BASICS OF INTEGRATED WEED CONTROL

Definition of a Weed

The definition of weeds is predicated on

human perception , desire and needs. 1.  A weed is a plant which interferes with

human activity or welfare.2. It is also defined as plant growing in a

place where it is not desired at a particular point in time.

Origin of Weeds

Weeds are classified as native or introduced according to their origin. Plant origin may help determine weed management strategies and the difficulty of control.

Native weeds have historic origins in the area and were not brought here by human activity. Natural enemies, competition from other plants, and environmental conditions hold many native weed populations in check.

Sometimes these plants may be troublesome, but native plants rarely become problem weeds.

Introduced Plants

Introduced plants came from other parts of the

a. country or world by accident or unintentionally.

b. The Pacific Northwest often lacks the weed’snatural predators or limiting factors such as climate to hold them in check.

c. Lack of natural control allows the weeds to flourish andspread. Most problem weeds are introduced plants.

Escaped Plants

Escaped plants were intentionally introduced by

humans as crops, forage, or ornamentals.1) They subsequently have spread beyond

their intended areas. Again, the natural mechanisms that suppress these weeds were not imported with them or are not present in the new environment.

2) Dalmatian toadflax, Scotch broom,and kochia are examples of escaped ornamental plants.

How Weeds Spread

Weeds spread when outside agents move or carry seeds or growing plant parts (roots, rhizomes) into new territory. Some invading weed species have evolved special seed shapes or structures to aid their movement by wind, water, or animals. Also, many plants have vegetative parts that resprout new roots or shoots. When carried into new areas, these plant fragments may grow and start new infestations

How Weeds Spread

Wind carries many seeds to new areas. Weeds such as dandelions have a parachutelike attachment that carries the seed in the wind

For other weeds, the entire plant moves or tumbles with the wind, dropping seeds as it rolls; tumble mustard and Russian thistle are common examples.

How weeds Spread

Water from rain or irrigation and the subsequent surface runoff transports many seeds. Some seeds have an oily coating or an air bladder to aid flotation. Rivers, streams, and irrigation canals move large numbers of seeds.

How weeds Spread

Mammals, birds, and humans carry seeds on their bodies, dropping them into new areas. Plants have seed shapes (burs, hooks, barbs) that cling to feathers, hair, and clothing. Some seeds are ingested and excreted by wild or domestic animals. The seeds often survive and subsequently germinate after they have passed through an animal’s digestive tract.

Effects of Cropping activities and their effects on biodiversity

Practices that Increases Biodiversity:• Intercropping • Crop Rotation• Cover cropping and Strip cropping

Decrease in Biodiversity can be caused by:•  Monocropping • Tillage and Herbicides use

Biodiversity leads to more stability for the ecosystem as a whole.  

Characteristics of Weeds

Harmful to humans, animal and crops Wild and Rank growth in an environment Exhibits persistency

High reproductive capacity: large number of seeds, possess diffeent types of propagules e.g. Seeds, tubers , rhizomes,

1. Seed Dormancy : could be innate, induced or enforced.2. Usually present in large populations.. Could be regarded as being useless, unwanted and

undesirable They exhibit spontaneous appearance without being

planted Some exhibit mimicry. (seed, vegetative and

biochemical )• Many weeds are aggressive and have rapid seedling

growth

Economic Importance of Weeds(contd.) 

• Reduction in quality of pasture land; it reduces the carrying capacity of grazing lands and pastures through their physical presence and weediness

• Reduction in quality of animal products;it affects the palatability of pastures, hay, silage etc. protein content in alfalfa wild garlic (Alliums spp) when eaten by cattle spoils the meat and the milk.

• Serve as alternate hosts for many plant diseases and animal pests e.g. insects, rodents, birds. Cyperus rotundus serve as alternate to nematodes and athropods

• Impose limitation to the farm size of a farmer• Can serve as sources of fire hazards

Beneficial Effects of Weeds

1. Reduce erosion problem through the production of protective cover

2. Help in nutrient recycling through decay of vegetative part.

3. Food/vegetables for humans e.g. leaves of Talinum triangulare, and tubers of Colocasia esculentus .

4. Serve as hosts and nectar for beneficial insects5. Beautification of the landscape e.g. Cynodon

dactylon

Beneficial Effects of Weeds (contd.)

