Weeds
Extent
of Weed damage
- Reduction in the
yield of crops by way of competition for water, nutrients and sunlight,
which are vital for survival.
- Loss in quality of
crop due to contamination with seeds and offensive odour
- Weeds harbour pests
and diseases by acting as alternate and collateral hosts
- Weeds interfere with
agricultural operations like sowing, irrigation, harvesting etc. thereby
increasing the cost of field preparation and inter-cultivation operations.
- Many of the weeds
like parthenium are allergic to human and cattle population. Poisonous
weeds kill livestock and human beings.
Characteristics of weeds
- Higher seed
production
- Smaller seeds with
lesser weight can disperse better
- Effective methods of
resting periods by undergoing dormancy
- Propagate faster
through vegetative propagation
- Prolific root
development provides better survival capacity
- Most of the weeds are
C4 plants which are efficient photo synthesizers and widely adopted to
harsh conditions
Crop Weed competition:
Initially both crop plants and weeds have low leaf
area index and root density. Therefore no competition is noticed between the
two types of crop plants. But as age advances, both compete
Competition above ground: Normally competition exists for light and carbon
dioxide
Light:
weeds grow faster and shade the slow growing crop plants thereby reducing the
vigor and strength of the crops
CO2: Since
most of weeds are C4 plants, they do effective photosynthesis even in low
carbon dioxide concentrations. Majority of the crop plants being C3 in nature,
they compete poorly with weeds thereby affected by the weed competition.
Below ground: Mainly both compete for nutrients and water
Nutrients: The
plant analysis shows that weeds contain more nutrients than the crop plants in
the same piece of land. Weeds uptake more nutrients and are also cause more
denitrification by associating with the denitrifying bacteria.
Water: The
amount of water required to produce unit amount of dry matter is more for weeds
compared to crop plants. Therefore weeds require more water to produce same
biomass. Thus deplete more moisture from the soil giving stiff competition to
weeds.
Allelopathic effect: Most of the weeds excrete certain chemicals, which
inhibit the germination and growth of other plants in their vicinity.
Critical period of weed competition:
“Shortest time span during the crop growth where
weeding results in highest economic returns. Normally it is 30 days for most of
the crops. The factors responsible for it are nature of crop, type of variety
and growing conditions.
Weed
control
Cultural Control:
Field
preparations:
- Cleaning of
irrigation channels to avoid movement of weed seeds from one field to
another
- Deep ploughing in
summer months exposes rhizomes, stolons and other underground vegetative
propagules of weeds to scorching sun and kills them.
- 2-3 Conventional
tillages followed by blade harrowing to be done
- Puddling reduces the
weed growth.
Planting methods:
- Sowing of seeds free
from weed seeds
- Sowing of crop seeds
with seed drill disturbs to layers of soil and control weeds
- Transplantation
control weeds by two means, first by the way of puddling and secondly by
the well grown seedlings which can overcome the weed pressure on heir
earlier growth.
Varieties: Short
statured and erect leaved varieties should be avoided. Good competing varies
should be selected for sowing in highly weed infested areas. Planting density
should also be maintained higher with higher rate of seed rate.
Crop rotation: Proper rotation of crops suitably reduces the weed infestation to a
greater extent.
Physical
control & Mechanical control
- Hand weeding: This is
done to temporarily reduce the weed infestation by engaging labour to
remove weeds by hand. Here the number of weeding and time interval between
two weeding is very important. However, this is not effective against
perennial weeds.
- Hand hoeing: removal
of weeds with mechanical implements.
- Mowing
- Dredging and Chaining
- Burning and flaming
- Mulches
- Inter-cultivation by
ploughing between the rows and use of rotary and blade type weeders.
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