LESSON 3,4 AND 5 Flashcards by Justy Malubay

A plant growing where it is not wanted (Oxford Dictionary)
● Any plant or vegetation, excluding fungi, interfering with
objectives or requirements of people (European Weed
Science Society)
● A plant that is especially successful at colonizing and
proliferating in disturbed site.

WEED

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is the study of weeds and their control.
● It is an offshoot of plant physiology having evolved from the study of
plant growth regulators.
● Its main goal is the formulation of most effective, economical, and
satisfactory methods of controlling weeds.

● Weed science

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The damage caused by weeds are not visible as those caused by
insects and diseases.
● Seldom results to total crop failure.
● Crops and weeds are always associated together.

Why weeds as pest are not recognized early

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break up the soil to make nutrients
more readily available

Modern hoe

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l first coined the term
“weed” in 1931 in the book
“Horse Hoeing Husbandry”
Weeds are the plants which
grow where they are not
wanted

Jethro Tull

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were largely replaced by monoculture systems
and chemical weed control however, crop rotation has become an integral
part of weed management in organic farming as well as integrated weed
management practices in conventional farming systems.

Rotation practices w

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different crops in recurrent succession

crop rotation

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Rotation of competitive crop and non-competitive crops
Use of weeds suppressing crops as cover crops
Under sowing cereals with fodder legumes
Use of catch crop or trap crop

Principle of crop rotation

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Chemical weed control was first mentioned when describing the effects of mainly—————and their ability to offer some form of
selective weed control.

inorganic substances

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● Some of the chemicals with herbicidal activity prior to the 1940’s were

salt, iron sulfate, sulfuric acid, and copper sulfate.

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● Weed science received a major boost as a valid scientific discipline with
the synthesis of

2,4-D effective herbicide.

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is an example of this technology; it was introduced during the
1970’s and offered excellent weed control at these lower use rates, and
with little harm to the environment as indicated in the ranking of the World
Health Organization

Glyphosate

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Characteristics of a Weed

A unique characteristic of a weed is its excellent adaptation to the
disturbed environment. They are able to occupy the ecological spaces
left open in those environments altered by man for his use.
● They have rapid vegetative growth.
● They reproduce rapidly and mature early.
● Most weeds are very prolific and produce abundant seeds.
● They have the ability to survive and adapt to adverse conditions.
● Propagules possess dormancy or can be induced to become dormant
under unfavourable conditions.
● Adapted to crop competition.

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Advantages/Benefits of Weeds

Weeds when ploughing under, add nutrients, organic matter.
● Weeds check winds or water erosion by soil binding effect of their roots
(underhand).
● Useful as fodder for castles (Hariyali) & vegetable by human beings
(Ghol, Tandulja)
● Have medicinal value, Leucas aspera is used against snake bite, oil of
satyanashi seed is useful against skin diseases, nuts of lavala are used
in making scents (Incense sticks).
● Weeds can also be valuable indicators of growing conditions in a field, for
example of water levels, compaction and pH.

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Disadvantages of Weeds

● Reduction in crop yield
● Increase in the cost of cultivation.
● The quality of field produce is reduced.
● Reduction in quality of livestock produce
● Harbour insect-pests & disease pathogens
● Check the flow of water in irrigation channels
● Secretions are harmful
● Harmful to human beings and animals
● Cause quicker wear & tear of farm implements.
● Reduce the value of the lands

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weeds that are more or less found in every farm but not
exceptionally injurious and are readily controlled by good farming practices.

Common weeds

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weeds that are particularly undesirable because of their certain
undesirable characteristics, like the presence of an extensive perennial
underground system which enable to resist the most determined effort to
control.

Noxious weeds

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Examples of Noxious Weeds

Crab grass (Digitaria spp.) Pig weed (Amaranthus spp.)

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Examples of Common Weeds

Siam weed (Chromolaena odorata)
Milk weed (Euphorbia heterophylla )

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Are members of the family Gramineae (Poaceae) which range from
small, twisted, erect, or creeping annuals and perennials.
● Stems are called culms with well -defined nodes and internodes.
● Leaves arise alternately in two rows from the nodes.
● The leaf is composed of two parts, the leaf sheath and leaf blade.

