Comprehensive Agronomy Study Guide: Weed Science and Management

Introduction to Weeds and Their Impacts

• Historical Origin & Global Context:

  • Crop cultivation began around $10,000\text{ BC}$, at which point any plant other than the intended crop was recognized as a weed.
  • Many modern crops (such as wheat) originated from wild grasses and were improved by human selection, yet wild relatives are still used in plant breeding for drought and disease resistance.
  • Out of approximately $250,000$ plant species worldwide, about $30,000$ species are grouped as weeds, and $18,000$ species cause measurable agricultural crop damage.
  • The term "weed" was first coined by Jethro Tull in $1731$ in his book Horse Hoeing Husbandry.

• Definitions:

  • A weed is any plant growing where it is not wanted (Jethro Tull, $1731$) or a plant out of place.
  • Weeds compete with crop plants for water, soil nutrients, light, and space (specifically $CO_2$), thereby reducing crop yields.

• Global Agricultural Losses:

  • Losses due to weed competition average $5\%$ in developed countries, $10\%$ in less developed countries, and $25\%$ in least developed countries.
  • In India, pest-induced losses breakdown as follows: weeds account for $45\%$, insects $30\%$, diseases $20\%$, and other pests $5\%$.

• Factors Influencing Crop Loss Severity:

  • Type of weed species present.
  • Severity and duration of infestation.
  • Crop-weed competitive ability.
  • Climatic conditions influencing crop and weed growth rates.

• Direct Yield Reductions Across Crops:

  • Direct yield reduction across major crops ranges from $34.3\%$ to $89.8\%$
  • Rice: $30\text{--}35\%$
  • Wheat: $15\text{--}30\%$
  • Maize, Sorghum, Pulses, Oilseeds: $18\text{--}85\%$
  • Sugarcane: $38.8\%$
  • Cotton: $47.5\%$
  • Sugar beet: $48.4\%$
  • Onion: $90.7\%$

• Indirect Agricultural Difficulties:

  • Harvesting operations are severely hindered by spiny species like wild safflower (Carthamus oxyacantha), Canada thistle (Cirsium arvense), and cocklebur (Xanthium strumarium).
  • Cowage (Mucuna pruriens) causes intense skin itching for farm workers during harvest.
  • Vine-like weeds such as field bindweed (Convolvulus arvensis) and morning glory (Ipomoea spp.) bind crop plants together, causing mechanical wear and tear on harvesting machinery and increasing processing costs.

• Erosion of Crop Quality:

  • Moisture stress caused by weeds forces food grains to shrivel.
  • Contamination of grain with weed seeds lowers market value; certified or foundation seed lots are rejected if weed seed content exceeds $2\%$
  • Parasitic weeds like Striga litura impair sugarcane quality.
  • Dendrophthoe falcata (Loranthus) and Mikania micrantha impair tea leaf quality in plantations.
  • Dry weed fragments adhere to cotton lint, hindering spinning.
  • Burrs of Xanthium strumarium entangle in sheep wool, degrading quality.
  • Contamination by Cirsium arvense reduces mint essential oil quality.

• Menace to Animal Husbandry:

  • Off-odors in milk occur when livestock feed on wild onion, wild garlic, Cichorium intybus, or Argemone mexicana.
  • Toxic constituents cause animal illness or death:
  • High oxalate content in Halogeton glomeratus causes fatal poisoning in sheep herds in arid regions.
  • Lantana camara contains "Lantradene", causing acute photosensitivity and jaundice in livestock.
  • Puncturevine (Tribulus terrestris) induces light sensitivity and skin punctures in sheep.
  • Rhododendron in Kashmir causes bloody diarrhea and milk staining.
  • Crotalaria spp. are fatal to chicks.
  • Sweet clover (Melilotus alba) contains dicumarin, an anti-coagulant that prevents blood clotting.

• Weeds as Alternate Hosts for Crop Pests and Diseases:

  • Insect Pests: Echinochloa and Panicum host rice stem borers; Chenopodium album hosts maize stalk borers, cutworms, and beetles in tomato; Crotalaria spp. host hairy caterpillars affecting castor.
  • Diseases: Wild mustard hosts club root (Plasmodiophora) and black leg in brassica crops; wild oats (Avena fatua) host stem rust of wheat, oats, and barley.

• Human Health Menace:

  • Airborne pollen from Ambrosia artemisiifolia (common ragweed) and Parthenium hysterophorus aggravates hay fever and asthma; direct contact causes severe dermatitis.
  • Poison ivy (Rhus sp.) causes severe skin allergies.
  • Aquatic weeds like water lettuce (Pistia lanceolata), Salvinia auriculata, and alligator weed (Alternanthera spp.) provide breeding habitats for mosquito vectors transmitting malaria, yellow fever, dengue fever, filariasis, and encephalitis.
  • Contamination of wheat flour with cocklebur seeds imparts a bitter taste and gastric irritation.
  • Contamination of mustard seed with Mexican poppy (Argemone mexicana) causes epidemic dropsy, glaucoma, blindness, or death due to the toxic alkaloids sanguinarine and $11\text{-oxotriacontanoic acid}$.

• Aquatic Ecosystems, Forest, and Land Impacts:

  • Aquatic weeds impair water aesthetics, impede navigation, hinder fishing, retard irrigation canal flow, and emit foul odors upon decomposition.
  • Parthenium hysterophorus has invaded extensive forest tracts in Karnataka and Andhra Pradesh.
  • Mikania micrantha affects $11\%$ of high forest and $38\%$ of plantation forest areas in West Bengal.
  • Heavy infestations of Cyperus rotundus and Cynodon dactylon render agricultural land unsuitable for cultivation, forcing land abandonment.

• Beneficial & Economic Uses of Weeds:

  • Fiber & Thatching: Typha spp. and Saccharum spp. are used for ropes and thatch boards.
  • Food & Flavoring: Cichorium intybus roots flavor coffee; Amaranthus viridis, Chenopodium album, and Portulaca spp. serve as leafy vegetables; Chlorella pyrenoides (algae) is consumed as a protein supplement in China and Japan.
  • Plant Breeding: Saccharum spontaneum was crossed to breed noble sugarcane varieties.
  • Nematicidal Effects: Soil incorporation of Crotalaria, Parthenium, Calotropis, and Eichhornia reduces root-knot nematode populations.
  • Forage: Grassland weeds like Cynodon dactylon, Cenchrus ciliaris, Dichanthium annulatum, and Eclipta alba provide animal forage.
  • Biological Control Hosts: Trichogramma chilonis parasitizes castor semilooper eggs while residing on weeds.
  • Indicator Geobotany: Commelina spp. indicate copper deposits; Eichhornia crassipes indicates heavy metals ($Cu$, $Zn$, $Pb$, $Cd$) in water bodies via satellite imagery.
  • Green Manure & Composting: Tephrosia purpurea and Croton sparsiflorus act as green manure; Eichhornia crassipes and Pistia stratiotes are utilized for composting.
  • Soil Reclamation & Conservation: Argemone mexicana reclaims alkali soils; wild mustard indicates $NO_2$ air pollution, whereas chickweed indicates $SO_2$ pollution; Agropyron repens controls soil erosion through extensive root systems; Dichanthium annulatum stabilizes field bunds; Opuntia dillenii serves as a live biological fence.
  • Medicinal Uses: Leucas aspera (snake bites), Calotropis procera (gastric disorders), Argemone mexicana (skin diseases), Phyllanthus niruri (jaundice), Striga orobanchioides (diabetes management).
  • Commercial Products: Incense sticks (agarbathis) from Cyperus rotundus tubers; essential aromatic oils from Andropogon spp. and Cymbopogon spp.

