Weed and there control

1. WEED AND THERE CONTROL
Plants are differentiated into crop which meet the need of man and
weed which compete with crops. The first person to use the word weed
in the present day meaning was JethroTull. Subsequently, several
definition was proposed as-
•A weed is a plant growing where is not wanted.
•Weed is an unwanted plant
•A plant out of place
•A plant that is extremely noxious, useless, unwanted or poisonous.
•Any plant or vegetation, excluding fungi, interfering with the
objectives or requirements of people
From the definitions, it can be seen that, as long as a plant is not
interfering with human activities, it is not called as a weed.

2. Weed problems:
Weed deplete crop’s environment of nutrients, water and light.
Crops yield are reduced considerably by weeds. In addition, weeds interfere
in cause inconvenience to agricultural operations. The quality of produce is
reduced by weed infestation.
Extent of losses in crop yield:
Extent of losses due to weeds depends on intensity of infestation,
time of occurrence and type of weeds. For example, wheat infested heavily
(435 plants/m2) with wild oats (Avena fatua) theyield loss occurred mainly
due to reduction in grain number/year followed by reduction in tiller
number. Losses due to weeds in other crops varied with crops.
Chillies was most sensitive to weeds followed by sesame, cowpea,
soybean and groundnut. The average losses due to weeds in different crops
are in the range of 30-40 % in soybean, maize , potatoes vegetables etc. and
15-20% in cereals.

3. Crop Pest Alternate host (weeds)
Red gram Green caterpillar Amaranthus, datura
Castor Hairy caterpillar Crotalaria sp.
Rice Stem borar Echinochaloa, panicum
Wheat Black rust Agropyron repens
Maize Downey mildew Sacharuns pontaneum
Pearlmillet Ergot Cenchrus cillaris
Harbour pests and diseases:
•Weed present in the off season on field bunds, wastelands, irrigation
channels etc.
•Harbor pests and diseases which attack the crop nearby.
Alternate hosts of some important pests and diseases

4. Classification of weeds;
There are over 30000 species of weeds around the world of which about 18000
cause serious losses to crops. Though weeds can be classified in different ways,
the important classification that helps in selection weed control methods are
based on cotyledonary number, morphological and other charecterstics;
•Duration: depending on the duration of life cycle, weeds are classified into
annuals, biennials and perennials. Most of the weeds present in annual crops
are annual weeds. There duration is short. They complete their life cycle within
a year or a season. They multiply by seeds and produce large number of seeds/
plant. Prevention of flowering or removal of annual weeds before flowering is
important for their control. Example of annual weeds are: Amaranthus spp.,
Argemone spp., Avenafatua spp., Chenopodium album etc.

5. Cont….
• Biennial weeds: complete their life cycle in two years or in two
seasons. In the first year, they remain vegetative and in the second
year, they produce flowers and seeds. They need two distinct
seasons, one for vegetative and another for reproductive growth.
Some of the biennial weeds are Alternantheraechinata,
Eichoriumintybus etc.
• Perennial weeds: They grow for more than two years. These
weeds propagate by plant parts in addition to seeds.
The most common and problematic perennial weeds are
Cynodon dactylon and Cyperus rotundus.

6. COTYLEDONS
•Base on the number of cotyledons, weeds are classified into monocotyledons
and die cotyledons.
•This classification is useful in chemical weed control.
•Some herbicides like 2,4 – D, MCPA etc. are more effective in dicotyledonus
weeds while dalapon, fluchoralin are more effective on monocotyledonus
weeds.
• Monocots have narrow and upright leaves whereas dicots have broad and
horizontal leaves with terminal bus visible at the tip.

7. Morphological characters:
Weeds are grouped into grasses, sedges and broad leaved weeds.
This is the most widely use classification as there leaves branches,
underground parts are different, they need different techniques to control
each group.
Almost all weeds that belong to family graminaceae are known as grasses .the
weeds which to belong to family cyperaceae are known as sedges.
While all dicotyledonus weeds are known as broad leaves weeds.

8. Weed
ranking
Botanical name Common name Type of weeds
1 Cyprus rotundus Nut grass Sedge
2 Cynodon dactylon Bernuda grass Grass
3 Echnichloa colonun Barnyard grass Grass
4 Echinochloa crusgalli Jungle rice Grass
5 Elusine indica Goose grass Grass
6 Sorghum halpense Jhonson grass Grass
7 Imperata cylindica - Grass
8 Eichornia crassipes Water hyacinth BLW
9 Portulaca oleracea - BLW
10 Chenopodium album Fat hen BLW
BLW= Broad leaf weeds

9. Characteristics of weeds:
Though crops and weeds are plants, they are certain distinct features of
weeds some of which are helpful to plan weed control strategy;
•Seed production: weeds produce large number of seeds/plant compared to
crops.
•Size of seeds: most of the weeds produce small sized seed. Since the seed
reserves are very limited, they can germinate only from the top layer of the
soils.
•Dormancy: most of the weed seeds have dormancy. Dormancy acts as a
safety mechanism against adverse environmental conditions. They germinate
only when sufficient amount of rainfall is received for them to survive. The
inhibitory chemicals from the seeds should be washed away for their
germination.

