1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Weed Management in Organic Rice
I.S.Hooda
Department of Agronomy
Haryana Agril.University
Hisar-125004, India
The major impediment in the cultivation of rice is the heavy weed infestation,
which compete with the crop to such an extent that the crop gets smoothered
by the weeds. The weeds shared not only the plant nutrients but transpire a lot
of valuable conserved water from the soil. The weeds also sometime serve as
host for breeding and development of certain disease and pests. The delay in
first weeding beyond 15-25 days after seeding sharply reduces the rice yield.
Rice field weed species may conveniently be classified into broad leaved,
grasses, sedges and others.
. High fertility accompanied by high moisture provides a situation where the
intensity & growth of weeds are high and in many cases may adversely affect the
production potential of crops. Let us take into consideration the world’s most
important food crop ‘Rice’ that is the staple food of over half of the world
population. The estimated yield losses in rice caused by weeds are given below:
Rice (transplanted) - 30-40% (Bhan(1994);
Rice (drilled)
70-80% (Balasuberamaniyan & Palaniappan)
The weed problem in upland rice is more serious as compared to transplanted rice.
The important weed species, which act as constraints in production potential of rice
are given below:
Sr. No.Scientific name
English name
Local name
1.
Echinochloa colonum Wild rice
chotta sawank
2.
Echinochloa crusgalli Banyard grass sawank
3.
Cyperus rotundus
Purple nut sedge
motha
4.
Cyperus irri
Yellow sedge
chhatriwala dila
5.
Cyperus difformis
Common sedge
motha
6.
Eclipta alba
false daisy
jal bhangra
7.
Celosia argentia
Cock’s comb
chilmil, sarai
8.
Dactylotenium aegypticum Crowfoot grass
makra
9.
Setaria glauca
Cattail millet
laptana, banra
10.
Scripus spp.
Club rushes
11.
Panicum spp.
12.
Paspalum spp.
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Weed management: Management of weeds is an important component of
production techniques as elimination of weeds is expansive and hard to achieve.
Presence of weeds is a constraint & their improper management further accentuates
the effect. In the past two decades work has been done on non-chemical
management techniques and environmentally safe alternatives to herbicides for
weed control. Such ecofriendly techniques for weed control in rice fields are given
below:
Preventive measures:
1. Always use well rotten and decomposed organic manure.
2. Avoid feeding of grain screenings or hay containing weed seeds without
destroying their viability by grinding or cooking, otherwise weed will spread
from dung or manure.
3. Clean all the implements & machinery properly after their use in infested
areas & before using in clean areas.
4. Keep irrigation and drainage channels free from weeds.
5. Watch seedlings in nursery carefully so that they do not get mixed with
weed seedlings & get carried to the fields.
6. Use of weed free seeds/seedlings for sowing/transplanting from a reliable
source. It is a preventive method against introduction of weeds.
Under these practices, use of clean seeds for sowing and of weed free seedlings for
transplanting should be practised. New exotic introduction of weed species perhaps
(seed/seedling contamination) is the one of the reason and if care is taken timely, it
can be managed easily.
summer ploughing: Ploughing in summer exposes the underground parts like
rhizomes and tubers of perennial and obnoxious weeds to scorching summer sun
and kills them. It also helps in improving the soil physical conditions. Conventional
tillage that includes 2-3 ploughings followed by harrowing decreases the weed
problem in upland rice.
Field preparation: Better land preparation (2 ploughing at 15 DBS and 2 at
sowing), timely sowing (last week of June), Optimum management of nutrients and
an additional hand weeding markedly decreased the infestation of all category of
weeds in rainfed upland rice. In lowland rice, puddling operation incorporates all
the weeds in the soil, which would decompose in course of time & that is the
reason for reduced weed infestation in transplanted rice as compared to direct
seeded rice.
Planting method: Sowing should be done 2-3 days after irrigation in upland rice.
Weeds already present in soil start germinating with 2-3 days. Sowing operation
with seed drill removes some of germinating weeds as blade harrow is run to cover
the seeds. During this process, all the surface soil to a depth of 2-3 cm is disturbed
2
1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
,thereby uprooting the germinating seeds. In addition due to loosening of surface
soil, it dries up quickly & does not allow weed seeds to germinate until subsequent
rain or irrigation. For this reason weed population is high when irrigation is given
after sowing or rain is received after sowing.
Transplanting is another operation that reduces weed population since the crop has
an additional advantage due to its age as a result covering the ground early. The
clean field preparation for transplanting helps in reducing weed germination. Weed
population is more in direct seeded rice as compared to transplanted rice.