Grasses

Echinocloa colonum Cynodon dactylon

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Are members of the family Cyperaceae has a narrow-elongated leaf but
differs from grasses by their distinguishing triangular stem.
● The leaves are mostly from the base having modified stem with or
without tubers.

Sedges

Cyperus rotundus Fimbristylis miliaceae

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Are members of the family Dicotyledonae and Monocotylidonae
characterized by shorter but wider leaves with either parallel or netted
venation like pickerel weed (parallel veins) or three-lobed morning glory
(netted veins).
● All dicotyledon weeds are broad leaved weeds

Broadleaves

Digera avensis
Portulaca oleracea
Tridax procumbens

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is one that matures in one growing season or within one
year. Example: barnyard grass and goose grass

Annual weed -

Barnyard grass (Echinochloa crus-galli) Goose grass (Eleusine indica)

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is one that matures in more than one growing season or
year. Example: purple nutsedge and bermuda grass

Perennial weed

Purple nutsedge (Cyperus rotundus)
Several purple nutsedge plants linked by a network of
rhizomes.

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is one that reproduces by means of seeds or seed propagules (is a plant part of which is capable of regenerating its own species). Example: barnyard grass

Sexual weed

is one that reproduces by means of vegetative propagules. Examples of vegetative propagules are:

Asexual weed -

– is a modified underground stem with buds and scales; Example: cogon grass

rhizome –

is a modified aboveground stem that creeps and roots at the side; Example: Bermuda grass

- stolon

is a short thickened underground stem; Example: purple nutsedge

- tuber

is a baby plant that arises from the mother plant; Example: water lettuce

- off-shoot –

stem stands upright; ex. Itch grass (Rottboellia cochinchinensis)

Erect –

– lying along the ground or along a surface, with the extremity curving upward. ex. Goosegrass (Eleusine indica)

Decumbent

having short stem and short internode;lying face down on the ground. ex. jungle rice (Echinochloa colona)

Prostate

day flower (Commelina communis)

Ascending

whose stems spread along the ground and root by means of adventitious roots. ex. bermuda grass (Cynodon dactylon)

Creeping

stem coils round the support. ex. three-lobed morning glory (Ipomea triloba)

Twining

is one that grows well in lowland or moist soil; Ex. barnyard grass and water lettuce

Wetland weed

s one that grows well in upland or drier soil; Ex. itchgrass and three-lobed morning glory

Dryland weed

is an inflorescence in the main axis with at least primary and secondary branching; Ex. barnyard grass

Panicle

is a compound structure whose members arise and diverge from the same point, like fingers of the hand; Ex. goosegrass

is more of less a conical inflorescence with flowers arising laterally from a common axis; Ex. itchgrass

Raceme

is an inflorescence in which a number of divergent flowers arise from the same point. Ex. purple nutsedge

Umbel

Weeds are classified:

Based on outward appearance or gross morphology ● Based on maturity period or life cycle ● Based on manner of reproduction ● Based on habit of stem growth ● Based on habitat or place for preferred growth ● Based on the inflorescence or floral structure of the plant

In many situations, the first one-third to one-half on the growth cycle of the crop is sensitive to weed competition. * This stage is called the

critical period of competition.

for nutrient, moisture, light and space

Direct competition

– through exudation and/or production of allelopathic chemicals.

Indirect competition

refers to the resumption of growth of the embryo in the seed or of the young plantlet in the tuber, bulb, or rhizome. * It marks the beginning of the battle for the survival of the weed.

Seed Germination

develops into the stem.

* Plumule

stores or absorbs food for the developing embryo.

Cotyledon:

first part of a seedling (a growing plant embryo) to emerge from the seed during the process of germination – primitive root

Radicle:

small pore on the seed coat for absorption of water.