Detailed Classification Systems of Weeds

• Morphological / Cotyledon Classification:

  • Monocots (Narrow-leaved):
  • Leaf traits: Narrow, upright, parallel venation, low herbicide spray retention.
  • Root system: Adventitious root system.
  • Internal structure: Open growing point; scattered vascular bundles (cambium).
  • Includes Grasses and Sedges (Note: Cyperaceae and Typhaceae are narrow-leaved monocots but are morphologically distinct from true grasses).
  • Dicots (Broad-leaved):
  • Leaf traits: Broad, horizontal leaves, reticulate venation, high herbicide spray retention.
  • Root system: Tap root system.
  • Internal structure: Open growing point; intact, arranged vascular tissue.
  • Examples: Amaranthus spp., Chenopodium album, Convolvulus arvensis, Phyllanthus niruri, Parthenium hysterophorus, Xanthium strumarium.
  • Distinction Between Grasses and Sedges:
  • Grasses: Hollow cylindrical stem except at nodes; ligulate leaves; alternate or opposite leaf arrangement (e.g., Digitaria, Cynodon).
  • Sedges: Solid, triangular/angular stem; lacks ligules; leaves arranged in three-ranked whorls around the stem (e.g., Cyperus, Scirpus).

• Habitat / Situation Classification:

  • Terrestrial Weeds:
  • Cropland Weeds: Echinochloa colona in rice fields.
  • Non-crop / Wasteland Weeds: Tribulus terrestris, Xanthium strumarium.
  • Grassland / Pasture Weeds: Vernonia spp., Rumex spp.
  • Lawns & Public Parks: Lippia nodiflora, Eleusine indica.
  • Orchard & Garden Weeds: Euphorbia geniculata, Imperata cylindrica, Acalypha indica.
  • Plantation Crop Weeds: Eupatorium spp., Mikania micrantha.
  • Parasitic Weeds: Loranthus spp.
  • Roadside Weeds: Euphorbia spp., Lantana camara, Hyptis suaveolens, Prosopis juliflora.
  • Aquatic Weeds:
  • Submerged: Hydrilla verticillata, Utricularia stellaris.
  • Emerged: Typha angustata, Nelumbo speciosum.
  • Floating: Eichhornia crassipes, Pistia stratiotes.

• Origin Classification:

  • Indigenous (Native) Weeds: Originate within the national/geographical boundaries (e.g., Acalypha indica, Abutilon indicum, Sorghum halepense, Cynodon dactylon, Echinochloa colona).
  • Exotic / Introduced / Alien (Anthrophytes when human-aided): Introduced accidentally or intentionally from other countries; often lack native natural enemies, making biological control highly effective (e.g., Parthenium hysterophorus, Acanthospermum hispidum, Eichhornia crassipes, Argemone mexicana, Lantana camara, Croton bonplandianus).

• Association Classification:

  • Season-Bound Weeds: Expressed exclusively in specific seasons regardless of crop species.
  • Summer Perennials: Sorghum halepense.
  • Winter Perennials: Cirsium arvense.
  • Winter Annuals: Phalaris minor, Avena fatua.
  • Crop-Bound (Parasitic) Weeds: Require a specific host crop for full or partial nutrition.
  • Complete Root Parasite: Orobanche cernua (broomrape in tobacco).
  • Partial Root Parasite: Striga spp. (witchweed in millets/sorghum).
  • Complete Stem Parasite: Cuscuta chinensis (dodder in lucerne/berseem).
  • Partial Stem Parasite: Loranthus longiflorus (in fruit orchards/tea).
  • Crop-Associated Weeds: Associated with specific crops due to mimicry, shared microclimate requirements, or seed contamination.
  • Phenotypic Mimicry: Echinochloa colona mimics rice; Avena fatua (wild oats) and Phalaris minor (canary grass) mimic wheat height and maturation schedule.
  • Specific Microclimate: Cichorium intybus (chicory) and Coronopus didymus (swinecress) require cool, shady, moist conditions provided by lucerne and berseem canopy.
  • Seed Contamination: Phalaris minor and wild onion/garlic (Allium spp.) mature at identical heights and times as the crop.

• Stem Nature Classification:

  • Woody (Brush Weeds): Shrubs/undershrubs developing thick bark tissue (e.g., Lantana camara, Prosopis juliflora, Ziziphus rotundifolia).
  • Semi-Woody: Partially lignified stems (e.g., Croton sparsiflorus).
  • Herbaceous: Green, succulent, non-lignified stems (e.g., Amaranthus viridis, Chenopodium album).

• Soil pH & Edaphic Classification:

  • Acidophiles: Thrives in acid soils (pH\text{ }4.5\n\text{--}6.5) (e.g., Rumex acetosella, Pteridium spp., Spergula arvensis, Cynodon dactylon, Borreria, Digitaria sanguinalis).
  • Basophiles: Dominates saline ($pH\text{ }7.4\text{--}8.5$) and alkaline soils (e.g., Taraxacum stricta, Salsola spp. in saline soils; Cressa erecta, Sporobolus diander, Puccinellia spp. in alkaline soils).
  • Neutrophiles: Prefers neutral soils ($pH\text{ }6.5\text{--}7.4$) (e.g., Acalypha indica, Lagascea mollis, Euphorbia spp.).
  • Soil Texture Specificity:
  • Coarse/Sandy soils: Tribulus terrestris, Euphorbia spp., Mollugo oppositifolia, Oldenlandia umbellata, Leucas aspera.
  • Heavy/Clay soils: Sorghum halepense, Trianthema portulacastrum, Amaranthus viridis.
  • Black Cotton soils: Aristolochia bracteata, Hibiscus vitifolius.
  • Red soils: Commelina benghalensis, Leucas urticifolia.
  • Laterite soils: Bidens pilosa, Lantana camara, Spergula arvensis.

