Weed Biology and Ecology

Introduction to Weeds

• Agricultural pests include insects, plant pathogens, weeds, vertebrate pests, and snails/slugs, existing in a dynamic environmental balance.
• Plants are essential, yet their uncontrolled spread can hinder human activities.
• Only a small fraction of the world's plant species are economically valuable.
• Volunteer vegetation competes with crops, impacting human and animal health and normal activities.
• Weeds are a major natural hazard alongside weather, insect pests, and plant diseases.
• Weed presence has been an issue since humans started cultivating plants, leading to ongoing battles and high agronomic costs.
• Weeds are considered a hidden enemy, causing substantial agricultural losses in India where they account for roughly 45% of all losses.

Defining Weeds

• Definitions vary, but center on how plants affect human interests.
• Though understood by weed scientists, there isn't one universally accepted definition.
• Jethro Tull (1731) defines weeds as plants growing where they are not wanted.
• The Weed Science Society of America defined a weed as "a plant growing where it is not desired" (1967), later revised in 1989 to include plants objectionable or interfering with human activities but also welfare.
• The European Weed Research Society defines a weed as “any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people" (1986).
• The Oxford English Dictionary (1973) defines a weed as a herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as preventing the growth of superior vegetation.
• W.S. Blatchley (1912) describes weeds as a plant out of place or growing where it is unwanted.
• A.E. Georgia describes it as a plant that is growing where it is desired that something else shall grow (1916).
• W.W. Robbins et al. (1942) describes "The obnoxious plants are known as weeds”.
• W.C. Muenscher (1946) weeds as harmful or objectionable plants growing where they are not wanted, usually where something else should grow.
• J.L. Harper (1960) described weeds as "Higher plants which are a nuisance”.
• E.J. Salisbury (1961) sees a Weeds as A plant growing where we do not want it.
• G.C. Klingman (1961) defines weeds as a plant growing where it is not desired, or a plant out of place.
• R.W. Emerson (1912) describes "A plant whose virtues have not yet been discovered”.
• F.C. King (1951) describes "Weeds have always been condemned without a fair trial”.
• A.H. Bunting(1960) sees Weeds are pioneers of secondary succession, of which the weedy arable field is a special case,.
• W.S. Blatchley (1912) describes "A plant which contests with man for the possession of the soil”.
• T. Pritchard (1960) Opportunistic species that follow human disturbance of the habitat”.
• Aldrich (1984), weeds are plants originating in natural environments that evolved to interfere with human activities.

Losses Caused by Weeds and their Impact

• Uncontrolled weed growth leads to crop yield reductions ranging from 34.3% to 89.8%. With weeding efforts, the reduction is still about 10% yield loss.
• Weeds cause losses of 5% in developed countries, 10% in less developed, and 25% in least developed countries.
• Weed-infested fields complicate operations like fertilizing and irrigation.
• Spiny weeds hinder harvesting. Some weeds bind crop plants, making harvesting difficult.
• Weeds shelter crop pests and diseases, acting as alternate hosts during and after crop season.

Weed Competition and Impact

• Yield losses in India due to weeds are greater than losses from pests and diseases.
• Yield losses vary by crop, with many weeds self-sown, competing vigorously during initial crop stages.
• Weed infestation can reduce yields by over 50% in some crops.
• Weeds deprive crops of vital nutrients, and reduce nutrient availability.
• Estimated nutrient uptake by weeds is 47% for N, 42% for P, 50% for K, 39% Ca, and 24% for Mg.
• Weeds harbor microorganisms that act as alternate hosts to insects, pests, and diseases.

Additional Effects of Weeds

• Weeds interfere with agricultural tasks, complicating sowing and harvesting, increasing costs for labor and chemicals.
• Aquatic weeds impede water flow in canals, disrupt transport, and hinder drainage, polluting water and threatening fish.
• Weeds pose fire risks along transport routes, forests, and industrial areas.
• Weed presence lowers farm produce quality such as food grains contaminated by seeds, leads to flour with bad odors, grain heating/rot, tea quality impairment, and spinning process interference, Datura or Argemone.