Bed planting or ridge planting systems also helps in reducing weed population in
rice. In these systems, we can easily control the weeds in the furrows between the
rows of rice crop by mechanical methods like manual weeding, blind tillage,
flooding, burning, mulching, solarization etc.
The main processes involved are germination and emerging of seedlings from the
seed or establishment of seedling after transplanting. The best method is bed
planting for both direct seeded and transplanted rice. It was found more economical
in terms of saving of water, weed management and finally the yield. Direction of
beds did not differ significantly. Crop-weed competition studies have shown that
biomass and reproductive potential of weeds is significantly reduced if the light
competition by the crop is improved ( Baumann D.T.,Bastiaans L., Kropff
M.T.,1999). Thus weeds remained under check on beds with two rows. Furrow is
irrigated + mulched with straw. The total population remained the same. Direct
seeding in time on flat method was also found good if weed free conditions are
maintained upto four weeks after germination of crop.
Relay planting at different places showed minimum weed population (Broadcasting
of rice seeds in standing wheat crop with last irrigation applied).
Intercropping:
Mixing of legumes will cover the ground quickly and many workers have observed
smoothering effect on weeds especially in direct seeded rice. In Operational
Research Project, the rice + sorghum, rice + Hibiscus spp or rice + teosinte seeding
geometry found to be the best because of uneven distribution of rainfall and
accumulation of flood water in plain zone of Haryana state of India.
One hand weeding 3 weeks after sowing recorded superiority in managing the
weeds.
Plant population: Plant population & row arrangement also affect the weed
growth. Higher plant population & narrow row spacing can put pressure on the
availability of space for weed growth. Closer planting of crops suppresses
germination and growth of weeds due to allelopathy and competition for growth
factors. A narrow (15cm) spacing was found superior to wide (30-45cm) spacing in
minimizing weed competition & increasing productive tillers & yield in upland
rice. Research showed that increasing the seed rate from 40 to 100kg/ha for direct
seeded rice decreased weed weight from 52 to 188g/m2 and increased rice yield
from 2.78 to 3.45t/ha. Higher of plant population in transplanted rice had lower dry
weights of weeds.
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Blind tillage/Stale seedbed: This method is extensively used to combat weeds in
rice in early stages before the sowing or just after sowing but before the crop
emergence. In this method, weeds are allowed to germinate and then their above
ground parts are destroyed by using various types of blade harrows as soon as the
weed appears.
Hand pulling and hand hoeing: Despite major advantages in chemical control,
hand removal of weeds still remains to be the most practical method of weed
control in many developing countries. Although back breaking & laborious, hand
weeding is quite effective if employed at the right time. Two aspects are important
in hand weeding: the no. of hand weeding to be done & interval between two hand
weeding. The number of hand weeding depends on crop growth, weed growth &
critical period of crop-weed competition.
Hand hoeing is done in upland crop, the entire surface soil is dig to shallow depth
with hand hoes, weeds are uprooted & removed. It also improves soil physical
condition.2-3 manual weeding at 25 and 45 DAS in direct seeded rice and at 15 and
30 DAT in transplanted rice is the best and most effective way of weed
management.
Post plant tillage: Rice rotary weeder is a specific tool for weeding in rice fields.
Single or double row weeders are also available. They are worked manually with
back and forth movement in between rice rows in wetlands. This rotary weeder is
less strenuous than hand weeding. At the time of weeding, it is essential to retain
1.5-2.0 cm depth of water. With the single row rice weeder, one labor can cover
about 0.5 ha/day. 2-3 weeding with paddy weeder at 15 and 30 DAT in
transplanted rice is the best and most effective way of weed management.
Mechanical methods:
Under this, different tools & implements have been tried. Irrigate the field before
planting and allow the first flesh of weeds to germinate which are controlled by the
bullock drawn disc harrow and then seeding the rice. In North India, in last week of
May, irrigate the field to a depth of 40mm of water and after 10 days, disc
harrowing (bullock drawn) is done to prepare the field. Then sowing of rice is to be
completed within two days. Consequently, threshold value is achieved.
Various types of hoe & weeder have been tried in the standing crop of rice. which
worked manually with back & forth movement in between rice rows. Just like a
wheel hand hoe and one person can weed 0.5 ha/day (8 hours work). Work is in
progress for sensor technique attached to mechanical weeder, which identify and
suppress the weeds.