10. Cont….
•Vegetative weed production; Some weeds propagate vegetatively by
rhizomes, stolon’s, bulbs etc. this is another mechanism for weeds to
withstand adverse conditions and help in persistence.
•Root stem: Monocotyledonous weeds depend on seminal roots during
seedling stage. Once nodal roots are developed at subsequent stage, they hold
the soil tightly. It is difficult to remove them either by hand weeding or by
inter cultivation. At this stage, it is difficult to kill them even by herbicides.
Dicotyledonous weeds have taproot system as a rule and as age
advances, taproot grows deeper into the soil, secondary thickening occurs and
food materials which are stored in the roots. At this stage, even if the shoot is
cutoff the weeds may survive.
•Type of photosynthesis: Most of the weeds have C4 type of photosynthesis. It
confers the advantage during moisture stress and also depletes carbon dioxide
to a very low level in the crop micro climate.

11. Crop weeds competition:
• Plants require nutrients, water, light and carbon dioxide for their growth.
Light and CO2 are used by the leaves.
• Competition of these growth factors is therefore, above the ground.
• The competition for nutrients and water is below the ground as these are
absorb by the roots.
• As the plants grow, LAI and root density increase leading to mutual
interference in the absorption of one or more growth factors.
• Growth rate falls below that obtains from plants grown without
interference.
• The close proximity of heats and their number causes sub-optimal
absorption of growth factors resulting in reduction in growth and yield of
crops.

12. Crop weed competition is for the following:
•Competition for nutrients
•Competition for light
•Competition for water
•Competition for CO2

13. Alleopathic effect:
Most of the plants excrete certain chemical into a soil which inhibit
germination and growth of other plants in their vicinity and this phenomenon
is called allelopathy.
The degree of alleopathic effect depends on the plants species. The important
weed species that show allelopathic effect are agropyronrepens, sorghum
halepense, lantana camara, Cyperusrotundus, Euphorbia naculata etc.
The extacts from these rhizomes of Ambrosia reduce germination and seedling
growth of several crops like wheat, oats, tomato and lucerne. Exudates of roots
of wild oat plants at 2 and 4 leaf stage are toxic to wheat crop.
Similarly extraction from Abutilon theophrasti decreased water status of
soybean plants by its effect on absorption of water. This induced water
shortage caused stomatal closure and break down of chlorophyll in soybean.

14. CRITICAL PERIOD OF WEED COMPETITION:
It kind be defined as the shortest time span during the crop growth when
weeding results in highest economic returns.
The crop yield level obtain by reading during this period is almost similar to
that obtained by the full season weed free conditions.
The critical period of weed competition is also defined as the period between
early growth during which weeds can grow without affecting crop yield and
the point after which weed growth does not affect the yield.
The critical period of weed competition is around 30 days for most of the
crops.

15. Factors influencing critical period of weed
competition:
The critical period of weed competition starts with the
beginning of interference from weeds and ends when the crop covers
80% of the soil.
The length of the critical period of weed competition depends
on the following:
•Nature of crop
•Varieties
•Growing conditions

16. Establishment of weeds;
The number of weeds that germinate at a particular time depends on three
factors.
1. Seed bank: The vast reservoirs of dormant and viable seeds in the top
layer of the soil decide the weed population. Approximately 73 genera of
weeds are found in this top layer of soil.
2. Dormancy: It may be due to water and oxygen impermeable seed coats,
low temperature requirements, special light requirement, presence of
germination inhibiting chemicals and immaturity of embryo. Weed seed
exhibit three types of dormancy i.e. enforced, innate and induces.
 Enforced dormancy is due to their presence in deeper layers absence of
light induces dormancy.
 Innate dormancy is due to geniticalfator
 Induced dormancy is due to high temperature, high CO2 , low oxygen
and water logging.
3. Longity: Unlike the seeds of crop plants weed seeds remain viable for a
long time and germinate when favourable condition prevail. Lotus seeds
found in a lake bed in Manchuria were approximately 1000 year old and
still viable.

17. Weed control methods
These are grouped into cultural, physical, chemical and biological methods.
Every method of weed control has its own advantages and disadvantages.
No single method is successful under all weed situations.
Many times a combination of these methods gives effective and economic
control than a single method.
•Cultural method of weed control
Cultural practices like tillage, planting, fertilizer application, irrigation etc.
are employed for creating favorable condition for the crop. These practices
if used properly help in controlling weeds.
In addition, aspects like selection of variety, time of sowing, cropping
system, cleanliness of the farm etc. are also useful in controlling weeds.