• Life Cycle / Ontogeny Classification:

  • Annuals: Complete life cycle within one year or single season; propagate exclusively via seeds.
  • Kharif / Summer Annuals: Trianthema portulacastrum, Digera arvensis, Setaria glauca.
  • Winter Annuals: Chenopodium album, Vicia spp., Avena fatua, Phalaris minor.
  • Multi-Season Annuals: Echinochloa colona, Eclipta alba, Eleusine indica, Phyllanthus niruri, Amaranthus viridis.
  • Ephemerals: Complete life cycle within $2\text{--}4\text{ weeks}$ (e.g., Phyllanthus niruri).
  • Biennials: Life cycle spans two years/seasons; 1st year produces vegetative rosette; 2nd year produces reproductive inflorescence ("bolting") (e.g., Daucus carota, Cirsium vulgare, Cichorium intybus, Taraxacum stricta, Alternanthera echinata).
  • Perennials: Live for more than two years; reproduce by seeds and vegetative organs.
  • Simple Perennials: Reproduce primarily by seed, but crown/root fragments regenerate if severed (e.g., Ipomoea carnea, Lantana camara).
  • Bulbous Perennials: Reproduce via underground bulbs/bulblets and seeds (e.g., Allium vineale, Allium canadense).
  • Creeping Perennials: Reproduce via rhizomes, stolons, creeping roots, and seeds (e.g., Convolvulus arvensis, Agropyron repens, Sorghum halepense, Cynodon dactylon, Cyperus rotundus).

• Specialized Classifications:

  • Poisonous Weeds: Cause severe illness or death in livestock or humans (e.g., Datura fastuosa, D. stramonium, D. metel, Withania somnifera berries, Abrus precatorius seeds, Lochnera pusilla, Solanum nigrum).
  • Noxious / Pernicious Weeds: Highly undesirable, persistent, difficult-to-control species possessing extraordinary reproductive and dispersal capacities (e.g., Cyperus rotundus, Cynodon dactylon, Cirsium arvense, Parthenium hysterophorus, Eichhornia crassipes, Lantana camara, Saccharum spontaneum, Imperata cylindrica, Striga spp.).
  • Objectionable Weeds: A noxious weed whose seeds are difficult to separate from crop seeds once contaminated.
  • Ecological Affinities:
  • Facultative Weeds (Apophytes): Native wild plants that adapt to crop fields and man-disturbed habitats (e.g., Opuntia dillenii).
  • Obligate Weeds: Confined exclusively to cultivated/disturbed land; vanish if land remains undisturbed for $2\text{--}3\text{ years}$ (e.g., Convolvulus arvensis).
  • Dryland Weeds: Deep-rooted, drought-tolerant species with hairy or mucilaginous stems (e.g., Tribulus terrestris, Convolvulus arvensis).
  • Wetland Weeds: Semi-aquatic habit, tender annuals surviving waterlogged conditions (e.g., Ammania baccifera, Eclipta alba).

Propagation, Seed Biology, and Dispersal Mechanisms

• Modes of Reproduction:
  • Sexual Propagation (Seeds):
  • Fusion of male and female gametes via fertilization.
  • Most weed species are monoecious; monoecious annuals/biennials produce vast quantities of seed.
  • Dioecious species (e.g., Cirsium arvense, Vallisneria spiralis) bear male and female flowers on separate plants, with seed set restricted to female individuals.

• Asexual Propagation:

  • Reproduction without gametic fusion via cell fission or spore formation (restricted to weedy algae, horsetails like Equisetum spp., and ferns).

• Vegetative Propagation:

  • Reproduction via detached vegetative structures; maintains genetic purity and enables rapid patch expansion in perennial vascular weeds.

  • Fecundity and Immediate Germination Characteristics:

• Seed Viability & Burial Depth:

  • Commelina benghalensis produces underground cleistogamous flowers and aerial chasmogamous flowers, placing seeds directly into the soil.
  • Rumex spp. seeds retain $63\text{--}90\%$ germination capacity after $2\text{--}7\text{ years}$ of burial at $30\text{ cm}$ depth.
  • Echinochloa, Poa, and Chenopodium spp. remain viable in soil for up to $25\text{ years}$.

• Vegetative Reproductive Structures:

  • Rhizomes & Rootstocks: Horizontal underground modified shoots with nodes, internodes, buds, and scale leaves (Imperata cylindrica, Cynodon dactylon, Agropyron repens [soboles]). Vertical downward-growing rhizomes are termed rootstocks (Sorghum halepense).

• Runners, Stolons, Suckers, Offsets:
  • Runners: Prostrate aerial branches trailing along soil surface, rooting at nodes (Cynodon dactylon, Oxalis corniculata, Centella asiatica).
  • Stolons: Arching horizontal stems hitting the soil surface to root (Rosaceae family).
  • Suckers: Horizontal stems creeping slightly below the soil surface (Hieracium spp.).
  • Offsets: Short, thick runners produced by floating aquatic weeds (Eichhornia crassipes, Pistia stratiotes).

• Tubers: Swollen terminal ends of wiry underground rhizomes/suckers containing nodes, scale leaves, and dormant buds (Cyperus rotundus).

• Bulbs: Compressed, disc-like stems surrounded by fleshy scale leaves and axillary buds (Allium vineale, Allium canadense).
• Bulbils & Bulblets: Modified aerial vegetative/flower buds capable of sprouting upon detachment (Oxalis corniculata, Bryophyllum pinnatum, Adiantum caudatum); turions in Hydrilla verticillata.
• Stem & Root Fragments: Detached stem segments of Cuscuta arvensis, Opuntia spp., and root cuts of Cirsium arvense, Convolvulus arvensis, Pluchea lanceolata regenerate complete plants. Propagules can be anchored up to $100\text{ cm}$ deep ($1\text{ m}$) in the soil profile.

• Dispersal Agents and Structural Adaptations:
  • Wind Dispersal (Anemochory):
  • Pappus: Persistent calyx modified into fine plume/parachute hairs (Taraxacum officinale, Asteraceae family, Typhaceae).

- Comose: Seeds covered partially or fully with silky hair (*Asclepias*, *Calotropis* spp., *Saccharum* spp.).

- Feathery Persistent Styles: Prolonged feathery style extensions (*Anemone* spp.).
- Balloon: Papery, inflated calyx enclosing fruit loosely with trapped air (*Physalis minima*).
- Wings: Winged seed/fruit appendages (*Acer macrophyllum*).