Recently zero-tillage technique has been successfully demonstrated at farmers’
field. Data collected indicate that weed weight in dry season is less under zerotillage as compared to conventional tillage seeding method. Similar trend was
followed to weed counts both for grassy & broad-leaved weed species. It is more
economical method in terms of time, energy and money (unpublished data).
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Solarization: It involves covering the soil surface with polythene sheets to increase
the soil temperature, which would be lethal to weed seeds. A mulching period of 26 weeks with clear polythene sheets has been reported to give effective control of
many annual weeds. Irrigation prior to solarization has a complementary effect, as
moisture-imbibed seeds are more sensitive to heat than dry seeds. The limitation
however is that normally weed seeds up to about 5 cm depth are only affected
implying that deep prepatory cultivation would nullify the effect. Although this
technique is limited by the cost of treatment, it may be made use of in controlling
weeds in nursery areas. Mulching by polythene sheets for 32 days decreased
emergence of Dactylotenium aegypticum and Cyperus rotundus by more than 90%
and the main solarization is restricted to the 0-5 cm layer of soil.
Mulching & mowing
Different types of mulch like straw, hay, saw dust, paper, plastic film etc. when
applied to soil surface, do not allow weeds to germinate or to grow as light does not
reach to the soil.
This has been practiced for obnoxious weed as well. It has been demonstrated that
harvesting the weeds with sickle between the rows and placing the harvested
material like mulch there and this gave good management of weeds. Moisture loss
and weeds remained under control with no investment. In bed-planting method of
rice seeding, the furrow is covered with straw mulch gave sufficient control of
weeds. Spreading of polythene sheets for 32 days in furrow checked the emergence
of Trianthema, Cyperus & Dactylotenium spp under direct seeded upland rice.
Critical period of crop-weed competition: This period indicate the critical crop
growth stage or the period during which the field must be kept weed free. It is that
shortest time span during the crop growth when weeding results in highest
economic return. Generally early one-third duration of the crop period should be
maintained weed free. The critical period also differs with the cultivars used. For
examples, 20 days of weed free growth appears best in short-statured variety of
rice. For an intermediate-statured variety, the weed free period should be extended
to the first 30 days after planting of rice. Generally weed competition is less under
flooded condition where rice is transplanted as compared to upland situation.
Varieties: Short statured, erect leaved varieties permit more light compared to tall
& leafy traditional varieties. Some of the crops grow quickly & produce canopy
early resulting in shading and thus suppress weed growth. In order to control the
weeds like Echinochloa colonum which one morphologically similar to rice, a
pigmented variety ‘kalashri’ (R260-292), grown in Orissa state, India, may be
useful in distinguishing weeds from rice in early growth stage.
New introductions do have impact on weed ecology in a region. Selection of crop
spp or varieties should be tested and tried in small protective areas. Genotypes
introduced from IRRI have changed weed flora in Northwestern plain zone of
India. Consequently today Phalaris spp has become most common resistant weed
in rice-wheat system of this zone due to spread of short duration & high input
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
responsive cultivars. Whereas, this weed is not a problem with traditional varieties.
Experiments at cultivator’s field (ECF) indicate that genotype in relation to
environment do have effect on weed flora of that field. ‘Kalinga-3’, a pigmented
tall rice variety is superior to smoother the weed E. colonum.
Crop rotation: In monoculture, a number of weed spp persist and expand rapidly
and increases the possibility of occurring of certain weed spp or group of spp. Crop
rotation helps to interrupt the life cycle of weeds and thereby can eliminate or at
least reduce difficult weed problems. Weeds like Cyperus rotundus (motha) can be
controlled effectively by including lowland rice in crop rotation in place of upland
rice. Add the smoothering crops in rotation like fodder grasses and majority of
pulses controls the weeds very effectively. In continuously transplanted rice fields
under irrigated conditions, Scripus maritimus persists. When upland crops are
rotated with rice, the population of this weed has been reduced drastically even
without adoption of weed control measures. Similarly,
Striga spp, a difficult weed in continuous flooded low land fields was effectively
controlled when rice was rotated with an upland crop.
The old common practice, which farmers used to follow, is the rotation and it needs
no explanation. Every farmer knows this technique round the globe. Under organic
farming it serve as the best tool for carrying out the various activities like change of
soil micro flora, release of toxin, which in turn improve soil fertility. The weed
seed banks also affected due to rotation and consequently weed infestation found at
lowest count. Many times it was recorded interaction between site condition and
cropping system. Sensitivity analyses clearly indicate count below the threshold
values for weeds in direct seeded rice. Under puddled condition (which is not
acceptable in general) scenario changed. Whereas, flooded rice cultivation have its
limitation for varied rotation to be followed.