Feed for livestock and wildlife and aquatic organisms in form of hay, silage and forage / pasture, fruit seeds and branches and whole plant.

1. source of pesticides e.g. Chrysanthemum cinerariifolium 2. Source of genetic material for useful traits in crop improvement.3. Medicinal use e.g neem ( Azadirachta indica), Ageratum

conyzoides4. Some serve as trap crop for parasitic weeds.5. Habitat for wildlife and plant species hence biodiversity

conservation.6. Major role in carbon recycling through carbon sequestration. Field

of exposed soil always suffers a net loss in organic matter and releases carbon dioxide, while a field covered with crops and/or weeds takes up carbon dioxide. This concept of carbon sequestration is an added advantage of sustainable and organic farming.

Benefits of some weeds:

Weeds can serve a purpose; here are benefits weeds offer:1. Soil fertility and water holding capacity. Perennial weeds such as thistles,

nightshades, and pigweeds are deep rooted and penetrate the subsoil. Root penetration breaks up the soil and allows water and nutrients to move up and down through the soil. Moisture seeps down and phosphorous and potassium are drawn to the surface for use by other plants.

2. Habitat for beneficial insects. Weeds in the sunflower (Asteraceae), parsley (Apiaceae), and mustard (Cruciferae) families are nectar sources for beneficial insects. Most weed flowers are shallow throated making their pollen and nectar easily available to beneficial insects.

3.  Trap crops. A border or row of weeds along a garden will attract pest insects that otherwise might concentrate in the garden and harm crops. You can encourage weeds that attract pest insects. Keep weeds lush and irrigated to attract insects, but cut back the top of weeds before they flower and set seed. Monitor weedy trap crops for insect eggs and pest insect larvae; handpick or spray pest insects in trap crops before they move into the garden.

4.  Soil protection. Low growing weeds that compete poorly with vegetables can protect the soil from drying out. As long as weeds are not competing with crops for moisture and nutrients, they can hold the soil in place and keep the sun

Classification of weeds

1. Most weeds occur in the division Anthophyta (Angiosperms, which are further divided into the classes dicotyledones (dicots) and monocotyledons (monocots)).

2. There are approximately 250000 species of plants in the world. However, less than 250 plant species, about 10%, are troublesome enough to be called universally throughout the world.

3. On the basis of the habitat, weeds are divided into terrestrial and aquatic categories and on the basis of duration of life, they are divided into annual, biennial and perennial.

4. Annuals complete their life cycle in a year or in a single growing season. They mostly propagate through seeds. Normally, they germinate with the crop and complete their life cycle before or along with the crop. Based on the season they are encountered, weeds may be called as summer and winter annuals. Some examples are:

Weeds Classification

1. Summer Annuals: Echinochloa colona, E. crus-galli, Amaranthus viridis, Trainthema portulacastrum, Eclipta alba, Digitaria sanguinalis, Panicum repens.

2. Winter Annuals: Phalaris minor, Avena ludoviciana, Poa annua, Chenopodium album, Melilotus indica, Lathyrus aphaca, Anagallis arvensis.

Biennials live longer than 1 year but less than 2 years. During the first growth phase, they develop vegetatively from a seedling into a rosette. After a cold period, the growth resumes into reproductive phase. Biennials are large plants when mature, and have thick fleshy roots. Relatively few weed species are biennials, e.g. Cichorium inttybus, Raphanus raphanistrum.

Perennials live longer than 2 years and may reproduce several times before dying. They are propagated through seeds and vegetative reproductive organs like rhizomes, suckers, bulbs and tubers, e.g. Cynodon dactylon, Saccharum sp

Weeds Classification

Aquatic weeds: They have been categorized further as floating, emerged and submerged. 

Floating weeds: These rests upon the water surface. Their roots hang freely into the water or sometimes attach to the bottom of shallow ponds or streams, e.g. water hyacinth, Pistia, Salvinia, etc.

 Emergent weeds:Typical plants of marshlands and are often found along the shorelines of ponds and canals. These plants stand erect and are always rooted into very moist soil, e.g. Typha, Scirpus, etc.

Submerged weeds:Grow completely under water, although a few stems and leaves may exist on the water surface, e.g. Hydrilla, Chara, Potamogeton, etc.

Parasitic weeds:These weeds parasitize on specific host plants and deprive them of water, nutrients and assimilates. They may be root parasites (Orobanche, Striga) or shoot parasites (Cuscuta). They are either partial parasites (Striga) or total parasites (Orobanche), depending on the degree of parasitism.