Impact on Animal and Human Health

• Growing with forage crops, weeds can cause farm animal sickness or mortality from alkaloids, tannins, or nitrates. Johnson grass can be poisonous to animal due to their high prussic acid contents. Tribulus terrestris sensitivity to light.
• Lantana causes photosensitivity/jaundice, carrot grass causes contact dermatitis, while leafy spurge is harmful to cattle, and Crotolaria spp. to chicks.
• Under drought, some weeds accumulate toxic nitrate levels.
• Weeds affect human health, causing allergies, asthma, rhinitis, and dermatitis. Vectors reproduce in weeds leading to diseases malaria, yellow fever and filariasis.
• Wheat with corncockel seeds tastes bitter and irritates the stomach, Mexican poppy seeds cause death, and milk from animals that consume weeds can negatively affect human health.

Impact to Aquatic Ecosystems, Industry and Public Aesthetic

• Aquatic weeds degrade aesthetics and recreational activities.
• Water flow is slowed in irrigation systems from aquatic and decomposing weeds.
• Fire risks from weed-ridden industrial sites and airfields leading to damage of pipelines/valves. Weed compromise rail tracks and airstrips.
• Regeneration of forests is impeded due to weed populations affecting aesthetic enjoyment.

Beneficial Effects of Weeds

• Soil moisture conservation and reduced erosion. Weed ground cover aids conservation and prevents nutrient leaching.
• Food/shelter for natural enemies. Weed cover enhances biological control.
• Weeds indicates growing conditons, such as water levels and pH.
• Weeds serve as food for wildlife, weeds such as Typha and Saccharum spp. are used for making cottage industry ropes while Cichorium intybus roots are used for adding flavour to coffee powder
• Certain weeds can form good leafy vegetables.
• Certain weeds has been used to donate genes to our crop plants, for instance,, Sacchumm spontaneum has been widely exploited for developing the present noble canes for North India.
• Fruit and rhizomes of certain weeds are used as vegetable and food material.
• Weeds like Crotolaria has nematicidal properties which their incorporation into the soil has reduce root-knot nematode pupulation
• Some entomologists have found that parasites and predators also survive on pests with certain such weeds helps in maintaining the lifecyle of certain parasite

Characteristics of Weeds

• These should not be confused with occurrences on farms that lead to whooping yield and quality.
• Weeds have special traits classifying them as unwanted, leading to development of different types of control methods.
• Rapid seedling growth. Pigweed can rapidly grow unlike crops.
• Quick maturation cycle.. Ephemerals live for 1 month before growing.
• Dual model reproduction. Many weeds reproduces through seeds or vegetatively as tubers.
• Environmental plasticity. Most Many weeds can tolerate and thrive in varied climatic conditions
• Weeds is often self-compatible unlike crops which requires pollinators
• Deitrimental environment factors. Unlike crops weed seeds resist decay.
• Many weeds exhibit several dormancies to escape danger from their environments.

Weed Proliferation Strategies

• Weed seeds have no requirements of growth to germinate.
• Weed seeds are viable and last longer.
• The seed shape of weeds makes physical cleaning and seperation difficult.
• Annual weeds releases more than once a year.
• Each generation produce large seeds that helps in dispersing through other environments
• Many weeds have structural adaption for Long and short range dispersal. Ballon Structures
• The roots of weed penetrates deep into the ground.
• roots and vegetative organ of weed is capable of storing large portions of food reserves.
• weeds adapt to the environment.
• Spines are adapted to repel grazing.
• weeds contain high level of nutrition and other materials.
• weeds are ubiquitous.
• weeds has high resistance to chemicals.