- Low Test Weight ($K$ = Voilure coefficient): Inverse density ratio; seeds with high $K$ values float easily. Example: $1,000\text{-seed}$ weight of *Polycarpaea corymbosa* is $0.03\text{ g}$ vs *Xanthium strumarium* ($194\text{ g}$).
- Tumbleweed Mechanism: High wind detaches whole mature dried plants of *Carthamus oxyacantha* and *Salsola kali*, rolling them across fields to drop seeds.
- Censer Mechanism: Wind sways rigid stems of *Argemone mexicana*, forcing mature capsules to scatter seeds through apical pores.

• Water Dispersal (Hydrochory):
  • Spongy aerenchymatous tissues keep aquatic seeds, fruits, and vegetative fragments buoyant in irrigation channels and streams.
• Animal Dispersal (Zoochory):
  • Endozoochory: Ingestion of seeds by animals with excretion of viable seeds. Excreta seed viability rates: Chicks $0.2\%$, Sheep $6.4\%$, Horses $8.7\%$, Calves $9.6\%$
  • Bird Dispersal: Lantana seeds disseminated by Indian myna and Chinese turtledove; Loranthus seeds carried on bird beaks and deposited on tree bark during beak-wiping.
  • Epizoochory: External attachment via hooks (Xanthium strumarium), stiff awns/barbs (Cenchrus spp.), sharp spines (Tribulus terrestris), scarious bracts (Achyranthes aspera), or sticky glands (Boerhavia repens).
  • Myrmecochory: Seed transport by ants attracted to specialized glandular seed secretions.

• Human Activity:
  • Aircraft tires introduced Tribulus terrestris into India.
  • Satellite Weeds: Seeds matching crop seed dimensions, maturity time, and height contaminate crop harvests (Phalaris minor and Avena fatua in wheat; Oryza spp. in rice; Cichorium intybus and Cuscuta in lucerne).
  • Farmyard Manure (FYM) & Silage: Convolvulus arvensis seed retains $4\%$ viability in FYM after 1 month of composting, $1\%$ after 2 months, and $0\%$ by 4 months.

Weed Ecology and Persistence Mechanisms

• Definitions:

  • Ecology is the study of inter-relationships between organisms and their environment.
  • Weed ecology deals with the growth characteristics, adaptations, and population dynamics enabling weeds to survive environmental changes.
  • Persistence is an adaptive mechanism enabling a weed species to repeatedly invade an environment even after targeted removal.
  • Hardiness refers to the physical ability to withstand local natural environmental stresses.

• Primary Mechanisms Driving Weed Persistence:

  • Prolific and viable seed production (e.g., Sonchus arvensis sets viable seed even if cut during flowering; uprooted Stellaria media and Portulaca oleracea mature viable seeds; Chenopodium flowers at $3\text{ cm}$ height under drought).
  • Seed Dormancy Mechanisms:
  • Enforced Dormancy: Caused by seed burial deeper than $5\text{ cm}$ due to tillage. In darkness, the phytochrome system remains in the inactive far-red absorbing form ($P_{fr} \rightarrow P_r$). Tillage brings seeds to the surface, exposing them to red light ($P_r \rightarrow P_{fr}$) and aeration, triggering germination (e.g., Lactuca spp., Xanthium spp., Rumex spp.).
  • Innate (Inherent) Dormancy: Genetically controlled resting state present upon seed shed; caused by immature/rudimentary embryos (Polygonum, Juncus, Eleocharis), hard seed coats impermeable to water/$O_2$ (Setaria, Ipomoea, Xanthium), or chemical germination inhibitors.
  • Induced (Secondary) Dormancy: Developed when non-dormant seeds encounter unfavorable conditions like elevated soil temperatures, high $CO_2$, low $O_2$, or soil waterlogging (e.g., Avena fatua, which exhibits innate, induced, and enforced dormancy).
  • Rapid seed dispersal and vegetative propagation.
  • Inherent Hardiness: C4 photosynthetic pathway in tropical weeds (Cyperus, Amaranthus) eliminates photorespiration; high transpiration efficiency; low nutrient requirement; rapid early shoot/root elongation.
  • Evasiveness: Bitter taste, bad odor, spiny protection, or morphological crop mimicry.
  • Self-Regeneration: Ability to germinate in uncultivated, non-friable seedbeds.
  • Selective Invasion & Environmental Adaptations:
  • Dryland shift: Tribulus terrestris, Argemone mexicana, Eragrostis, Euphorbia hirta, Celosia argentea dominate.
  • Irrigated shift: Trianthema portulacastrum, Phalaris minor, Commelina benghalensis emerge.
  • Paddy shift: Echinochloa, Eclipta, Caesulia axillaris dominate.
  • Weed Succession / Shift & Chemotypes:
  • Repeated selection pressure from continuous herbicide usage selects for resistant agricultural ecotypes ("chemotypes").
  • Continuous isoproturon use in wheat shifted weed flora from Phalaris minor to Avena fatua in Punjab.
  • Continuous butachlor use in rice shifted weed flora from Echinochloa spp. to Cyperus spp. in Tamil Nadu.

• Environmental Factors Regulating Persistence:

  • Climatic Factors: Light intensity/duration/photoperiod, temperature thresholds for dormancy release, rainfall distribution, wind velocity, atmospheric humidity.
  • Edaphic Factors: Soil moisture, aeration, soil pH (acidophiles, basophiles, neutrophils), soil fertility status.
  • Biotic Factors: Crop canopy competition, grazing, crop root exudates/toxins, and cultural management practices (e.g., shallow tillage reduces nut sedge; continuous flooding kills Cynodon dactylon).

Crop-Weed Competition and Allelopathy

• Principles of Competition:

  • Competition occurs when crops and weeds draw upon shared, limited growth resources: nutrients, moisture, light, and space ($CO_2$).
  • A $1\text{ kg}$ increase in dry weed biomass generally results in a $1\text{ kg}$ decrease in dry crop biomass.