WEED ECOLOGY

Ecology is the study of the relationship of plants and animals to their physical and biological environment. Physical environment like light, heat solar radiation, moisture, wind, oxygen, carbon dioxide, nutrient soil, water and atmosphere. Biological environment includes organism of the same kind as well as other plants and animals

Weed ecology is generally about the growth characteristics (ii) adaptation (iii) survival mechanism of weed that enables them to exploit environmental resources and successfully colonize new habitat often at the expense of other neighboring plants

Weeds Habitant

A habitat is a dwelling place or a kind of environment occupied by the individuals of a species. Habitat may imply places like rubbish dump, farm land or other sites occupied by weeds.

Niche

• Niche: it is the condition in a location under which a species can live successfully. Within the habitat, organisms occupy different niches.

•  A niche is the functional role of a species in a community—that is, its occupation, or how it earns its living. For example, the scarlet tanager lives in a deciduous forest habitat. Its niche, in part, is gleaning insects from the canopy foliage. The more a community is stratified, the more finely the habitat is divided into additional niches.

WEED-CROP ECOSYSTEM

• Ecosystem is the energy driven complex system in which the living organisms interact with themselves and the environment.

• Weed- crop ecosystem involves weed-crop interaction as well as the nature and the function of that ecosystem. This will assist in understanding the impact of crop production and husbandry on the shifts in weed flora, for instance the persistent of weed in given weed- crop ecosystem.

Persistence and survival mechanism of weeds

• Weed persistence is a measure of the adaptive potential of weeds that enables them to survive in disturbed environment such as i. Crop land ii. Recreational site iii. Irrigation canal and iv. Pastures

• The adaptive features or survival mechanisms of annual weeds include: i. Production of large quantities of seeds ii. Seed dormancy and iii. Periodicity of seed germination and short life span.

• The adaptive features of perennial weeds include: i. Deep rooting ii. Dormancy iii. characteristics of buds on rhizome iv. Other modified stems and v. Fragmentation of parts

Types of peennating and reproductive vegetative structures in perennial weeds:1) 1. Rhizome – underground, horizontal stem (quackgrass, swamp smartweed) 2) 2. Stolon – aboveground, horizontal stem (bermudagrass) 3) 3. Tuber – swollen stem tissue (yellow nutsedge) 4) 4. Bulb – stem with shortened internodes and fleshy modified leaves (wild garlic)  5) 5. offset6) 6. bulbils7) 7. corm8) 8. runners9) 9. suckers

Persistence and survival mechanism of weeds(contd.)

Storage organs may act as 'perennating organs‘ These are used by plants to survive adverse periods in the plant's life-cycle (e.g. caused by cold, excessive heat, lack of light or drought). During these periods, parts of the plant die and then when conditions become favourable again, re-growth occurs from buds in the perennating organs. For example geophytes growing in woodland under deciduous trees (e.g. bluebells, trilliums) die back to underground storage organs during summer when tree leaf cover restricts light and water is less available.

Crop mimicry

Crop mimicry is an example of the extent to which weeds have adapted themselves to survive in that frequently disturbed site.

Crop mimicry is defined as the phenomenon whereby weeds develop morphological and or biochemical close resemblance to some phases in the life history of a crop as to be mistaken for the crop and thus evade eradication.

Types of crop mimicry

1. Vegetative mimicry:A situation where close similarity in appearance occurs between weeds and crops at seedling and vegetative stages.e.g. wild rice (Oryza longistaminata) in cultivated rice ; wild sorghum (Sorghum halepense) in cultivated sorghum, wild sugarcane (Saccharum spontaneum) in sugarcane.

2. Seed mimicry:This is a situation whereby the similarities between weeds and crops is observed in seed, weight, size and appearance. e.g. similarity in seed size between seeds of upland rice and those of itch grass (Rottboellia cochinchinensis).

3. Biochemical mimicry:This is a situation in which a weed develops resistance to a herbicide that has been used previously for selective control in a given crop.

Factors affecting weed persistence

Weed persistence can be affected by:1. Climate e.g light, temperature, water,

and wind2. Soil (edaphic)3. Biotic factors e.g. plants and animals

WEED-CROP INTERACTION

When plants grow close to each other, they interact in various in ways.