Growing of inter crops not only produce higher return but also help in reducing
of weed infestation in row crop. Growing of cowpea in between the rows of upland
rice resulted in an energy saving up to 315MJ/hacompared to recommended
practice and specific energy requirement is 42.0MJ/ha than hand weeding (Gogoi
and Saikia, 1997)
Water management: Depending upon the method of irrigation, weed infestation
may be increased or decreased. Frequent irrigation or rain during initial stages of
crop growth reduces several flushes of weeds. In lowland rice, where standing
water is present most of the time, germination of weeds is less, which is mainly due
to puddling & impounding of water.. Weed population and type of seeds changes
with the depth of water in rice fields. Continuous submergence with 5cm water
results in reducing weed population whereas under upland situation, weed
population and weed dry matter is very high.
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Effect of irrigation levels on weed growth: ( Reddy and Reddi,1995)
Irrigation level
Submergence-5cm
Submergence to saturation
FC to 20% DASM
FC to 50% DASM
No. of weeds/m2
Grasses BLW
5
0
12
5
382
11
494
16
Weed dry matter(kg/ha)
Sedges
0
0
51
9
6.83
4.36
73.43
72.15
DASM- Depletion of available soil moisture,FC- Field Capacity,BLW- Broadleaved weeds
Under submerged condition, tubers of Cyperus rotundus killed due to lack of
oxygen. Flooding upto 10-20 cm early in the season reduced the infestation of
many weeds including Echinochloa crusgalli. Drainage or alternate wetting &
drying induce germination and increases weed problem in rice. Under irrigated
conditions, planting crop-seeds in the moisture zone in an otherwise dry seedbed
and delaying the first irrigation reduces the weed infestation quite substantially.
Biological controls: It involves deliberate introduction & establishment of natural
enemies in areas where they did not previously occur. Good control of a variety of
aquatic weeds by Chinese grass carp (Ctenopharyangdon idella) has received
worldwide attention. In Japan, there is limited use of tadpole shrimp (Triopus spp)
to control weeds in flooded rice fields. At the IRRI, the cover effect of Azolla spp
is being evaluated as a means of weed control in rice. Several species of
herbivorous fish feed on aquatic weeds. The most important fish are Tilapia
melanopleura, T. zilli, T. nilotica, and Puniux gonianatus. In Japan, farmers also
used some species of ducks for weed control in natural rice farming.
*Bioherbicides:
Rust fungi have been tried for control of Cyperus spp.
Synthetic derivatives if naturally occurring compounds as herbicides:
Methoxyphenone is the herbicide developed as a synthetic analog of the microbial
toxin, anisomycin by employing bio-rational chemical synthesis of an herbicide
based on a natural chemistry. In Japan, this herbicide is marketed as a selective
herbicide for the control of banyard grass (Echinochloa crusgalli) in rice is easily
degraded in soil.
Natural herbicides: Solutions containing rice hull extract inhibits the germination
of barnyard grass (Echinochloa crusgalli ). Japanese farmers uses rice bran (200
g/m2) for weed control.
Many crop residues including straw from wheat, barley, rye and sorghum contain
allelopathic compounds. Volatilization, leaching, root exudation, and
decomposition of plant residues release these compounds.
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1st RDA/ARNOA International Conference
“Development of Basic Standard for Organic Rice Cultivation”
12-15 November 2002, RDA and Dankook Univ. Korea
Corn gluten meal (CG) is a by-product of corn wet milling process and has been
patented as a natural pre-emergence herbicide.
References
Balasubramaniyam,P and Palaniappan S. P.(2002)In principles and practices. Of
Agronomy, Agrobios, Jodhpur printed H S offset New Delhi
Baumann,D.T.,Bastiuans,L.andkropff,M.J.,1999)Effect of light competition. On
seed production and viability of seed of common. Sel, Proc.11thEWRS
symp.Basal,Swatzerland,p.52
Bhan, V.M. (1994) Weed infestation in rice. Indian J.Agron.39: 345
Gogoi, A.K.and Saikia, M.(1997)Biomass: A source for sustainable weed
Management in rice, Proc.3rd IFOAM-Asia Conf. Bangalore, India, pp.158
Reddy, T.Y.and Reddi, G.S.(1995) Principles of Agronomy,Kayani publisher.
Shadhara, New Delhi.
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