• Specific Resource Competition Dynamics:

  • Nutrients: Weeds absorb nutrients faster and in larger quantities than crop plants.
  • Nitrogen: Amaranthus spp. accumulate over $3\%$ N in dry matter (nitrophiles).
  • Phosphorus: Digitaria spp. accumulate over $3.36\%$ P.
  • Potassium: Chenopodium album and Portulaca spp. accumulate over 4.0\%\text{ }K_2O$.\n - Zinc: *Setaria lutescens* accumulates 585\text{ ppm} Zn (3 times that of cereal crops).\n - **Moisture:** Consumptive water use of *Chenopodium album* is 550\text{ mm}$versus$479\text{ mm}$for wheat. Weeds extract soil moisture evenly down to$90\text{ cm}$depth, whereas early wheat moisture uptake is restricted to top$15\text{ cm}$. Weeds on fallow land consume$70\text{--}120\text{ ha}\cdot\text{mm} of soil water.\n - **Light (Solar Energy):** Shading during early growth severely reduces crop photosynthesis. C4 weeds outcompete C3 crops under high light and temperature.\n - **Space (CO_2$):** More efficient$CO_2 fixation in C4 weeds gives them a growth advantage in crowded canopies.\n\n- **Critical Period of Crop-Weed Competition:**\n - Defined as the shortest time span in the crop growth cycle during which weed competition causes maximum yield loss, and during which keeping the crop weed-free yields the highest economic return (generally the first 1/4$to$1/3$of total crop life, or first$20\text{--}30\text{ days} for annual crops).\n\n- **Critical Periods Across Major Field Crops:**\n\n| Crop | Critical Weed-Free Period |\n| :--- | :--- |\n| Transplanted Rice | Up to 6 weeks after transplanting (WAT) |\n| Direct Sown Rice | Up to 6 weeks after sowing (WAS) |\n| Wheat and Ragi | Up to 5 WAS |\n| Maize | 2\text{--}6 WAS |\n| Bajra (Pearl Millet) | Up to 4 WAS |\n| Jowar (Sorghum) and Sesame | 2\text{--}5 WAS |\n| Sugarcane | 12\text{--}14 WAS |\n| Sunflower | 2\text{--}4 WAS |\n| Groundnut | 4\text{--}7 WAS |\n| Green Gram and Black Gram | 4\text{--}6 WAS |\n| Red Gram (Pigeonpea) | Up to 7 WAS |\n| Horse Gram | 3\text{--}6 WAS |\n| Cotton | 6\text{--}9 WAS |\n| Castor | 3\text{--}9 WAS |\n| Tobacco | Up to 9 WAT |\n\n- **Factors Influencing Competition Intensity:**\n - Weed and crop density (yield loss vs. weed density displays a sigmoidal relationship; jointvetch or barnyard grass at >10\text{ plants/m}^2$reduces rice yield by$2,000\text{ kg/ha}$and$1,100\text{ kg/ha}, respectively).\n - Narrowing row spacing in wheat from 20\text{ cm}$to$15\text{ cm}$reduced dry matter yield of *Lolium* and *Phalaris* by$11.8\%$and$18.3\%, respectively.\n - Crop species/varietal competitiveness: Barley > Rye > Wheat > Oats. Spreading groundnut cultivars (TMV-3) suffer only 15\%$yield loss from weeds compared to$30\% loss in bunch cultivars (TMV-2).\n\n- **Allelopathy (Teletoxy):**\n - Termed by Hans Molisch in 1937; refers to the suppression of one plant species by another through the production and release of secondary chemical metabolites (allelochemicals such as benzoic acid, cinnamic acid, phenolic acids, coumarins, hydroquinones, benzoquinones).\n - Autotoxy: Self-inhibition of parent plants by allelochemicals released by daughter plants (*Parthenium*).\n - Release Pathways: Volatilization from leaves (Lamiaceae, Asteraceae); foliar leachates (*Eucalyptus*); root exudates (*Cirsium arvense* roots releasing parahydroxybenzoic acid); tissue decomposition.\n - Specific Interactions:\n - Dodder (*Cuscuta*) powder and carrot grass (*Parthenium*) powder exert toxic effects on water hyacinth and aquatic weeds.\n - Marigold (*Tagetes erecta*) and *Cassia* spp. (*Cassia sericea*, *Cassia tora*) exert strong allelopathic suppression over *Parthenium hysterophorus*.\n - *Eucalyptus* leaf leachates suppress *Cyperus rotundus* and *Cynodon dactylon*.\n - Root/tuber exudates of *Cyperus esculentus* and *Quackgrass* reduce dry matter and nutrient uptake in maize.\n - *Imperata cylindrica* inhibits emergence of *Borreria hispida*.\n - Root exudates of wheat, oats, and peas suppress *Chenopodium album*.\n - Germination stimulants: Corn root exudates stimulate *Orobanche minor* and *Striga hermonthica*; sorghum root exudates release kinetin stimulating *Striga asiatica*; cotton roots exude *Strigol*, triggering witchweed seed germination.\n\n# Methods of Weed Management\n\n- **Historical Evolution Timeline:**\n - 10,000\text{ BC}: Hand removal at the dawn of agriculture.\n - 6,000\text{ BC}: Primitive hand tools.\n - 1,000\text{ BC}: Animal-drawn weeding implements.\n - 1920\text{ AD}: Tractor/machine-drawn weeding equipment.\n - 1930\text{ AD}: Biological control agents introduced.\n - 1947\text{ AD}: Chemical control via organic herbicides.\n\n- **Core Principles of Weed Management:**\n - **Prevention:** Preventing the entry, introduction, and establishment of new weed species into uninfested areas.\n - **Eradication:** Complete destruction and elimination of all live plants, vegetative propagules, and viable seeds from a defined geographic area.\n - **Control:** Reduction of weed infestation and growth below the economic injury threshold.\n\n- **Preventive Measures:**\n - Sowing certified weed-free crop seeds.\n - Preventing contamination of livestock feed, hay, and grain.\n - Applying fully decomposed farmyard manure (FYM); maintaining compost pit temperatures at 65\text{--}90^ ext{o}\text{C}$for$4\text{--}5\text{ months} or treating manure with acrocyanamide, SMDC (metham), DMTT (mylone), ammonium thiocyanate, or synthetic urea.\n - Cleaning farm machinery, harvesters, and tools when moving between fields.\n - Keeping non-crop lands, bunds, and irrigation/drainage channels clean.\n - Enforcing strict national and international plant quarantine laws.\n - Crop Seed Cleaning Techniques: Sieving (mesh screens), salt solution dipping (5\text{--}10\%$$NaCl solution causes light weed seeds to float), winnowing, pneumatic/aspirator specific gravity separators, velvet rollers/conveyer belts (separates rough weed seeds from smooth crop seeds), magnetic separators (iron powder adheres to rough weed seeds), electrical separators (voltage field separation based on surface charge), dented plates, photo tubes.