Integrated weed management in rice
By Dr M. Farooq, Dr
Shahzad Basra & Basharat Ali Saleem
WEEDS reduces the yield and quality of a crop. It also
raises the cost of production. In case of paddy, the yield
drops by 15-20 per cent and sometimes, even 50 per cent.
A crop badly infested with weeds, normally fails in
totality. Every year there is huge loss of paddy crop while
the quality of produce from weed infested fields is also
inferior. These also obstruct harvesting.
Weed-crop competition: Weeds compete with desirable plants.
Competition denotes a relationship between the same or
different species which leads to the flourishing of one at
the expense of the other or at the expense of both.
While weed competition with rice does not normally lead to
death of either species, it almost results in decreased
yields. Weeds are known but the extent of problem they pose
is not recognized. Farmers acknowledge the problem in their
fields but the high labour cost of hand weeding discourages
adequate response.
The competition between weeds and rice depends on the
following influences:
i. Relative growth stages of rice and weed.
ii. Nature of stand establishment (transplanting versus
direct seeding).
iii. Density of planting.
iv. Rice variety.
v. Moisture and nutrient availability.
In irrigated systems, rice seedlings are transplanted into
puddled soil. This gives rice a substantial head-start on
weeds and initially, competition is minimal. Competition
increases as growth progresses, particularly in stands of
direct-seeded rice, since weeds germinate at the same time
and compete for light and nutrients with the rice seedlings.
Weed competition generally takes three forms:
Competition for light: Weeds that are shorter than rice crop
throughout growth period compete a little or not at all with
rice for light. However, weeds that are taller can reduce
the light available to rice by as much as 50 per cent. Since
sunlight provides the main source of energy utilized by
plants for manufacturing food, shading by tall weeds can
significantly stunt growth and reduce yields.
Competition for water: Where water is plentiful, competition
between rice and weeds is minimal but during shortage the
situation is quite different. If weeds consume significant
portion of water then tillering, flowering, and grain
filling are delayed or impeded.
Competition for nutrients: Weeds have a large nutrient
requirement. They are big feeders and can, if left
uncontrolled, absorb more of the soil nutrients than the
crop. Fertility increases, though fertilization is generally
accompanied by increase in weeds, which can result in larger
yield reductions.
Reduction of grain quality: Weed seeds in grain reduce the
price. Weed seeds in grain can also cause uneven moisture in
the grain causing loses in quality due to the formation of
molds and/or due to cracking losses during milling.
In addition to competing with rice for sunlight, water, and
nutrients, weeds pose another problem. Many weed species act
as alternate hosts for insect pests and disease-causing
organisms, and their presence among the crop or along bunds
and peripheries can increase losses due to insect or disease
attack.
Why are some weeds successful?
Weeds become successful because of their characteristics
that give them the ability to:
i. Set seed before the crop ripens.
ii. Produce large quantities of seed (e.g., Cyperus
difformis can set 100,000 seeds per plant).
iii. Seeds survive in the soil.
iv. Reproduce vegetatively, which aids their dispersal and
makes them difficult to control.
v. Mimic the crop (e.g., red rice – which cannot be
distinguished from the crop in early stages but which sets
seed and then shatters before the crop is harvested).
vi. Grow vigorously which allows them to out-compete the
crop.
Integrated weed management: Integrated weed management makes
use of a combination of different agronomic practices to
manage weeds, so that reliance on any one weed control
technique is reduced.
Reducing the reliance on one or two specific weed control
techniques means that those techniques or tools will be
effective for future use. The object of integrated weed
management is to maintain weed densities at manageable
levels while preventing shifts in its populations to more
difficult-to-control ones. Losses caused by weeds will be
minimized without reducing farm income.
Controlling with one or two techniques gives the weeds a
chance to adapt to those practices. For example, the use of
herbicides with the same mode of action, year after year,
has resulted in weeds that are resistant to those
herbicides.
Integrated management uses a variety of techniques to keep
weeds “off balance”. Weeds are less able to adapt to a
constantly changing system that uses many different control
practices, unlike a programme that relies on one or two
control tools. Integrated weed management practices in rice
include:
Land preparation: Thorough land preparation can
significantly decrease the incidence of weeds in rice by
destroying all weeds and weed seeds to provide weed-free
conditions at the time of planting, and providing a good
environment for rapid growth of rice seedlings.
Water management: Many weeds cannot germinate or grow in
flooded soils, making water management an extremely
effective tool for controlling, particularly grasses and
sedges.
When the transplanted seedlings have established themselves
(approximately one week after transplanting), completely
flood the plot to a depth of 3”-4” to inhibit weed growth.
As the rice grows, gradually increase the depth to 6”. The
soil must be submerged completely and uninterruptedly if
flooding is to be effective.
Hand weeding: Hand weeding is time consuming and tedious.
When weeds are large enough to be gripped, they are pulled
out of the soil and discarded. Smaller weeds can be
hand-pulled. Early hand weeding is better, since any delay
will enable the weeds to absorb nutrients.
A common fallacy is that small weeds do not affect the rice
but they certainly do, as a simple weeding demonstration
will show.
Hand hoeing: Hand hoeing is used as a method of weed
control, particularly where line-planting is practiced. Hand
hoeing is faster than hand weeding and works well against
creeping perennials.
Crop rotation: Since every crop has its own characteristic
weeds, continued cultivation of the same crop in one plot
allows these weeds to build up. Rotation of rice with kharif
crops may result in reduced infestations of water tolerant
weeds in the subsequent crops.
Crop rotation with allelopathic crops and rice cultivars:
Some crops such as sorghum, pearl millet and maize may
drastically suppress the weed population and reduce its
biomass. Pearl millet may exhibit residual weed suppression
in the following crop. The inclusion of these fodder crops
before the rice crop in a rice-wheat rotation may provide
satisfactory weed control and can minimize the use of
herbicides. It is obviously necessary to evaluate whether
these crops can be grown successfully.
Herbicides: The importance of herbicide use is closely
related to the cost and availability of labour. Herbicides
are one of the first labour saving technologies to be
adopted as labour costs rise. As a consequence, the use of
herbicides varies considerably between countries. Herbicides
replace hand weeding and enable direct seeding rather than
transplanting which is less labour demanding.
Direct seeding is linked to the use of herbicides, as
without their use the weeds grow so rapidly in the stages
before the fields can be flooded, that manual means of
control are often not feasible.
Herbicides are also used in the transplanted systems. The
costs involved with herbicide use are likely to remain a
major constraint to their widespread adoption. Herbicides
may be classified as non-selective or selective, and
pre-and-post emergence.
Most herbicides used in rice production are selective,
controlling some or most weeds, while having a limited
effect on the crop. Selectivity is not necessarily dependent
upon the compounds, but also on the rates, timing and
methods of application, and hence it is important to follow
the manufacturer’s recommendations.
Non-selective herbicides such as glyphosate are sometimes
used before establishing rice, on weed infestations such as
wild rice which are difficult to control with selective
herbicides.
Pre-emergence herbicides are applied to the soil and control
weeds before they emerge, while post-emergence are applied
to weeds after they emerge. Among the amide group are the
herbicides butachlor, pretilachlor and propanil. Butachlor
can be applied either as pre-emergence or early post
emergence to give control over a wide range of annual
grasses and some broadleaf weeds.
Courtesy: The DAWN
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