Classification of Weeds

• Weeds are classified based on ontogeny/life cycle, seed type/morphology/cotyledon character, nature of shoot/stem/plant, association, habitat and origin, reaction of soil, response to photoperiod
• Weeds can grouped by Annuals, Biennails and Perennails.

Annuals

• Weeds that completes a years of their germination in one season,

Characteristics of Annuals

• Annual seeds reproduce by abundance seeds, for example, Allium
• Can be grown from bulbs.
• Can not be grown when is cut
• They prossess crown buds when they sprouted into shoots.
• is easy to control.
• Produce thousands of plants and is buried in the soil for several years.

Biennial Weeds

• Completes their life cycle in 2 years and before the years and they stored nutrients.

Perinnial Weeds

• This type of weeds can grow up to 3 years.
• Usually flowers for the second of growth and then regulary
• They reproduce through seeds or specialized vegetative organs.
• Remians green during tropical area.

Adpative measures for Perrennials

• Longevity of tubers and stolons roots and bulbils.
• Prosess deep roots _ Presecnece of buds grows at the base and below the ground.
• Stems with buds to grown when cut.

Classification Simple Perennials

• Simple Perennials, reproduces though predominantly by seeds. Some sowthistle, Rumx

Creeping Perennials

• Have seeds propagation reproduces through vegetative means like bulbs, tubers etc.
• Has the most difficult method.

Woody Pernennials

• Grown at the season consistenty giving rise too woody biomasses
• Lentala camera are examples

Depth of roots

• Eg. Cyoondon, Bermagrass e.t.c roots in the furrows slice in 6 inch of soil.
• Cyperous purple nutsedge e.t.c deep roots
• Control or Perrenail weeds are called paracoius

Classification by Seed type

• Mono cots
• Weeds has one cotyledon, split into halves narrow grass
• family Graminae is the most important. Cyoondon daction

Die Cot

• Weed has to and cannot split Herbaceous Spp.

Classification by Association

• Season Bound grow for one specific season for either winter or summer time
• Crop bounds. Dependent on hosts
• Orobanche
• Based on roots.

Parasitism Based Classification

• total roots will provide them with energy but not food.

Partical Hemi-

• will also provide them with energy but not enough, the plant turns green and produces its own food.

Based on Shoot-Stem-Parasitism

• Total stems will take away food but will also provide food.

Classification of parasitize

• Species of weeds which usually Parisitz the host.

Need of specific micro climates, for example, cicorium, it requires high amounts of moist.

Mimcary

• wild oat will require similarity to suive

Vegetative

• Weeds needs similar morphology separated with stem leaves.

Chronical

• Simcalar heiight and ripening

Bio chemical

• crop is tolerate to herbicides the weeds develop reisstance to herbicides

Ready Contamination

• contamination for some species
• Digera- crop lands species

Aquatic. Are weeds that grow in.

• Submerged weeds. Grows underneath water with true roots leaves
• Immersed species with are rooted with top mud in the water.
• Marginal in moisture in areas with water

Origin of the weed Antrophytes. Alien and introduce. Country Naturalized weeds adapted to new environment growing Indenginous origin from where the weeds grow from Acidopile acidity and bassfile bascially

Classification by Habitat

• Crop land
• Fallow Land
• Grazing Field
• Aquatic
• Forest woodland
• Lawn garden
• Orchard plantation
• roadside canal irrigation Aquatic

Classifaction according to soild PH reaction

• acidophile weeds such as Rumex acetosella tend to dominate. Bassopile polygonum

• 17. Weeds have great competitive ability for nutrients, light, and water and can compete

• Weeds resist control/eradication, including resistance to herbicides.

• Annuals weeds completes its seed cycle within two to four weeks known as ephemerals

• Biennials usually feature limits their dispersal and so get harvest

Explore the key characteristics and adaptations of weeds that enable them to thrive in diverse environments. Learn about seed persistence, rapid spread mechanisms, and reproductive strategies of annual weeds. Understand the implications of these factors for agriculture and forest management.