\n\n- **Physical and Mechanical Control Methods:**\n - **Hand Weeding:** Manual uprooting or removal using a sickle or khurpi.\n - **Hand Hoeing:** Cutting weeds at the soil surface/crown level using hand hoes.\n - **Spudding:** Hand hoeing supplemented with a sharp-edged sickle.\n - **Sickling:** Cutting tall weed growth above ground level using a sickle to prevent seed production.\n - **Digging:** Excavating deep root systems/propagules using a crowbar or pickaxe.\n - **Mowing & Cutting:** Repeated cutting of vegetation at uniform height in non-crop areas using lawn mowers or power cutters.\n - **Dredging & Chaining:** Aquatic weed removal; dredging scrapes bottom mud and rhizomes; chaining drags a heavy iron chain along ditch bottoms using tractors to sever rooted weeds.\n\n\n\n - **Flooding:** Submerging fields under water for 2\text{ weeks}$or more excludes$O_2$, suffocating terrestrial weeds (e.g., *Haveli* system in MP: deep flooding of fallow land during the monsoon for$2\text{--}3\text{ months}).\n - **Burning & Flaming:** Thermal destruction of weed protoplasm; flaming exposes weeds briefly to 1,000^ ext{o}\text{C} using flame throwers in crop rows (cotton, onion, fruit orchards) or to control dodder (*Cuscuta*) in lucerne.\n - **Soil Solarization:** Covering moist soil with thin (20\text{--}25\text{ }\mu\text{m}$) transparent polyethylene (PE) film during the hottest summer months for$2\text{--}4\text{ weeks}$; raises top$5\text{ cm}$soil temperature by$10\text{--}12^ ext{o}\text{C}, desiccating seeds of *Phalaris minor* and *Avena fatua* (Note: hard-seeded *Melilotus* spp. resist solarization).\n\n\n\n - **Cheeling:** Scraping weed growth and topsoil using a long-handled spade ("cheel") in tea plantations.\n - **Mulching:** Covering soil surface with continuous black plastic sheets or organic materials (paddy husk, groundnut shells, sawdust) at a thickness of 10\text{--}15\text{ cm} to exclude sunlight; *Tripsacum laxum* (Guatemala grass) and *Cymbopogon* spp. (citronella grass) are used as mulches in coffee and tea.\n\n- **Cultural & Crop Husbandry Methods:**\n - Maintaining optimum plant population and early seedling vigor.\n - Selective Crop Stimulation: Applying soil amendments (gypsum/lime), localized fertilizer band placement, and seed inoculation with Rhizobium or phosphate-solubilizing bacteria to selectively favor the crop over weeds.\n - Proper Planting Methods: Furrow planting in dry summer months restricts irrigation water to the furrow, keeping ridges dry and weed-free.\n - Adjusting Planting Time: Sowing slightly earlier or later than the peak weed emergence flush.\n - Crop Rotation: Rotating crops disrupts satellite weed cycles (e.g., rotating wheat with pea/gram breaks *Avena fatua*; lucerne with grains breaks *Cuscuta*; lowland flooded rice breaks *Cyperus rotundus*).\n - Stale Seedbed Technique: Irrigating or rain-soaking a prepared seedbed to induce a flush of weed germination; germinated weed seedlings are destroyed using non-residual contact herbicides (paraquat) or light shallow tillage prior to crop sowing.\n\n\n\n - Smother / Competitive Crops: Quick-germinating, dense-canopy crops (*Vigna unguiculata* [cowpea], lucerne, berseem, millets) shade out weeds.\n - Summer Fallowing / Summer Tillage: Off-season tillage during April, May, and June exposes rootstocks and tubers of *Cynodon dactylon* and *Cyperus rotundus* to intense solar heat and desiccation.\n\n# Biological Control and Bioherbicides\n\n- **Biological Control:**\n - The deliberate utilization of natural living organisms (insects, herbivorous fish, plant pathogens, competitive plants, snails, mites) to reduce weed population densities below economic injury levels.\n\n- **Merits and Demerits:**\n - Merits: Environmentally safe, no toxic chemical residues, self-perpetuating, cost-effective long-term control, highly suited for non-crop areas and water bodies.\n - Demerits: Slow acting, cannot achieve total eradication, bio-agent rearing/multiplication can be expensive, risk of non-target crop attack if host-specificity tests fail.\n\n- **Biological Control Approaches:**\n - **Classical Biological Control:** Introduction of an exotic bio-agent from the native home of an invasive alien weed. The agent population multiplies, suppresses the weed, drops in number due to food starvation, and eventually stabilizes in a balanced cycle with the target weed at low density.\n - **Bioherbicide / Mycoherbicide Philosophy:** Native or exotic plant pathogens are mass-reared artificially and applied in sprayable formulations using standard spray equipment to knock down current season weed flushes without self-perpetuating cycles.\n\n- **Classical Biological Control Agents & Success Stories:**\n - *Lantana camara*: Controlled in Hawaii (1902) by the moth *Crocidosema lantana*; seedfly *Ophiomyia lantanae*, Hispine beetles (*Octotoma scabripennis*, *Uroplata girardi*), and tingid bug (*Teleonemia scrupulosa*).\n - *Opuntia* spp. (Prickly Pear): Controlled in Australia by the moth *Cactoblastis cactorum*; controlled in Tamil Nadu and Maharashtra, India by the cochineal scale insect *Dactylopius tomentosus* and the spider mite *Tetranychus desertorum*.\n - *Alternanthera philoxeroides* (Alligator Weed): Controlled by flea beetle *Agasicles hygrophila* and thrips *Amynothrips andersoni*.\n - *Eichhornia crassipes* (Water Hyacinth): Controlled by the water hyacinth moth *Sameodes albiguttalis* and curculionid weevils *Neochetina eichhorniae* and *Neochetina bruchi*.\n - *Salvinia molesta* (Water Fern): Controlled in Kerala, India by the curculionid weevil *Cyrtobagous salviniae* and grasshopper *Paulinia acuminata*.\n - *Parthenium hysterophorus*: Controlled in India by the leaf-eating beetle *Zygogramma bicolorata*, stem-galling moth *Epiblema strenuana*, and stem-galling beetle *Conotrachelus* sp.