WEED SCI

MEABDetalla’s Weed Science Lecture Study Notes

What to Expect in Weed Science

• Overview of Weed Science

  • Definition and acknowledgment of weeds as a significant study area

  • Various topics covered in this discipline

Weed Science as a Discipline

• The study of weeds and their control:

  • Evolves from plant physiology, specifically from the study of auxins and plant growth regulators

  • Developed after the discovery of 2,4-D in the 1940s

• Goal of Weed Science:

  • Development of an effective, cost-efficient, and environmentally friendly weed control program

• Key areas of focus:

  • Weed biology, weed control strategies, herbicide biochemistry, and herbicide physiology and resistance

• Historically aligned with the use of herbicides as the primary control method

Definition of Weeds

• Weeds are:

  • Plants out of place, interfering with human activities

  • Grown in undesired areas

  • Described as unwanted, useless, and undesirable

  • Weighed down by harmful effects exceeding benefits

Characteristics of Weeds

• Weeds exhibit:

  • Rapid vegetative growth and high root production

  • Early and efficient reproduction

  • Resilience to adverse conditions (e.g., drought, poor soil)

  • Dormancy in propagules that can be induced

  • Ability to cause significant damage even at low densities

Importance of Weeds

• Pervasive nature of weeds impacts everyone; they are a general concern due to their harmful effects, leading to significant yield losses if not managed effectively.

Harmful Effects of Weeds

• Weeds can:

  • Reduce crop yield quantity and quality

  • Serve as alternate hosts for other pest species

  • Increase farming production costs

  • Clog waterways and irrigation systems

  • Release harmful chemicals that adversely affect other vegetation

  • Decrease land value

Beneficial Effects of Weeds

• Despite their negative aspects, weeds can:

  • Reduce soil erosion in hilly areas

  • Provide food sources for humans and animals

  • Prevent nutrient leaching and contribute to soil nutrient return

  • Serve as sources of biopesticides and drugs

  • Offer germplasm for crop improvement

  • Provide habitats for insect predators

Classification of Weeds

• Weeds can be classified based on:

  1. Origin

  2. Habitat

  3. Lifespan

  4. Stem structure

  5. Growth habit

  6. Gross morphology

Classification Based on Origin

• Two main types:

  • Endemic/Native Weeds:

  • Native to their growing region

  • Exotic/Introduced Weeds:

  • Non-native to their location

Classification Based on Habitat

• Divided into:

  • Upland/Terrestrial Weeds:

  • Grow in dry soils

  • Lowland/Aquatic Weeds:

  • Lifecycles completed in flooded areas, further classified into:

    • Floating

    • Emerged

    • Submerged

Classification Based on Lifespan

• Annuals:

  • Complete their life cycle within a year

• Perennials:

  • Live more than a year, categorized into:

  • Simple perennials

  • Creeping/Rhizomatous perennials

Classification Based on Stem Structure

• Herbaceous:

  • Soft stems, common in annuals

• Woody:

  • Hard, barked stems, typical of perennials

Classification Based on Growth Habit

• Categories include:

  • Erect:

  • Grows upright without support

  • Twining:

  • Requires support to grow vertically

  • Creeping:

  • Grows horizontally along the ground

  • Variants include:

    • Procumbent

    • Decumbent

Classification Based on Gross Morphology

• Includes:

  • Grasses:

  • Narrow leaves with parallel veins, fibrous roots

  • Sedges:

  • Similar to grasses but with triangular stems

  • Broadleaves:

  • Wide leaves and various stem structures, often with taproots

Weed Establishment

• Factors determining establishment include:

  • Weed Seeds in Soil: The presence of seeds within the soil

  • Weed Seed Germination: Successful germination of seeds

  • Competition: Involves the interplay of various weeds in the soil seed bank influenced by:

  • Magnitude of Weed Seed Production: Number of seeds produced per plant cycle

  • Longevity & Dormancy of Weed Seeds: Ability to remain viable over time

  • Cultural Practices & Environment: Practices that affect growth dynamics

Seed Production Capacity of Common Weeds

Factors Affecting Seed Production of Weeds

1. Photoperiod: Influences flowering times

2. Vegetative Growth: Differences in growth patterns of monocots vs. dicots

3. Soil Nutrient Status: Early flowering occurs in nutrient-poor soils, promoting sexual reproduction

4. Temperature: Affects key reproduction processes like anthesis and pollination

5. Growth Regulators: Substances like auxin or gibberellic acid impacting reproduction

Factors Affecting Asexual Reproduction by Weeds

1. Soil Texture: Coarser soils encourage better tuber/rhizome development

2. Daylength: Short day lengths promote tuber formation whereas favorable day lengths for flowering do not

3. Light Intensity: Greater light intensities enhance vegetative propagule development

4. Soil Nutrient Status: Affecting the type of reproduction favored

Common Vegetative Propagules of Weeds

• Tubers/Bulbs: Examples include Cyperus rotundus and Scirpus maritimus

• Rhizomes: Found in Imperata cylindrica and Paspalum distichum

• Stolons (Runners): As seen in Cynodon dactylon

• Offshoots (Suckers): Examples include Pistia stratiotes and Eichhornia crassipes

Dormancy in Weeds

• Defined as the inability of seeds or vegetative propagules to germinate under favorable conditions

• Causes:

  • Physical Factors: e.g., hard seedcoats

  • Physiological Factors: E.g., immature embryos needing after-ripening

  • Germination Inhibitors: Chemicals leaching into the soil

• Dormancy explains patterns of continual weed emergence in fields

Germination Process

• The resumption of active growth in seeds or plantlets includes:

  1. Water imbibition

  2. Enzyme activation

  3. Initiation of embryo growth

  4. Seedcoat rupture

  5. Seedling emergence

Dispersal of Weeds

• Defined as the movement of seeds and plants from one area to another

• Dispersal Unit: Any structure capable of perpetuating its species, such as seeds and vegetative propagules

Agents of Dispersal

1. Wind: Lightweight seeds with various structural adaptations

2. Water: Seeds that are buoyant

3. Explosive Mechanisms: Seeds that shatter and spread

4. Animals: Seeds with edible fruits or structures for attachment

5. Humans: Seeds spread through clothing, shoes, or cultural practices

Competition Among Plants

• The struggle for limited resources defined as:

  • Interspecific Competition: Between different species

  • Intraspecific Competition: Between same species

• Allelopathy: Refers to chemicals released by one plant negatively affecting another

• Interference: General disturbances in normal plant growth due to competition or allelopathy

Competition for Water

• Water Use Efficiency (WUE): Measured as the amount of dry matter produced per unit of water used:

  • Corn: 370 L water/kg DM

  • Portulaca: 253 L water/kg DM

  • Rice: 682 L water/kg DM

Competition for Light

• Leaf Area Index (LAI): Measurement of leaf area relative to ground area

• Light Transmission Ratio (LTR): Ratio of light intensity beneath plant canopy to light above it

Critical Period of Competition

• Duration of crop life most affected by weed competition; typically during the first 1/3 to 1/2 of the crop lifespan

• Practical indicator: Closure of the canopy

• Critical Threshold Level: Density of weeds beyond which crop yield significantly declines

Factors Affecting Competition

• Key influences on plant competition include:

  1. Weed species

  2. Weed density

  3. Duration of competition

  4. Crop variety

  5. Seeding method

  6. Soil nutrient availability

  7. Environmental conditions

Weed Control Techniques

• The aim of weed control is to limit weed populations below critical levels, not to eradicate them

• Weed Management: Incorporation of various control methods to minimize weed populations

• Aims of Weed Management: Shift crop-weed balance in favor of crop survival

Principles of Weed Control

1. Reduce weed populations to non-damaging levels

2. Focus control measures on reproductive organs of weeds

3. Initiate control strategies early and maintain them

4. Ensure efficiency in controls with minimal harm to crops/environments

Methods of Weed Control

• Manual: Hand weeding or pulling

• Mechanical: Use of tools (e.g., rotary weeders)

• Cultural/Ecological: Practices such as mulching, flooding, crop rotation

• Biological: Utilizing organisms (like insects) to combat weeds

• Chemical: Application of herbicides

Chemical Weed Control

• Herbicide Definition:

  • Chemical substance to kill/inhibit plant growth, derived from Latin terms: Herba = plant + Caedere = to kill

• Notable Herbicides:

  • 2,4-D (2,4-dichlorophenoxy acetic acid), originated in the USA

  • MCPA (2-methyl, 4-chlorophenoxy acetic acid), prevalent in Britain

• Discovered during the 1940s and classified as part of the phenoxy group of herbicides

Classification of Herbicides

1. Based on Target Plants / Selectivity

   • Selective Herbicides: Kill specific plants (e.g., butachlor, 2,4-D)

   • Non-Selective Herbicides: Affect all plants (e.g., glyphosate)

2. Based on Time of Application

   • Pre-plant: Before crop planting

   • Pre-emergence: Before emergence of crops/weeds

   • Post-emergence: After crops/weeds have emerged

3. Based on Movement within Plants

   • Contact/Localized: Site of action is where applied (e.g., paraquat)

   • Translocated/Systemic: Herbicide moves to specific action sites (e.g., glyphosate)

4. Based on Rapidity of Phytotoxicity

   • Acute Toxicity: Quick phytotoxic response

   • Chronic Phytotoxicity: Delayed toxic response

5. Based on Chemical Structure

   • Numerous classes include phenoxys, triazines, sulfonylureas, and dinitroanilines

Herbicide Selectivity and Resistance

• Herbicide Selectivity: Property enabling a herbicide to differentiate between beneficial crops and harmful weeds

• Types of Selectivity: True selectivity and placement selectivity

• Herbicide Resistance: Ability of some plants to survive exposure to herbicides that would typically eliminate them

Pesticide Terms

• Toxicity: The property of a pesticide causing harm

• Hazard: Risk of adverse effects

• Active Ingredient (a.i.): Component responsible for the pesticide's phytotoxic effect

• Recommended Rate: Dosage of a.i. or formulated product needed for effective application

• Adjuvant/Surfactant: Adds performance aids to pesticides, enhancing effectiveness

• Pesticide Formulation: Mixtures designed to control pests, prevent growth, or mitigate harmful effects

• LD50: The median lethal dose that kills 50% of test subjects (mg/kg body weight)

Combining Pesticides Effects

1. Additive: Combined effects equal sum

2. Synergistic: Together, they have a greater effect than their individual sums

3. Antagonistic: Combination less effective than the individual compounds

Regulatory Bodies

• FPA (Fertilizer and Pesticide Authority): Established through PD No. 1144 in May 30, 1977

• IPM (Integrated Pest Management): Created via DA Special Order No. 495, Series of 1997

Pesticide Toxicity Category

Thank You!

• End of Lecture

• Good Day!