1. Interference: It is the detrimental effects of one species on another resulting from their interactions with each other. When plants are far apart they have no effect on each other. Interaction generally involves competition and amensalism.

2. Commensalism: This is the relationship between unrelated organism (different species) in which one derives food or benefit from the association while the other remains unaffected.

WEED-CROP INTERACTION

3.Competition (allelospoly): It is the relationship between two plants (weed/crop, crop/crop, weed/weed) in which the supply of a growth factor falls below their combined demand for normal growth and development. The growth factor competed for include water, nutrients, light, space and air/gasses (oxygen, carbon dioxide).

Types of competition

1. Above-ground (Aerial) competition : Takes place in the leaves and the growth factors involve are light and carbon dioxide.

2. Below-ground(Subterranean) competition: Takes place mainly in the roots while the growth factors involve are water, nutrients and oxygen.

3. The perceived consequence of competition with crop is reduction in the economic yield of affected crop plants.

Forms of competition:

Intraspecific competition: competition for growth factors among individuals of a plant species

Interspecific competition: competition for growth factors between two different plant species i.e crop/weed, weed/weed,or crop/crop

Critical Period of Weed competition/interference: This is the minimum period of time during which the crop must be free of weeds in order to prevent loss in yield . it represents the overlap of two separate components (a) the length of time weeds can remain in a crop before interference begins (b) the length of time that weed emergence must be prevented so that subsequent weed growth does not reduce crop yield.

Factors affecting weed-crop competition

Competitiveness of weed species1) Weed density and weight2) Onset and duration of weed-crop association3) Growth factors4) Type of crop and seeding rate5) Spatial arrangement of crops6) Plant architecture7) Growth factors availability8) Cropping patterns9) Crop type (C3 or C4 plants)10) Crop variety( tolerance, resistance, aggressiveness)

Factors affecting weed-crop competition (contd.)

Environmental factors1. Climatic factors e.g. rainfall,

temperature, wind, light etc2. Tillage3. Ground water management4. Soil (Edaphic)

Allelopathy

• Amensalism (Allelopathy)• Allelopathy is the production of chemical(s) or

exudates by living and decaying plant species which interfere with the germination, growth or development of another plant species or microorganism sharing the same habitat.

• There are two types of allelopathy:(True and Functional )

• True allelopathy involves the release into the environment compounds that are toxic in the form they are produced. Functional allelpathy involves the release into the environment substances that are toxic as a result of transformation by microorganism.

Amensalism (allelopathy) (contd.)

Allelochemical complex commonly encountered in plants include:coumaric acid, terpenoids, - syringic acid, butyric acid, flavonoids,

phenolic compounds.Examples of allelopathic plants:1. Black walnut (Juglans nigra)2. Gmelina arborea3. Soghum bicolor4. Casuarina 5. Lantana camara6. Imperata cylindrica is allelopathic on tomato, cucumber, maize

rice, glnut, olera, cowpea, pepper.7. Cyperus esculentus– is allelopathic on rice, maize8. C. rotundus – is allelopathic on barley. 

Parasitism

ParasitismIt is a relationship between organisms in which one lives as a parasite in or on another organism.

Parasitic weeds are plants that grow on living tissues of other plants and derive part or all of their food, water and mineral needs from the plant they grow on (host plants)

1. Hemi parasite (Semi parasite) a plant which is only partially parasitic, possessing its own chlorophyll (green colour) and photosynthetic ability (may be facultative or obligate). E.g Striga hermonthica

2. Holo parasite – a plant which is totally parasitic, lacking chlorophyll thus unable to synthesize organic carbon. E.g Orobanche spp

3. Obligate parasite – a plant which cannot establish and develop without a host

4. Facultative parasite – a plant which can grow independently but which normally behaves as a parasite to obtain some of its nutrition.

Predation

Predation: It is the capture and consumption of organisms by other organisms to sustain life.

1. Mutualism: it is an advantageous relationship between two organismsof different species that benefits both of them. It is obligatory and the partners are mutually dependent. Both partners are stimulated when the interaction is on. Example is the case between fungus and algae. The fungus protects the algae while the algae provide carbohydrate for the fungus.

2. Neutralism: This is the situation where plant exert no influence on one another.

3. Protocooperation: This is a condition whereby two plants interact and affect each other reciprocally. Both organisms are stimulated by the association but unaffected by its absence.