CPRT 42 (Weed Science)

• Weed Science

  The study of weeds and their control, an offshoot of plant physiology
• Main Goal

  The formulation of the most satisfactory, most efficient yet less expensive method of controlling weeds
• Two main groups of weed control

  
  

  Non-chemical methods

  Chemical method/Herbicidal method
• Non-chemical methods

  • Manual
  • Mechanical
  • Cultural and ecological methods
  • Flaming
  • Biological control
• Manual

  Essentially hand weeding, the oldest method of weed control
• Tools used for manual weed control

  
  

  Small garden tools like sticks, trowels, or garden forks
• Handweeding

  Tedious job, climate dependent
• Forces favouring hand weeding

  • Availability and low cost of labor
  • Increasing cost of herbicides
• Mechanical

  The use of implements such as cultivator and rotary weeder, requires the use of bigger and expensive machines
• Factors to be considered in choosing farm implements for weed control

  • Life cycle of the crop and weed
  • Depth and spread of root system
  • Age and area of infestation
  • Soil type and topography
  • Weather conditions
• Rotary weeder and cultivators are applicable only to straight-row planted crops, not applicable in broadcast-seeded crops
• Cultural and ecological methods

  
  

  Growing conditions are altered with the aim of suppressing weed population directly
• Cultural and ecological methods

  • Mulching
  • Flooding
  • Cover cropping
  • Change in crop or variety
  • Change from monocropping to intercropping or multiple cropping
  • Crop rotation
  • Increasing crop density and nutrient level
  • Thorough land preparation
• Flaming

  
  

  Method of weed control employing fire, may be used in the form of a "flame cultivator" or weed burner, another age-old method of weed control
• Biological control

  Use of living organisms or natural enemies to control a weed species, includes insects, mites and fungi, the most desirable control measure
• Approaches used in biological control

  
  

  • Classical biological control
  • Use of endemic or native biological control agents
• Characteristics of a good biological control agent

  • Ability to kill the plant or prevent its reproduction
  • High ability to disperse and locate host plant
  • Good adaptation to weed host and environment
  • Sufficient reproductive capacity
• Biological control agents

  • Teleonemia scrupolosa used to control Lantana camara but attacked Sesanum indicum in Uganda
  • Chrysolina quadrigemina ineffective on Hypericum perforatum in Australia but highly successful in California
  • Success stories using same agents on Opuntia in Australia, Hypericum in USA, Lantana camara and Tribulus in Hawaii
• Herbicides

  
  

  Pesticides used to kill plants in general, coined from Latin words 'herba' (plant) and 'caedere' (to kill)
• Historical events leading to discovery of herbicides

  • 1896 - Discovery of Bordeaux Mixture
  • 1896-1910 - Many chemicals discovered for weed control
  • 1930s - Start of use of organic selective chemicals
  • 1940s - Weed control revolutionized with discovery of 2,4-D
• Pokorny (1941) reported synthesis of 2,4-D which was first tried as an insecticide and fungicide but unsuccessful
• Zimmerman & Hitchcock (1942) tested 2,4-D as a growth regulator
• Martin & Mitchell (1944, USA) reported selective weed control of 2,4-D in blue grass lawn
• Hammer & Tukey (USA) used 2,4-D successfully in the field
• Discovery of selective herbicide property of 2,4-D

  Considered a major breakthrough in agriculture
• Herbicides

  One of the most labor-saving innovations developed in weed control
• Classification of herbicides based on selectivity

  • Non-selective or general weed killers
  • Selective
  • Physical - contact between crops and herbicide minimized
  • Physiological - uninhibited contact between herbicide and plant
• Classification of herbicides based on time of application

  • Pre-plant - applied before planting
  • Pre-emergence - applied after planting crop
  • Post-emergence - applied after crop or weed has emerged
• Advantages of using herbicides

  • Applied before crop germinates provide good weed control during early growth
  • Applied over large areas in short time
  • Avoid drudgery of weeding and make farming more attractive
  • Can replace need to plow and harrow
• Disadvantages of using herbicides

  • Importation of herbicides by developing countries
  • Requires appropriate equipment with initial capital expenditure
  • Requires skill
  • Improper application can create hazards
• Naming of herbicides

  • Common/generic name
  • Trade name
  • Chemical name
• Classification of herbicides based on movement in plants

  • Contact herbicides - phytotoxic effects at spray sites
  • Translocated herbicides - phytotoxic effects at and away from spray sites
• Classification of herbicides based on selectivity

  • Nonselective herbicides - kill all plants
  • Selective herbicides - kill some plants leaving others unharmed
• Herbicide formulations

  The active ingredient in this formulation dissolves readily in water.
• Mode of action

  The sequence of events from when the herbicide comes into contact
• Mechanisms of action

  The primary site of action of the herbicide
• Resistance

  The ability of a plant to withstand the phytotoxicity of a chemical
• Tolerance

  Reduced susceptibility or enhanced resistance
• Selectivity

  The property of a herbicide that enables it to kill some plants
• Mechanisms of selectivity

  • Differential absorption
  • Differential translocation
  • Differential metabolism
  • Differential rate of metabolism