\n - Herbivorous Fish: White amur / Chinese grass carp (*Ctenopharyngodon idella*) consumes massive quantities of aquatic weeds (>5\text{ kg/year}$growth, reaching$50\text{ kg}); Common carp (*Cyprinus carpio*) controls submerged aquatic vegetation.\n\n\n\n\n\n- **Commercial Bioherbicides / Mycoherbicides:**\n\n| Product Trade Name | Biological Agent Constituent | Target Weed Species & Application |\n| :--- | :--- | :--- |\n| **De-Vine** | Liquid suspension of fungal spores of *Phytophthora palmivora* | Causes root rot in strangler vine (*Morrenia odorata*) in citrus orchards. |\n| **Collego** | Wettable powder containing spores of *Colletotrichum gloeosporioides* f. sp. *aeschynomene* | Causes stem and leaf blight in jointvetch (*Aeschynomene* sp.) in rice fields. |\n| **Bipolaris** | Fungal spore suspension of *Bipolaris sorghicola* | Controls Johnson grass (*Sorghum halepense*). |\n| **Biolophos** | Microbial fermentation toxin from *Streptomyces hygroscopicus* | Non-specific general vegetation knockout. |\n| **Luboa-2** | Fungal spores of *Colletotrichum gloeosporioides* f. sp. *cuscutae* | Controls dodder (*Cuscuta* spp.). |\n\n- **Integrated Weed Management (IWM):**\n - An integrated system combining cultural, physical, biological, and chemical weed management methods into a unified, eco-friendly framework to maintain weed populations below economic injury levels while preventing weed shifts and herbicide resistance.\n\n# Herbicide Classification, Formulations, and Adjuvants\n\n- **Herbicide Usage Trends:**\n - Herbicides account for over 45\%$of total pesticide consumption in developed countries, compared to$8\% in India.\n - Average annual herbicide consumption: India 40\text{ g/ha}$, advanced nations$675\text{--}1,350\text{ g/ha}$, Japan$5,000\text{ g/ha}$. Annual Indian production capacity is approximately$6,000\text{ metric tonnes}$, with$3/4 applied in plantation crops.\n\n- **Classification Based on Chemical Nature (31 Major Groups):**\n - **Inorganic Herbicides:** Acids (arsencomous, sulfuric acid) and inorganic salts (sodium chlorate, copper sulfate, sodium arsenate).\n - **Organic Herbicides:**\n 1. Aliphatic acids: Dalapon, TCA, Glyphosate, Methyl bromide, Cacodylic acid, MSMA, DSMA.\n 2. Amides: Alachlor, Butachlor, Propachlor, Metolachlor, Diphenamid, Propanil.\n 3. Benzoics: 2,3,6\text{-TBA}, Dicamba, Tricamba, Chloramben.\n 4. Bipyridyliums: Paraquat, Diquat.\n 5. Carbamates: Propham, Chlorpropham, Barban, Dichlormate, Asulam.\n 6. Thiocarbamates: Butylate, Diallate, EPTC, Molinate, Triallate, Thiobencarb (Benthiocarb), Metham.\n 7. Dithiocarbamates: CDEC, Metham.\n 8. Nitriles: Bromoxynil, Ioxynil, Dichlobenil.\n 9. Dinitroanilines: Fluchloralin, Trifluralin, Pendimethalin, Nitralin.\n 10. Phenols: Dinoseb, DNOC, PCP.\n 11. Phenoxy acids: 2,4\text{-D}$,$2,4,5\text{-T}$, MCPA, MCPB,$2,4\text{-DB}, Dichlorprop.\n 12. Triazines: Atrazine, Simazine, Metribuzin, Ametryn, Terbutryn.\n 13. Ureas: Monuron, Diuron, Linuron, Metoxuron, Isoproturon, Methabenzthiazuron.\n 14. Uracils: Bromacil, Terbacil, Lenacil.\n 15. Diphenyl ethers: Nitrofen, Oxyfluorfen, Nitrofluorfen.\n 16. Aryloxyphenoxy propionates: Diclofop, Fenoxaprop-P, Quizalofop-P, Haloxyfop-P, Fluazifop-P.\n 17. Cyclohexanediones: Sethoxydim, Clethodim, Tralkoxydim, Cycloxydim.\n 18. Imidazolinones: Imazapyr, Imazamethabenz, Imazaquin, Imazamox, Imazethapyr.\n 19. Isoxazolidinones: Clomazone.\n 20. Oxadiazoles: Oxadiazon.\n 21. Oxadiazolides: Methazole.\n 22. N-phenylphthalimides: Flumiclorac.\n 23. Phenylpyridazones: Sulfentrazone.\n 24. Phthalamates: Naptalam.\n 25. Pyrazoliums: Difenzoquat, Metflurazone.\n 26. Picolinic acids / Pyridines: Picloram, Dithiopyr, Pyrithiobac, Fluridone, Thiazopyr.\n 27. Quinolines: Quinclorac.\n 28. Sulfonylureas: Bensulfuron, Chlorimuron, Metsulfuron, Sulfosulfuron, Triasulfuron.\n 29. Triazolinones: Pyridates.\n 30. Cineoles: Cinmethylin.\n 31. Others: Pyrazon, Endothall, Amitrole, Anilofos.\n\n- **Classification Based on Application Method and Timing:**\n - **Soil-Active vs. Foliage-Active:**\n - Soil-Active: Applied to soil to kill germinating seeds or sprouting tubers/rhizomes (e.g., simazine, alachlor, trifluralin, EPTC).\n - Foliage-Active: Applied directly to emerged plant foliage (e.g., paraquat, glyphosate, propanil).\n - **Soil Application Methods:**\n - Surface Application: Uniform surface spraying/broadcasting.\n - Soil Incorporation (PPI): Mechanical incorporation into topsoil to prevent volatile or photodecomposition losses (e.g., EPTC, fluchloralin, trifluralin).\n - Sub-Surface Layering: Concentrated band placement 7\text{--}10\text{ cm} below soil surface to control perennial weeds (*Cyperus rotundus*, *Convolvulus arvensis*).\n\n\n\n - Band Application: Herbicide applied in a narrow band over the crop row, leaving inter-rows untreated to save costs.\n - Soil Fumigation: Volatile chemical application by soil injection (chloropicrin), plastic tarp containment (methyl bromide), or surface drenching (metham).\n - Herbigation: Application of herbicides via irrigation water (e.g., fluchloralin in tomatoes/chillies; EPTC via sprinklers in lucerne).\n - **Foliage Application Methods:**\n - Blanket / Over-the-top: Uniform spray over crops and weeds (requires high crop selectivity, e.g., 2,4\text{-D} in wheat).\n - Directed Spraying: Spraying directed between crop rows to avoid crop foliage.\n - Spot Treatment: Treating localized patches of noxious perennial weeds.\n - Protected Spraying: Using physical shields/hoods over spray nozzles or plants to prevent spray contact with crop foliage.\n - Brush / Tree Methods: Foliage treatment (
  • Classification Categories:
  • Algae:
    • Phytoplankton / Water Blooms: Single-celled/colonial blue-green algae causing water blooms, $CO_2$ release, and $O_2$ depletion.
    • Filamentous / Non-planktonic: Thread-like, net-like forms changing water taste/odor (Chara, Nitella, Spirogyra, Cladophora).