Micropyle

outer seed coat, protective layer

* Testa:

During germination, the cotyledons are raised above the ground where they continue to provide nutritive support to the growing points. * This is the characteristics of bean seeds * During the establishment, the hypocotyl begins to elongate in an arch that breaks through the soil, pulling the cotyledon and the enclosed plumule through the ground and projecting them into the air. * Afterwards, the cotyledon open, plumule growth continues and the cotyledon wither and fall to the ground

Epigeal germination

* The cotyledon remain below or at the surface of the ground as in grasses and sedges. In both species plumule is enclosed in the coleoptile. * The epicotyl is the rapidly elongating structure. The cotyledons provide a food source for the growing seedling, eventually shriveling up. * The first leaves are formed from the plumule and are sometimes referred to as ‘true leaves’. This characteristics of pea seeds and all grasses and as corn and many other species. * During germination, the cotyledons or comparable storage organs remain beneath the soil while the plumule pushes upward and emerges above the ground

Hypogeal germination

absorption of water accompanied by swelling

Imbibition

cell division and cell elongation proceed at a fast rate supported by a rapid synthesis of materials. The end result is the perceptive growth of the embryo.

Period of rapid metabolic activity

– under field condition the radicle or root-like structure break through the seeds and grow into the soil.

Emergence of root –

commonly, this is considered the first sign of weed growth, the shoot grows above the ground.

Emergence of the shoot

the dependence on food reserve ceases and the seedling start to manufacture its own carbohydrate. From this stage on, the seedling is on its way to establishment

Period of independent growth

* Seedling stage is the most important stage in weed establishment * It is the most sensitive stage to the environmental influences. * The demand for nutrients and water is high at this stage so that in a crop weed association the weed seedling competed strongly for these factors with the crop seedlings.

Seedling growth and development

* The most vulnerable and most practical stage for control. * The stage where damage against the crop is at the low level and any control measure can take advantage of the sensitivity of the weed seedling. * Most susceptible stage to herbicide action. The best time to apply pre-emergence herbicide. * Leaves at the early seedling stage are tender succulent, lack cutin and waxes allowing easy penetration of the herbicide. * Roots of seedlings have still thin epidermal walls making herbicide absorption more efficient.

Seedling stage

seedling stage is characterized by rapid metabolic activity which creates a big demand for the needed soil resources. Most weeds are more efficient than crops in drawing nutrients from the soil. Amaranthus accumulates calcium, Cleome and Pistia accumulate potassium.

Nutrient level –

salinization of areas near sea shore appear to favor weeds than crop.

Salinity

shading reduces the amount of light available to the plant reducing photosynthetic activity and consequently reducing dry matter production.

Light

– the ability to adapt to adverse conditions is responsible for the rapid spread and wide distribution of weeds.

Adaptation to growing condition

most weeds derived their competitive power from their rapid development manifested by: a. Rapid root growth and development b. Rapid leaf production c. Multiple shoot development d. Formation of large and expansive foliar-type of cotyledon allowing early photosynthetic function. e. Formation of toxin, which may be toxic to other higher plants as in Imperata and Rottboellia or toxic to microorganisms as in Tagetes erecta.

Competitive power of the weed

It includes both detrimental and beneficial biochemical interactions among all classes of plants, including microorganisms.

Allelopathy

published a book on allelopathy. Defined as “any direct or indirect harmful or beneficial by one plant (including microorganisms) on another through the production of chemical compounds that escape into the environment”

Rice (1984)

Knotweed affect the wheat plant growth and development

Weed on Crop

Imperata cylindrica (Cogon grass) inhibits the emergence and growth of an annual broadleaf weed i.e. Borreria hispada (Button weed) by exudation inhibitory substances through rhizomes.

Weed on Weed

Oat, Pea, Wheat suppress the growth of Chenopodium album (lams quarter)

Crop on Weed

substance produced by a microorganism and effective against another microorganism.

Antibiotic

substance produced by a microorganism and toxic against higher plants.

Marasmin -

- substance produced by a higher plant and effective against microorganism.

Phytoncide -

substance produced by a higher plant and effective against another higher plant.

Koline

– is the release of substances that are toxic in the form in which they are produced in the plant.

True allelopathy

– is the release of substances that are toxic or a result of transformation by microorganism.

