth of plant life when present in the soil
• Effect lasts from a few months to several years
• Examples: bromacil, tebuthiuron, atrazine at high rates.

Herbicides classification on time of application

1. Pre-Plant Soil Incorporated – these herbicides are applied to the soil before the crop
   is sown. They are incorporated in the soil to prevent loss due to vapourization and
   breakdown by sunlight. Trifluralin and EPTC are examples.
2. Pre-Emergence – applied to soil prior to seeding or after the crop is sown but before
   crop and weeds emerge. In most cases, the weeds germinate in treated soil while the
   crop germinates below the herbicide zone.
3. Post-Emergence – sprayed directly on the weeds after they are up and growing.

Herbicides classification on Chemical group

1. Inorganic Ex: CuSO4 (dicots)
   H2SO4 (Cereals), Naclo3 (deep rooted perennials)
2. Organics Ex: Amides – Butachlor
   Dinitro anilines – Fluchloralin
   Thio carbamates – Benthiocarb
   Bi pyridyliums – Paraquat
   Triazines – Atrazine
   Phenoxy acetic acids – 2, 4-D
   Substituted ureas – Diuron, monuron, isoproturon.

Herbicide Selectivity and resistance

Herbicide Selectivity – It is a phenomena by which in a given mixed crop stand some species eg weeds are preferably controlled while others (eg crops) remains unaffected or marginally affected when a herbicide is applied to them.

If SI <2 it is safe for use.

Herbicide resistance is a phenomenon by which a plant/biotype is capable to survive and
reproduce seeds even after a exposure to a dose of herbicide normally lethal to the wild type.
Herbicide resistance is measured by Resistance Factor (GR50) values – based on the %
inhibition of growth / dry wt. of plant.

Allelopathy

Allelopathy – Refers to beneficial or harmful effects of one plant on another through biochemicals (allelochemicals).

Affects both crop and weed species.

Term Greek-derived “allelo” and “pathy” (meaning “mutual harm” or “suffering”)

First used in 1937 by Austrian scientist Hans Molisch in his book.