- Vascular Hydrophytes:
  - Emerged: Rooted in bottom mud with leaves/flowers elevated above water (*Nelumbo lutea* [lotus], *Nymphaea odorata* [water lily], *Trapa natans* [water chestnut], *Nymphoides*).
  - Submerged: Rooted (*Hydrilla verticillata*, *Vallisneria spiralis*, *Potamogeton* spp.) or Suspended non-rooted (*Ceratophyllum demersum*, *Utricularia*).

  - Floating: Free-floating (*Eichhornia crassipes*, *Pistia stratiotes*, *Salvinia*) or Rooted-floating (*Nymphaea*).

  - Marginal: Thrives in saturated soil or shallow shorelines up to $60\text{--}90\text{ cm}$ water depth (*Typha angustata*, *Polygonum*, *Alternanthera*, *Ipomoea aquatica*).

• Aquatic Weed Herbicidal Control Protocols:

  • Acrolein: $4\text{--}7\text{ ppmv}$ for submerged weeds (Potamogeton).
  • Dalapon: $15\text{--}20\text{ kg/ha}$ for cattails; Dalapon ($5\text{--}10\text{ kg/ha}$) + Amitrole-T ($2.0\text{ kg/ha}$).
  • Amitrole-T: $5\text{--}10\text{ kg/ha}$ for cattails, water hyacinth, and alligator weed.
  • $2,4\text{-D}$: $4\text{--}8\text{ kg/ha}$ for floating and emerged weeds (Note: mandatory $30\text{-day}$ waiting period before consuming fish from $2,4\text{-D}$ or $2,4,5\text{-T}$ treated waters).
  • Diquat & Paraquat: $0.5\text{--}1.0\text{ ppmv}$ for submerged weeds and algae.
  • Endothall: $1.0\text{--}5.0\text{ ppmv}$ for submerged weeds.
  • Fenac: $15\text{--}20\text{ kg/ha}$ to prevent root/rhizome regeneration.
  • Algicides: Copper sulfate ($0.1\text{ ppm}$ for Microcystis water blooms, $1.0\text{ ppm}$ for algal scum, $5.0\text{ ppm}$ for Chara spp.); Simazine, Diuron, Monuron ($1.0\text{--}2.0\text{ ppmv}$).

• Profiles and Management of Specific Problematic Weeds:

  • Water Hyacinth (Eichhornia crassipes - Pontederiaceae):
  • Free-floating perennial introduced to India in $1896$ from Brazil; glossy leaves, bulbous spongy petioles, purple-black feathery roots, lavender/pink aerial flower spikes. Reproduces via stolons and seeds (viable for $>15\text{ years}$ in bottom mud; $3,000\text{--}4,000$ seeds per flower; 1 plant covers 1 acre in a year).
  • Control: Mechanical sieves/harvesters; $2,4\text{-D}$ sodium/amine/ester ($2.0\text{--}8.0\text{ kg/ha}$); $2,4\text{-D}$ ($4.0\text{ kg/ha}$) + Paraquat ($0.5\text{ kg/ha}$); Amitrole-T ($0.5\text{--}1.5\%$ concentration); Bio-agents Neochetina eichhorniae, N. bruchi, and Sameodes albiguttalis.
  • Utilization: Biogas generation, paper pulp/board manufacture, non-ruminant animal feed, organic compost/mulch, roof thatching.
  • Cat Tail (Typha angustata - Typhaceae):
  • Perennial grass-like marsh weed, $1.5\text{--}2.0\text{ m}$ tall, thick leaves $5\text{--}12\text{ mm}$ wide lacking a midrib; unisexual flowers on a single stem forming a brown cylindrical female spike $10\text{--}40\text{ cm}$ long; produces $10,000\text{--}20,000$ minute hair-attached anemochorous seeds per spike; extensive creeping rhizomes.
  • Control: Summer water desiccation; competitive displacement by Panicum purpurascens or Brachiaria mutica (Para grass); Dalapon ($2\%$ spray); Dalapon + Amitrole ($15 + 3\text{ kg/ha}$); Amitrole + TCA ($5 + 10\text{ kg/ha}$); pre-emergence Simazine or Diuron on canal banks.
  • Swamp Morning Glory (Ipomoea aquatica - Convolvulaceae):
  • Trailing/floating herbaceous vine with hollow stems $>3\text{ m}$ long, milky sap, rooting at nodes; funnelform white/pink flowers; woody oval capsules containing 1-4 hairy seeds.
  • Control: Chinese grass carp (Ctenopharyngodon idella); $2,4\text{-D}$ sodium salt ($4.0\text{ kg/ha}$) + Paraquat ($0.5\text{ kg/ha}$).
  • Bermuda Grass (Cynodon dactylon - Poaceae):
  • Deep-rooted creeping perennial grass with rhizomes and stolons; 4-5 terminal purple spikes ($10\text{ cm}$ long); a single shoot covers $2.5\text{ m}^2$ in $150\text{ days}$.
  • Control: Summer deep tillage ($7\text{--}14\text{ days}$ desiccation); polyethylene solarization; selective graminicides (Sethoxydim, Fluazifop, Clethodim); Glyphosate or Amitrole-T ($1.0\text{--}2.0\text{ kg/ha}$) applied 1 week prior to deep tillage.
  • Purple Nut Sedge (Cyperus rotundus) & Yellow Nut Sedge (C. esculentus) - Cyperaceae:
  • Perennial sedge ranked as the world's worst weed (infests 52 crops across 92 countries); underground wiry runners form chains of black, round, dormant tubers in the top $10\text{--}30\text{ cm}$ soil; 10-30 florets per spikelet.
  • Control: Summer deep tillage; black solarization film ($1000\text{ gauge}$); $2,4\text{-D}$ or MCPA ($2.0\text{--}5.0\text{ kg/ha}$) + Trifluralin; Glyphosate ($1.0\text{ kg/ha}$); Atrazine for seedlings; Metham or Methyl Bromide soil fumigation in nurseries.
  • Carrot Grass / Congress Grass (Parthenium hysterophorus - Asteraceae):
  • Noxious thermo- and photo-insensitive annual alien weed covering 5 million ha in India; grows $0.5\text{--}2.0\text{ m}$ tall; finely dissected basal rosette leaves; creamy white flower heads ($4\text{ mm}$ across); produces $5,000\text{--}10,000$ pappus-armed seeds per plant (viable for $20\text{ years}$ in soil).
  • Contains the sesquiterpene lactone toxin parthenin, causing severe allergic contact dermatitis, asthma, and mental depression in humans.
  • Control: Manual uprooting prior to flowering; competitive biological replacement by sowing Cassia sericea, Cassia tora, Cassia auriculata, Amaranthus spinosus, Sida acuta, Tephrosia purpurea, Stylosanthes scabra, or marigold (Tagetes spp.); $2,4\text{-D}$ esters ($2.0\text{--}5.0\text{ kg/ha}$) or $15\text{--}20\%$ common salt drench; pre-emergence Atrazine, Alachlor, Chlorimuron ($0.2\text{--}0.4\text{ kg/ha}$), or Metsulfuron ($0.003\text{--}0.0045\text{ kg/ha}$); bio-agents Zygogramma bicolorata (leaf-eating beetle) and Epiblema strenuana (stem-galling moth).