Functional allelopathy

Ways of releasing Allelochemicals

Allelopathic chemicals are released from plants as: * Vapor – from root and leaf (through stomata) * Foliar * Root * Breakdown/decomposition of dead plants * Seed extract

Factors affecting Allelopathic effect

Allelopathic effects might also depend on a number of other factors that might be important in any given situation: * Varieties * Specificity * Auto-toxicity * Crop on crop effects * Environmental factors

Advantages of Allelopathy

1. Limit competition for space, light, nutrients, water 2. Autotoxicity (Chemical produced inhibits germination of its own seeds) 3. Allelopathic weed suppression through the use of cover crops

Crop allelopathy can be effectively used to control weeds in the field. * By intercropping those field crops which can suppress weeds of next crops.

Arrangement of cropping systems

* Allelopathic applications such as straw mulching, provide sustainable weed management * The allelochemicals from decomposed straw can suppress weed growth in farmlands, and reduce the incidence of pests and diseases.

Straw Mulching

* It is the inability if the seed or any vegetative propagule to germinate under favorable conditions. * It can be acquired as the seed or organ develops or matures (primary or natural dormancy) or * Induced through encounter with unfavorable conditions (induced or secondary dormancy)

Dormancy

this involves the impermeability of the thick seed coat to imbibe water and oxygen, hence even the embryo is viable it will not germinate because of the barrier

Physical (Exogenous dormancy) –

can be accounted to immaturity of the embryo or to the presence of substances inhibitory to the germination of weed seeds. This type of dormancy is induced because of chemical changes within the seed’s embryo. One reason for a plant to not germinate during endogenous dormancy is that the embryo is underdeveloped at maturity

Physiological (Endogenous dormancy)

It is a condition when seeds cannot germinate even when exposed to favorable growth conditions. The inability is sometimes due to the embryo being immature at the time of germination. Innate dormancy can also be imposed chemically by inhibitory compounds in the seed coat or the embry

Innate Dormancy

Found when seeds are deprived of favorable growth conditions such as sufficient moisture, oxygen, light, and suitable temperature. In enforced dormancy, no physiological mechanism is involved in inducing dormancy. The seeds are said to be in the ‘quiescent’ stage.

Enforced Dormancy -

Caused by extremely unfavorable growth conditions. Such plant species have no innate dormancy but acquire them with time. After a certain period, the seeds permanently fail to germinate.

Induced Dormancy

It is the process of removing the seed coat by mechanically rupturing them. These make a hard seed coat permeable to water and gases.

Scarification

for a short duration makes the seed coat soft, allowing water and oxygen to enter.

Treating seeds with hot water or concentrated sulfuric acid

Seeds of tobacco and tomato revokes dormancy when exposed to white light after the seeds have imbibed 30-40% moisture.

Exposure to white light

Exposure to a very low intensity of light is sufficient to overcome dormancy. Even a brief exposure to the red light of wavelength 660u is effective for germination overcoming dormancy. In contrast, exposure to far-red light of wavelength 730u induces dormancy and thus prevents germination.

Exposure to red light

Application of gibberellic acid and kinetin can substitute to the red-light requirement of lettuce seeds. Thus, seeds can germinate in the presence of these hormones in total darkness.

Treatment with plant growth hormones

Dormant seeds contain an inhibitory hormone called gibberellic acid, which disappears during stratification. At this point, gibberellic acid is secreted, which promotes seed germination. Thus, the chilling requirement can be replaced through the application of gibberellic acid.

Presence with gibberellic acid

Chemicals such as potassium nitrate, thiourea, and ethylene break seed dormancy and induces germination. Similarly, the application of cytokinin and ethylene also promotes seed germination.

Use of growth-promoting chemicals

or approaches that reduce production and germination of weed seeds and vegetative propagules and minimize their establishment and spread. * Use of high quality and disease and weed-seed free planting materials. * Control of weeds before they produce seeds or vegetative propagules.

. Preventive Methods –

directly destroy weeds or involve the manipulation of the environment in the field to control weeds.

Physical/Cultural methods

Initial plowing buries weeds and stubbles from the previous crop, however, the process allows a batch of weed seeds to germinate which is called a weed flush

Land preparation and management of weed seeds and vegetative propagules reserve banks in the soil –

weeds are controlled by uprooting or with the use of various small local hand implements by slashing

Hand weeding/slashing –

this method can be accomplished in less time compared with uprooting the weeds

Hoe weeding

directly destroy weeds or involve the manipulation of the environment in the field to control weeds.