18. •Physical method of weed control:
physical force either manual, animal or mechanical power is used to
pull out or kill weeds.
Depending on weed and crop situation one or combination of the
following are used:
•Hand weeding
•Hand hoeing
• Digging
•Mowing
•Cutting
•Dredging and Chaining
•Burning and flaming
•Mulches
•Intercultivation

19. Biological weed control
Now a days interest is increasing in biological controls of weeds, especially for
equatic weeds as other methods involve much labour and cost.
These methods have to be repeated as all the weeds or atleast those which
reproduce vegetatively or have long seed viability, are difficult to control.
Bioagents like insects, pathogen etc. and other animals are used to control
weeds.
Insects and pathogens influenced weeds and they either reduce growth or kill
weeds.
In biological control method, it is not possible to eradicate weeds, but weed
population can be reduced.

20. Outstanding and feasible examples of biological weed control:
•Insects: Prickly-pear weed (opumtia) was control in India
dactylopiustomentosus, a scale insects by
•Fish: Common carp and chinese carp control aquatic weeds
•
•Mammals: mantee or sea-cow is very effective in controlling water hyacinth
•
•Snales: Merisa sp. and other fresh water snales feed on submerged weeds
like coontail and algea
•Fungi: water hyancinth can be control by rhizoctinia blight
•Mites: a spider mites (Tetranychus sp.)I found to be useful in controlinf
prickly pear
•Plants: cowpea sown in between sorghum rows effectively reduces the
growth of weeds.

21. Chemical weed control:
The commercial product of herbicides contains the actual herbicides,
adjuvants and other filling materials are used in chemical weed control.
The percentage of adjuvants and other inert material used as fillers depends
on formulation and the company which is manufacturing the particular
herbicide.
Active ingredient:
The chemical in commercial product that is directly responsible for the
herbicidal activity is called active ingredient.
Amount of commercial product to be is used = Recommended dose x 100
a.i.

22. HERBICIDES ARE USED IN FOLLOWING WAYS:
Pre-planting application: Application of herbicide before sowing (or along with
sowing) termed as pre planting or pre plant application.It is use for controlling
perennial weeds such as Cyperusrotundus
Pre-plant soil incorporation (PPI): A herbicide is also required to be
incorporated in a soil soon after its application it is term as pre plant soil
incorporation. It is use for controlling perennial weeds such as
Cyperusrotundus
Pre-emergence application: A herbicide is applied to soil soon after soon, crop
before emergence of weeds is termed as pre- emergence applications.
Normally,the herbicides are sprayed within three days after sowing.
Post planting application: In this method the herbicides are applied after
planting the crop. The herbicides are generally applied at a particular stage of
standing crop.
Post-emergence herbicides: when herbicides are applied in young weed
seedling standing among the crop plant it is called as post emergence
application. In this method the herbicides are applied 20 to 30 days after
sowing or transplanting.

23. Weed control in important crops
Herbicide Dose (kg ai/ha) Time of application
Rice
2,4-D Na salt
2,4-D ester
Benthiocarb
Butachlor
Pendimethalin
0.8
0.9
2.0
1.25 to 2.0
0.75 to 2.0
21-28 DAT/DAS
5-7 DAT/DAS
2-3 DAT/DAS
2-4 DAT/DAS
3-5 DAT/DAS
Wheat
2,4-D amine
2,4-D Na salt
2,4-D ester
Isoproturon
0.8 to 1.0
0.4 to 1.0
0.45
1.0
21 DAS
28-35 DAS
30-45 DAS
30-35 DAS

24. Herbicide Dose (kg ai/ha) Time of application
Sugarcane
Atrazine
Nitrofen
Simazine
0.5 to 2.5
2.5 to 5.0
1.0 to 2.0
Pre to crop and weed
Pre to crop and weed
Pre to crop and weed
Maize
Atrazine
Simazine
2,4-D ester
1. to 2.0
0.5 to 1.0
0.9
Pre to crop and weed
Pre to crop and weed
2-3 DAS
Sorghum
Atrazine
Simazine
2,4-D ester
0.25 to 1.5
0.5 to 1.0
0.9
Pre to crop and weed
Pre to crop and weed
2-3 DAS

25. Herbicide Dose (kg ai/ha) Time of application
Groundnut
Alachlor
Fluchloralin
Nitrofen
Pendimethalin
1.5 to 2.5
1.25 to 1.75
3.00 to 4.00
0.75 to 1.50
Pre to crop and weed
PPI
Pre to crop and weed
Pre to crop and weed
Cotton
Diuron
Oxadiazon
2.0
1.5
Pre to crop and weed
Pre to crop and weed