Weed biology is the study of the establishment, growth, reproduction, and life cycles of weed species and weed societies/vegetation. Weed biology is an integrated science with the aim of minimizing the negative effects, as well as using and developing the positive effects, of weeds.

1. Weeds
• Weed is a plant grown out of place and interferes
with man’s objectives
-Economic losses
-Quantitative and qualitative loss of
crop/yield
-Loss of land values etc.
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2. Characteristics of weeds
• Efficient reproduction by producing larger number
of seeds
• Adaptation for different dispersal mechanisms
• Ability to compete with the crop well
• Possess the weed seed dormancy
• Possess extensive root systems
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3. Why weeds are troublesome
• Compete with crops for, Light, Water, Nutrient and Space.
“Struggle between two or more spices to acquire a source
which is limited is known as competition”
• Photorespiration is the process which caused to make
difference between C3 and C4 plants.
• C4 are better competitors than C3 plants, 90% of weeds
are C4 plants.
• Majority of crop plants are C3plants
• Maize, Sorghum, Sugarcane are the major C4 crops, they
are having higher efficiency in Co2 fixation.
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4. • Weeds are Alternative Host for insect pest and pathogens.
• Parasitism - obtain resources directly from other plants.
• Allelopsthy – Production and release of compounds from
one plant that are toxic to other plants,
• Allelocchemicals - The toxic which are produces and
release to the environment.
• Phytotoxins - The toxic which are produces and do not
release but stored.
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5. • Prolong dormancy :
• Echinochla crus – Galli - 3 -4 Months
• Parthenium hysterophorous - 2 – 20 years
• Memosa pigra - 23 -50 years
• Non – synchronous breaking of dormancy
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6. Weed
Classification
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9. Classification of weeds
1. Classification based on morphology
2. Classification based on life spans
3. Classification based on number of cotyledons
4. Classification based on nature of stem
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10. 1. Classification based on
morphology
-Grasses
- Sedges
- Broad leaves
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14. Grasses
• Belong to family Poaceae
• Stem -cylindrical in shape
-solid nodes
-hollow internodes
• Leaf -leaf sheaths and leaf blade are present
• Inflorescence is present
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15. • Scientific name: Echinocloa colonum
• Common name: Jungle rice
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16. Sedges
• Belong to family Cyperaceae
• Leaves are mostly emerged from the base having
modified stem with or without tubers
• Stem is solid
• No nodes or internodes are present
• Cross section is angular shape
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17. • Scientific name: Cyperus rotundus
• Common name: Nut sedge
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18. Broad leaves
• All dicotyledonous weeds are broad leaved weeds
• Stem is solid and rounded in shape
• Leaves are broad in shape
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19. • Scientific name: Flavaria australacica
• Common name: Clustered yellow tops
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20. 2. Classification based on life
spans
-Annuals
-Biennials
-Perennials
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21. Annuals
• Weeds that complete their life cycle within one year
• Propagated commonly by seeds
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22. • Scientific name: Commelina benghalensis
• Common name: Wandering jew
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23. Biennials
• Weeds that complete the vegetative growth in the
first year, flower and set seeds in the second year
• Found mainly in non-cropped areas
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24. • Scientific name: Daucus carota
• Common name: Wild carrot
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25. Perennials
• Weeds that complete their life cycle for more than
two years
• Have different types of propagation methods
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26. • Scientific name: Sonchus arvensis
• Common name: Field sow thistle
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27. 3. Classification based on
number of cotyledons
-Monocot
-Dicot
• All grasses and sedges are monocots
• All broad leaved weeds are dicots
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28. Monocots
• Scientific name: Panicum flavidum
• Common name: Yellow watercrown grass
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29. Dicots
• Scientific name: Crotalaria verucosa
• Common name: Blue rattle pod
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30. 4. Classification based on
nature of stem
- Woody weeds
- Semi-woody weeds
- Herbaceous weeds
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31. Woody weeds
• Scientific name: Lantana camera
• Common name: Wild sage
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32. Semi-woody weeds
• Scientific name: Croton sparsiflorus
• Common name: Ban tulasi
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33. Herbaceous weeds
• Scientific name: Amaranthus viridis
• Common name: Slender amaranth
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35. Control of Negative impact of Weeds
1. Prevention – prevent entering of weeds to the field
through machineries, vehicles, crop seeds, water, insect
and wind (weed seeds).
• Preventive measures
1. Cleaning of irrigation cannels
2. Recommended to use certified seeds
3. Quarantine regulations
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36. 2. Eradication – Removal of weeds up to zero level.
• Weeds will not be invasive
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37. Weed Management
1. Physical / Manual methods - Hand weeding, Burning
2. Agronomic / Cultural Practices – Ploughing, Harrowing
Mulching
3. Mechanical / use of instruments - Tractors, Harrowers
4. Chemical methods – Herbicide ( Herb – Plant Cide –Killer )
5. Biological methods
6. Integrated weed management
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39. Classification of herbicides
• Synthetic chemicals used to control or suppress the growth and
development of weeds.
• Can be classified in to different groups based on,
1. Mode of action - Contact – Propanil, Paraquat (Gramoxone)
Systemic - Glyposate, MCPA,
Glufosinate, Basta
2. Time of application – pre emergent , post emergent
pre planting / post planting
3. Selectivity – selective , non selective
4. Place of application – soil, foliage 39

40. Biological Methods
• Salvinia molesta - Cyrtobagous salviniae
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41. • Eichhornia crassipes – Neochetina eichhorniae
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42. • Chromolaena odorata – Pareuchaetes pseudoinsulata
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43. Integrated weed management
• Integration of two ore more methods for controlling weeds.
1. Proper land preparation
2. Use weed free planting materials
3. Proper crop establishment techniques
4. Proper agronomic practices
1. Clean implementation
2. Fertilization
3. Water management
4. Weed free organic fertilizers
5. Timely use herbicides - proper selection, effective dosage,
correct method.
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