Physical/Cultural methods

this may done with animal or tractor-drawn implements

. Interrow cultivation

provides considerable control of some weeds by reducing light available to the crops and serving as a barrier to weed emergence.

Mulching

this method is effectively used in transplanted lowland rice.

Water management or flooding

– directly destroy weeds or involve the manipulation of the environment in the field to control weeds.

Physical/Cultural methods

a variety adapted to the locality which is characterized by an early rapid growth provides some measure of weed control through its competitive advantage.

Use of competitive crop varieties or cultivars

this may involve spatial (intercropping) or temporal (crop rotation or sequential cropping) combinations of crops in an area in one year).

Multiple cropping

– is the deliberate use of a weed’s natural enemies, such as insects or pathogens to suppress the growth or reduce the population of the weed. The types of biological control are:

Biological control –

use of host-specific damaging biotic agents from the country of origin of the imported problem weed species.

Classical approach

this includes augmentation and bioherbicide approaches.

Inundative approach

– natural enemies already present in the area where the problem weed species occur often do not cause sufficient damage to effect adequate control of the weeds because their population is low.

Augmentation approach

involves the application of inoculum of a weed pathogen in a manner analogous to a chemical herbicide.

Bioherbicide / mycoherbicide approach

Mycoherbicides--------are commercially available for the control of specific weeds in the USA.

devine and colego

involves the use of herbicides for selective control of weeds with minimum or no injury to the crop * The use of herbicides substantially reduces the labor requirement for weed control and provides early season control of weeds within crop rows. * Safety precautions on the label should be strictly observed. * Improper use of herbicides may result in crop injury, poor or ineffective weed control and drift or residue problems.

. Chemical control –

herbicide applied after land preparation but before the crop is planted. The herbicide is volatile and needs to be incorporated in the soil before planting. Examples are fluchloralin, trifluralin, EPTC

Preplant herbicide

the chemical is applied after the crop is planted but before the weeds of the crop emerge. Examples are pendimethalin, atrazine, alachlor, metolachlor, butachlor, nitrofen, oxadiazon, clomazone, chlorimuron-p-ethyl

Preemergence herbicide

– the herbicide is applied after the crop or the weeds have emerged. Examples are 2,4-D, butachlor, atrazine, isoproturon, fluazifop-p-butyl, fenoxapropp-ethyl, clodinafop-propargyl, chlorsulfuron, sulfosulfuron, lactofen, tralkoxydim

Postemergence herbicide

herbicide whose phytotoxic effects are manifested at the sites of the plants where the spray droplets are deposited. Examples: bromoxynil, ioxynil, paraquat, diquat, propanil

Contact herbicide

erbicides whose phytotoxic effects are manifested at and away from the sites on the plant where the spray droplets are deposited. The herbicide is moved to other parts of the plant. Examples: pendimethalin, trifluralin, fluchloralin, 2,4-D, isoproturon, glyphosate, glufosinate-ammonium, atrazine, metribuzin, nitrofen.

Translocated/Systemic herbicide

herbicide that kills all plants. Examples are Paraquat, glyphosate, glufosinate-ammonium, acrolein, picloram, amitrole, chlorfenac/fenac, metham, sodium chlorate.

Non-selective herbicides

– herbicides that kill some plants leaving others practically unharmed. All pre-planting, pre-emergence and post emergence herbicides meant for application in crops are selective ones.

Selective herbicides

control a particular group of weeds like grasses, broad-leaved or sedges. Their range of activity is narrow. Examples: 2,4-D, diclofop-methyl, flufenacet, fluazifop-p-butyl, fenoxaprop-pethyl, clodinafop-propargyl

Narrow spectrum

control a wider array of weeds constituting grasses, broad-leaved and/or sedges. Their range of activity over the population of weeds is broad. Examples: metribuzin, isoproturon, chlorsulfuron, atrazine, pendimethalin.

Broad spectrum –