Weed Control & PGR Lecture 13

PGR and Weed Control

• Weed control is the most widespread practical use of Plant Growth Regulators (PGRs).
• Many types of PGRs are used, with auxin types being the most important.
• Auxin types of weedicides/herbicides include phenoxy-type compounds, benzoic acids type, and heterogeneous compounds.

Importance of Herbicides

• Weeds are costly and compete with crops for water, nutrients, and light, leading to a reduction in yield and quality.
• Over 75% of total weed control is achieved by applying three groups of compounds.

2,4-D

• 2,4-D is the most widely used systemic herbicide for controlling broadleaf weeds.
• It is the third most widely used herbicide in North America and the most widely used herbicide globally.
• 2,4-D was the first commercially successful selective herbicide, introduced in 1946.
• It is sold in various formulations under different brand names and continues to be used due to its low cost.

Types of Herbicides

• Herbicides can be classified into several basic types, each targeting certain weeds:
  • Pre-emergent herbicides to stop germination of plant seeds
  • Post-emergent herbicides, which can be:
    • Non-selective herbicides that kill most plants
    • Selective herbicides that kill specific types of plants, usually grasses or broad-leaf weeds
• Roundup is a total vegetative, non-selective, post-emergent herbicide.

Surfactants and Selectivity

• Surfactants can be used to increase the efficiency of herbicides when spraying on plants.
• Auxin-type herbicides are used to control weeds without injuring crops, due to their unique toxic properties that can "select" and kill weeds.
• These herbicides can be applied to foliage as foliar spray or to soil as soil drench, and their movement generally follows the translocation of photosynthate in the phloem.

Chlorophenoxy Herbicides

• Major phenoxy herbicides include 2,4,5-T, MCPA, silvex, and 2,4-DB, which have the same advantages as 2,4-D.
• These herbicides have morphogenetic and biochemical effects on plants.
• Morphogenetic effects include different growth responses, stoppage of elongation, increase in width of plant parts, cell division stimulation, and disorganization of tissues leading to death.
• Biochemical effects are very complex and affect changes in plant metabolic status, including composition of carbohydrates, lipids, nitrogen, auxin, water, minerals, nucleic acids, and enzymes.

Compounds and Formulations

• Phenoxy herbicides are usually formulated as acids, esters, salts, and amines.
• Forms include dust, solid forms (granular and pellet), and pre-emergent and aquatic weed control formulations.

Chlorobenzoic Herbicides

• Chlorobenzoic herbicides are derivatives of chlorinated benzoic acids used extensively for perennial weed control in uncultivated areas.
• They are effective on a wide range of plants, especially those resistant to chlorophenoxy acid.
• Soil application requires a higher application rate than chlorophenoxy herbicides.
• 2,3,6-TBA and 2,3,5,6-TBA are the most toxic.

AUXIN Herbicides (other than Chlorophenoxy and Chlorobenzoic Acids)

• Notable AUXIN herbicides include NPA, N-aryl phthalamic acids, and picloram.
• NPA is used for pre-emergence applications, killing both monocot and dicot weeds, and upsets the geotropic response of roots.
• Picloram induces the same physiological responses as benzoic acids, is absorbed by foliage and roots, and accumulates in developing tissues.

This quiz covers the importance of herbicides in weed control, types of PGR, and their practical use. It also discusses the effects of weeds on crop yield and quality.